coax length and SWR from a CB perspective

[n6yg]

While I was doing a search for so239 barrel connector insertion loss I ran into this interesting bit of information at a CB website.

Is it just me or does anyone else think that if varying the length of your coax changes your SWR you might have an RF ground issue.

From CB world informer at http://cbworldinformer.com/cb_world_960901.htm
================================================== ===================
Mobile and base systems use different formulas. Base antennas work best using multiples of 1/2 wave lengths of cable. Mobile antennas work best using multiples of 3/4 wave lengths of cable. You may recall that one antenna that seemed impossible to match, this could have been the problem. I've had truckers come in and say they've tried everything, even new coax, and nothing works. The first question I asked; How long was the coax cable? The response usually was 6-12 feet and once it was replaced with an 18 foot piece of RG-58-A/U the match came down, usually below 1. 5: 1.

Here are the formulas used to calculate the coax length:

1. Base Coax = 468 + Freq. in Mhz x Velocity of Propagation
2. Mobile Coax = 702 + Freq. in Mhz x Velocity of Propagation
================================================== ===================

I guess if you are using multiple antenna in a directional array then coax length maters.

[AB6ND]

Some one's in the business of selling new coax cables. If the customers are happy, let them get on with it.
73
ab6nd

[KL1ZB]

I will admit I am only a tech and some of us techs are stupid. But..

Doesn't everything(including coax) resonate? When dealing with a fixed frequency I always thought coax length could affect SWR, in theory?

[ve2nsm]

Quote[/b] (KL1ZB @ Feb. 28 2007,05:43)]I will admit I am only a tech and some of us techs are stupid. But..

Doesn't everything(including coax) resonate? When dealing with a fixed frequency I always thought coax length could affect SWR, in theory?
In simple words, yes.
IF the load (antenna) is not matched with the feedline (coax), then the feedline *may* act as a transformer and change the apparent SWR.
Usually if the feedine is kept as a multiple of 1/2 wavelength, then no transformation occurs.

[ab0wr]

Quote[/b] (ve2nsm @ Feb. 28 2007,05:31)]Quote[/b] (KL1ZB @ Feb. 28 2007,05:43)]I will admit I am only a tech and some of us techs are stupid. But..

Doesn't everything(including coax) resonate? When dealing with a fixed frequency I always thought coax length could affect SWR, in theory?
In simple words, yes.
IF the load (antenna) is not matched with the feedline (coax), then the feedline *may* act as a transformer and change the apparent SWR.
Usually if the feedine is kept as a multiple of 1/2 wavelength, then no transformation occurs.
There is no "may" about it. The impedance seen at the end of a piece of coax connected to a mismatched load *is* dependent on the length of the coax.

Changing the length of coax does not change the VSWR on the antenna system. It only changes the impedance seen at the end of the coax. That is why it is important to actually measure the VSWR at the feedline/antenna junction if you want to actually find out what is going on with the antenna system.

You can match the antenna system to the transmitter if the coax is the right length but this will be only for a very small bandwidth.

tim ab0wr

[N5LRZ]

CB if full of MYTH and LEGEND.

In re to VE2NSM he is right. A half wave length of coax as calculated for a "Specific Frequency" will have the same charastic on the output as the input (become invisible). BUT that is for ONE frequency. The farther away from that one perfect frequency you get the more it is not mirror image.

And "WHO" among us operates on one single frequency?

If your radio has a 50 ohm match to a 50 ohm coax of any length then match the antenna to the coax, NOT the coax to the antenna. Always consider the signal system (radio, coax, amp, tuner (in/out), coax, balun (optional), antenna termination) as an entire system to be tuned and considered as an entire system. If the antenna (one part of the entire system) does not match the system make the antenna match the system NOT the system to the antenna. There are many matching systems for antennas. Reference the ARRL Antenna Book.

IF you cannot tune your antenna to the coax at the feedpoint of the antenna then use a transmatch aka antenna tuner.

I know some people hate antenna tuners but sorry guys the world is an imperfect place and you are just have to going to learn that sometimes an antenna tuner is a 'Requirement' not an option.

N5LRZ
R Arceneaux

[VE1IDX]

This is the reason I have 1/2 electrical wavelength coax cables made up for 6,10,15,20,40, and 80m where I have resonant antennas. I use the MJF-269 and the proper cable to tune the antenna and then run whatever it takes for cable back into the shack.The SWR is always the same at the end of 125 feet as it was at the end of 12 feet (or whatever).

[N5LRZ]

Re KL1ZB

Technicians are NOT dumb. They are the smart ones.

I have a bud who was a technician for Slumberger who prepared their down hole tools and such.

His stories to me about how college educated EE s did certain things convinced me that EEs sometimes are the dumb ones. Sometimes so dumb as to be unable to pour water out of a boot with the instructions in the bottom of the boot. ;)

LOL one particular story illustrates. The company was having an RF Leakage problem on something. The EEs wanted to buy a sniffer costing multiple thousands of dollars to find it. My bud found it with a 6 buck am/fm transistor radio. LOL

Oh by the way he did not get a thanks for saving money bonus from the company>>>>>downer.

Its the technicians who actually know how to fix things who are the SUPERIOR ones.

Later

N5LRZ

[WB2WIK]

The tools used to resolve a problem should depend on the goal.

If you use a $6 BC radio to find an RF leakage problem, that's great: But although you might find the source, you won't have any idea how strong it is, what frequency it's on, and what other leakage there is below the strongest signal you stumbled across. For hams, the $6 BC radio to "find RFI" might be perfect; in the commercial/industrial world, this may be a completely unacceptable solution that leaves more questions unaswered, which will still require the purchase or rental of more sophisticated test equipment.

An engineer with more education in these matters would be my pick, over the technician with the $6 radio.

It's kind of like "how do you make a hole in the wall to bring coax into the ham shack?" For some, hitting the wall with a big hammer to make the hole might be perfectly acceptable, cost nothing and take five seconds. In the commercial or industrial world, this probably won't cut it.

Some of what's posted above is misleading. SWR on a transmission line remains constant at all locations along that line; it never changes. A line terminated in a load equal to the characteristic impedance of the line will have SWR = 1.0 everywhere from load to source. It's only when dealing with mismatched loads that the line can become a conjugate matching network, and act to transform impedance. If you do it right, it can work in your favor. However, even while doing so, the SWR along the entire line remains constant. The SWR "measured" at the source end of the line is reduced by line loss. A transmission line having 6 dB loss, not at all uncommon when using mismatched antennas, can never "measure" an SWR over 3:1 at the source, regardless of what the load is...a 6 dB lossy line can be terminated in a short circuit, or an open circuit, and will still measure SWR = 3.

WB2WIK/6

[w5alt]

Quote[/b] (N5LRZ @ Feb. 28 2007,08:58)]Technicians are NOT dumb. They are the smart ones.

I have a bud who was a technician for Slumberger who prepared their down hole tools and such.
That's great and please tell them to keep it up. I've made most of a career of cleaning up after Schlumberger and I'm not ready to retire yet. Of course I charge double to cleanup after others rather than do things right the first time, so I'm a big fan of Schlumberger.

73,
Walt, W5ALT

[ky5u]

Quote[/b] (w5alt @ Feb. 28 2007,10:24)]Quote[/b] (N5LRZ @ Feb. 28 2007,08:58)]Technicians are NOT dumb. They are the smart ones.

I have a bud who was a technician for Slumberger who prepared their down hole tools and such.
That's great and please tell them to keep it up. I've made most of a career of cleaning up after Schlumberger and I'm not ready to retire yet. Of course I charge double to cleanup after others rather than do things right the first time, so I'm a big fan of Schlumberger.

73,
Walt, W5ALT
And I am a big fan of "Smart Technicians".

[N2RJ]

Quote[/b] (N5LRZ @ Feb. 28 2007,07:58)]Re KL1ZB

Technicians are NOT dumb. #They are the smart ones.

I have a bud who was a technician for Slumberger who prepared their down hole tools and such. #

His stories to me about how college educated EE s did certain things convinced me that EEs sometimes are the dumb ones. #Sometimes so dumb as to be unable to pour water out of a boot with the instructions in the bottom of the boot. ;)

LOL one particular story illustrates. #The company was having an RF Leakage problem on something. #The EEs wanted to buy a sniffer costing multiple thousands of dollars to find it. #My bud found it with a 6 buck am/fm transistor radio. #LOL

Oh by the way he did not get a thanks for saving money bonus from the company>>>>>downer.

Its the technicians who actually know how to fix things who are the SUPERIOR ones.

Later

N5LRZ
EE's (and engineers in general) are not dumb.

However many of them go strictly by the book because they lack practical experience.

However, many technicians (not tech ham licensees) have this prejudice against the edjumacated folks because they gots the book smarts. And as such the technicians come off with a false sense of superiority because they're more accustomed to doing maintenance and repairs.

But put them to design and tell you exactly how a device works, many of them will be scratching their heads.

And I think a lot of it is just sour grapes. #

An engineer's job isn't to implement and maintain, it's to plan and design.

The guy who designs say your car's engine on a computer probably won't be the best person to change the cylinder head gasket. #But the mechanic who can change the cylinder head gasket probably won't be able to design an engine from scratch.

[n6yg]

Am I the only one that caught the error in the formula ??
1. Base Coax = 468 + Freq. in Mhz x Velocity of Propagation
2. Mobile Coax = 702 + Freq. in Mhz x Velocity of Propagation

should it not read something like this

468 /F x V
V= Velocity of Propagation
F= Frequency in Mhz

where the Frequency is divided by 468 not added to it.

[WB2WIK]

Quote[/b] (KF6PYF @ Feb. 28 2007,11:12)]Am I the only one that caught the error in the formula ??
1. Base Coax = 468 + Freq. in Mhz x Velocity of Propagation
2. Mobile Coax = 702 + Freq. in Mhz x Velocity of Propagation

should it not read something like this

468 /F x V
V= Velocity of Propagation
F= Frequency in Mhz

where the Frequency is divided by 468 not added to it.
Wouldn't matter if you add, subtract, divide, multiply or take the cube root...the formulas are not applicable because there is no magic transmission line length, period.

The proper length for a transmission line is always enough to reach from the equipment to the antenna.

To use any "trimmed" or "calculated" length implies you're trying to use the line as a transformer to overcome a mismatch at the load end, and then any formula used only applies at one frequency; and to find what formula to use, you need to know the actual load impedance creating the mismatch.

468/f or whatever doesn't tell you any of this.

WB2WIK/6

[n6yg]

Quote[/b] (WB2WIK @ Feb. 28 2007,11:45)]Quote[/b] (KF6PYF @ Feb. 28 2007,11:12)]Am I the only one that caught the error in the formula ??
1. Base Coax = 468 + Freq. in Mhz x Velocity of Propagation
2. Mobile Coax = 702 + Freq. in Mhz x Velocity of Propagation

should it not read something like this

468 /F x V
V= Velocity of Propagation
F= Frequency in Mhz

where the Frequency is divided by 468 not added to it.
Wouldn't matter if you add, subtract, divide, multiply or take the cube root...the formulas are not applicable because there is no magic transmission line length, period.

The proper length for a transmission line is always enough to reach from the equipment to the antenna.

To use any "trimmed" or "calculated" length implies you're trying to use the line as a transformer to overcome a mismatch at the load end, and then any formula used only applies at one frequency; and to find what formula to use, you need to know the actual load impedance creating the mismatch.

468/f or whatever doesn't tell you any of this.

WB2WIK/6
I understand that, That's why I brought this whole subject up in the first place.
Aside from the fact that it does not matter, I was trying to point out that the formula for determining a 1/2 wave length of coax was also wrong.

[n6yg]

I probably should not go here http://www.qrz.com/iB_html/non-cgi/emoticons/tounge.gif
But maybe now is the time to point out the obvious. To all those people who claimed eliminating the code would cause a huge influx of CBers. That If a so called CB expert can't figure out stuff this simple. What makes you believe the average CBer has the brain cells necessary to memorize the 1800 question's needed to get his Tech and then General ticket.

Some how I know I'm going to regret this http://www.qrz.com/iB_html/non-cgi/emoticons/smile.gif

[ne6ao]

A half wave repeats what it see's at the ant,it he ant feed point is 100 ohms ina half wave coax the rig will see a 100 ohm load.If the ant is 40-60 ohms it will be 40-60 ohm at the end that the rig see's.The 1/4 wave stub is a type of transformer to match a rig to an ant when the ant is say 100 ohms like a loop for 6 mtrs.A 1/4 of coax cut for the design freq with the velocity factor used will match the ant to the rig for 6 mtrs.So a coax that is used for a mobile rig for CB will, if the ant and the coax are 50 ohms be 50 ohms at the rig.As long as the antenna feed point is 50 ohms ,any length will show 50 ohms at the rig.I ran cb for 30 yrs and All I had to do was make the ant 50ohms and used all kinds of different lengths mobile and base.I remember one time I had a flat tire near palomar observatory.I had a cb and coax but no ant.I had no jack either.So I got a piece of wire abt 9 ft long 1/4 wave .Had some rope and so I threw the rope onto a branch above the car and raised the wire .I was able to use the cars ground plane and a guy heard me and he called the friend I had been visiting and he was quite happy to bring me a jack.When he saw the makeshift antenna He looked at me and said only you tom could get this to work.ha ha Never ran power and so it really is the antenna that needs to be resonant.BTW these and other questions used to be on the advanced and extra tests ,for those of you who want to learn this stuff its in every arrl hand book and also the antenna handbook.I know my theory and also the common sense that only will come from actually bulding stuff and then learning abt what works in the real time arena.Every single day cb guys and "techs "who havent built this stuff,parrot stuff they heard from some other know it all who never read anything abt antennas.They get taken advantage of because they wont read the books and verify what the antenna system needs to work.

[K9STH]

Actually, the figure 468 already takes into consideration a velocity factor of 95 percent (the approximate velocity factor for 12 gauge wire on HF) for a half wave. To calculate the wavelength in meters the formula is 300/F where F is expressed in MHz. In the "English" system the formula is 984/F where F is expressed in MHz.

To get the wavelength you divide the speed of light by the frequency and the speed of light has been determined to be 299,792,458 (+/- 1.2) meters per second or 983,569,089 (+/- 3.397) feet per second. Now for all practical purposes when dealing with frequencies in the MHz range the 299+ figure can be rounded off to 300 and the 983+ figure can be rounded off to 984.

Glen, K9STH

[K4KWH]

Sacrilege!!! Sacrilege, I say! How DARE y'all question how them thar CBers git rid of them SWR-zzzzzzzzzzzzzzzzzzzzzz thar!!!http://www.qrz.com/iB_html/non-cgi/emoticons/tounge.gif http://www.qrz.com/iB_html/non-cgi/emoticons/biggrin.gif

I've had this argument with CB guys when I threw a monkey wrench into their CB "gospel" of coax length. #I simply explained about a screwdriver antenna and how, if all that garbage was "true", then I would have to have TEN lengths of coax each #a different length. To wit, (all figures approximate), 60 feet for 3.9 MHZ, 'bout 33 feet for 7 MHZ, 16 feet for 14 MHZ, 12 feet for 18 MHZ, about 10-11 feet for 21 MHZ, and 8 feet or so for 28 MHZ. #I mentioned that IF radio theory is good for CB, then it is good for ALL radio, and not only that, I work a bunch of frequencies outside the ham bands as well. #So WHERE would I put all those coaxes in a small pickup--particularly a 60 foot one?http://www.qrz.com/iB_html/non-cgi/emoticons/biggrin.gif http://www.qrz.com/iB_html/non-cgi/emoticons/laugh.gif # Yet, I pointed out that screwdriver I built has ONE coax...................um, about 7 feet long and I didn't bother to try to figure how long it needed to be. And I work ALL those frequencies without even THINKING about coax length!

Ya know, with the exception of one or two with cocka-maimy (sic) explanations, NOTONE of the CB coax length gurus have ever successfully refuted my explanation. Most of them remain silent.

In FORTY + years of installing HF radios I have NEVER considered coax length in the equation UNLESS the antenna design calls for it!
While there ARE special circumstances for using certain lengths, I consider it mostly horse hockey!!! And it has WORKED all these years!

73

[ka5piu]

Quote[/b] (K4KWH @ Feb. 28 2007,21:24)]In FORTY + years of installing HF radios I have NEVER considered coax length in the equation UNLESS the antenna design calls for it!
While there ARE special circumstances for using certain lengths, I consider it mostly horse hockey!!! And it has WORKED all these years!

73
Hello.

You hit the nail on the head.
I have an antenna that I roll out and hang up for 11 meters all the time.
There is 102" of stripped of shield but insulated with heat shrink tubing and 102" of intact coax and a PL-259.
This is the Super antenna, the "secret weapon" I use to make a 4 watt AH-100 talkie do "100 watts or better".
Guess what it is called?
I use 2 Francis antennas, one mounted upright and one mounted down for somewhat the same application.
The Coax feeding it is 102" long.
There are people who will argue the point all day long that the coax is a critical part of the equation.
The reason that the coax is 102" long is so that I can use a UHF barrel and 102" of wire if need be.
So,

[WA7KKP]

Quote[/b] (KL1ZB @ Feb. 28 2007,02:43)]Doesn't everything(including coax) resonate? When dealing with a fixed frequency I always thought coax length could affect SWR, in theory?
No. Coax should be operated at 1:1 or close to it, and length should have no bearing. If you have large values of SWR, say over 2:1, then certain lengths will appear at the radio to be a perfect match -- when in reality it isn't.

The SWR bridge for optimum results should be located at the antenna feedpoint -- not at the transmitter end of the coax.

Gary WA7KKP

[ne6ao]

JUST GET THE FREAKING ARRL ANTENNA HAND BOOK AND READ THE SUCKER.THERE IS ONLY ONE WAY TO TRANSMIT,50 OHM FEEDPT 50 OHM COAXANY LENGTH,50 OHM PORT ON THE RIG.

[K4KWH]

Quote[/b] (ne6ao @ Mar. 01 2007,11:58)]JUST GET THE FREAKING ARRL ANTENNA HAND BOOK AND READ THE SUCKER.THERE IS ONLY ONE WAY TO TRANSMIT,50 OHM FEEDPT 50 OHM COAXANY LENGTH,50 OHM PORT ON THE RIG.
Bu.....bu......but how will them fellers git of them thar SWR-zzzzzzzzzzzzzzzzzzzzzzz if they ain't got 18 feet of coax (thar). http://www.qrz.com/iB_html/non-cgi/emoticons/biggrin.gif #http://www.qrz.com/iB_html/non-cgi/emoticons/tounge.gif #http://www.qrz.com/iB_html/non-cgi/emoticons/laugh.gif Why even them "bird watts" won't work if they ain't got the "right" length of jumper to them leenyars!

[n6yg]

Quote[/b] (K4KWH @ Mar. 01 2007,15:27)]Quote[/b] (ne6ao @ Mar. 01 2007,11:58)]JUST GET THE FREAKING ARRL ANTENNA HAND BOOK AND READ THE SUCKER.THERE IS ONLY ONE WAY TO TRANSMIT,50 OHM FEEDPT 50 OHM COAXANY LENGTH,50 OHM PORT ON THE RIG.
Bu.....bu......but how will them fellers git of them thar SWR-zzzzzzzzzzzzzzzzzzzzzzz if they ain't got 18 feet of coax (thar). http://www.qrz.com/iB_html/non-cgi/emoticons/biggrin.gif http://www.qrz.com/iB_html/non-cgi/emoticons/tounge.gif http://www.qrz.com/iB_html/non-cgi/emoticons/laugh.gif Why even them "bird watts" won't work if they ain't got the "right" length of jumper to them leenyars!
Funny you brought that up.

That same article states that you need to use a foot or less of coax between the transceiver and a Linear. If a foot won't reach then you MUST use a 1/2 wave length section of coax between the linear and the transceiver. Where the heck do they come up with this garbage? http://www.qrz.com/iB_html/non-cgi/emoticons/laugh.gif

[N3ATS]

Huh?

So let's cut our 40M mobile coax to half-wave multiples. What do we do with the extra 50'? Coil it up behind the seat?

Jeez, talk about loss...

[W5HTW]

For a short time in the early 1970s I managed a Lafayette Radio Associate store, where I served as well as the bench technician (having a 2nd class, at the time, commercial ticket.) I repaired CB and marine HF radios, as well as some business band.

The CB myths were so enjoyable I have to admit I furthered them often by smiling and nodding, while dishing out coax in great quantities! It was pointless to waste my time trying to teach them radio theory. Just sell them what they want and move on. Which is precisely what I did.

As to technicians, (NOT talking about Technician Class amateurs of any era) there is a great deal of difference between techs and engineers. I was a tech. I could use test equipment and dive into a piece of electronics (it did not have to be a radio) and find and replace faulty components. I did this with television, consumer audio, all sorts of HF and VHF radios, and some specialty devices such as laser measuring systems for contractors, alarm systems, and the like. If I had a schematic, I was a fair tech. Without a schematic, I was most of the time a fair tech, but once in a while not so good.

I was not an engineer. I was not going to design anything, or write any books on how complicated electronics worked.

I learned the difference. At a location I was an operator/tech. We had a rather large and prototype sort of comm station. As an operator I understood the general principles of the several racks of equipment. I knew "what they did." And I knew how to use them.

We had a failure in one of the units. This was, as noted, prototype stuff. There was nothing very standard in it. My job was to tell the engineer "it ain't doing what it am supposed to be a-doin.' And that is what I did. I described the problem to him, in a conference meeting, away from the equipment.

When we returned to the site he did not go over and turn on the equipment. If he had done so, he would have seen what I had seen. And that's it.

Instead, he went to the shelf that held the dozen thick manuals on the equipment. He pulled a couple of them down, spread them on the table, and began browsing. After some time, perhaps a half hour, maybe a bit longer, he closed the books, came over to the equipment, pulled out a 'module unit', opened it, replaced a small pc board inside it, put the module back and told me to turn on the equipment.

Problem resolved.

Could I have done that? Yeah, given a few years to read all the books and learn the theory. But he had helped design the equipment.

To me that made all the difference in the world. Could he have gone into an R390A and found a problem in the IF stage? I doubt it. I could. Could he have gone into a 10 KW transmitter and repaired a driver stage? I doubt it. I could. Maybe he could have, too, but he would have had to take extra time to learn the theory. I already knew RF theory.

I have known engineers who could talk a tech through, on the phone, some very sophisticated procedures to find and isolate a problem, and then to fix it. Yeah, some techs can do it, too, but at a different level.

I think, in short, I consider them like this:

Techs equals hands on folks

Engineers equals heads on folks

It takes both.

Ed

[n6yg]

Quote[/b] (N3ATS @ Mar. 01 2007,16:08)]Huh?

So let's cut our 40M mobile coax to half-wave multiples. What do we do with the extra 50'? Coil it up behind the seat?

Jeez, talk about loss...
Using their logic if we cut our coax for 40 meters our screwdriver antennas would not tune 17 meters. I guess us hams have been doing it wrong all along. what we need is a coax switch and a bunch of coax cut for different bands http://www.qrz.com/iB_html/non-cgi/emoticons/tounge.gif

[ka0gkt]

Quote[/b] (ne6ao @ Mar. 01 2007,11:58)]JUST GET THE FREAKING ARRL ANTENNA HAND BOOK AND READ THE SUCKER.THERE IS ONLY ONE WAY TO TRANSMIT,50 OHM FEEDPT 50 OHM COAXANY LENGTH,50 OHM PORT ON THE RIG.
Oh...Then...you mean...

My Rhombic isn't a good antenna because it isn't a 50-Ohm feedpoint antenna?

Or my Zepp with 600-Ohm feed isn't a good antenna either?

Funny, I never read anything in the ARRL antenna book that said 50-Ohm feedpoint antennas, 50-Ohm transmission and that the tank circuit on my HF rig and Linear couldn't match to anything other than a 50-Ohm impedance line.

Being one of those dumb Engineers who don't do nothin' (Been in the Broadcast Biz for more than a quarter century and still get my hands dirty on a daily basis), I guess I don't know about antennas.

Remember, One sould beware of BOTH CB and HAM RADIO wives tales, urban legends and "Common" Knowlege.

73 DE KAØGKT/7

--Steve

[ne6ao]

Quote[/b] (ka0gkt @ Mar. 01 2007,16:48)]Quote[/b] (ne6ao @ Mar. 01 2007,11:58)]JUST GET THE FREAKING ARRL ANTENNA HAND BOOK AND READ THE SUCKER.THERE IS ONLY ONE WAY TO TRANSMIT,50 OHM FEEDPT 50 OHM COAXANY LENGTH,50 OHM PORT ON THE RIG.
Oh...Then...you mean...

My Rhombic isn't a good antenna because it isn't a 50-Ohm feedpoint antenna?

Or my Zepp with 600-Ohm feed isn't a good antenna either?

Funny, I never read anything in the ARRL antenna book that said 50-Ohm feedpoint antennas, 50-Ohm transmission and that the tank circuit on my HF rig and Linear couldn't match to anything other than a 50-Ohm impedance line.

Being one of those dumb Engineers who don't do nothin' (Been in the Broadcast Biz for more than a quarter century and still get my hands dirty on a daily basis), I guess I don't know about antennas.

Remember, One sould beware of BOTH CB and HAM RADIO wives tales, urban legends and "Common" Knowlege.

73 DE KAØGKT/7

--Steve
Lets not get out our my antennas bigger than yours.I was stateing the requirements of the standard feed point for a gnd plane or mobile unit,Ive built 80 mtrloops and I use only a home built tuner and yes rombics need that little resister and etc.etc.Ive built mobil 80mtr bugcacherwhere I used a center load and a coil at the feed point to give a 50 feed to the radio.I been a tech and speak eng,no degree needed.The theory is in the books.I merely state that no matter what kind of match as long as the rig see'e apprx 50 ohms it'll put out ,and the conjagate match rules.I ran CB for 30 yrs.Built my own ants.And for the record never ran power,so the ant and line had to be as efficient as possible to reduce loss.I think you knew what I meant but oh well you got bored hi hi?73's Let the good sunspots roll!By the way I made my own ladderline for the full wave loop and worked all the hf bands with it.

[n6yg]

80 meter loops ??

How do you guys run 80 meters when your supposed to have 133 feet of coax between your radio and tuner. Oh my what happens if you add a linear now you have 266 feet of coax in the shack. I guess you could coil it up under the desk, http://www.qrz.com/iB_html/non-cgi/emoticons/biggrin.gif

[kc4ylv]

Quote[/b] ]I guess you could coil it up under the desk,

I used 133 foot chunks of ladder line and strung it all over my ceiling and put model train cars on it http://www.qrz.com/iB_html/non-cgi/emoticons/tounge.gif http://www.qrz.com/iB_html/non-cgi/emoticons/tounge.gif

Gotta remember that I have to flip the engines over to the K line whenever I transmit on 75 meters or I blow locomotives

[KE5FRF]

I think the problem with the CB myths is that a lot of CBers are tinkerers just like hams, but they lack the resources that hams have. In other words, in the ham community, if I have a question about antennas, I know at least a dozen local guys with engineering, physics, or electronics degrees who I can ask a question. I am a licensed Journeyman in an electronics trade myself, so I can offer insight on certain things. We are a body of knowledge. In the CB community, you have a bunch of doods who don't know their butt from a hole in the ground, with the OCCASIONAL guy who knows just enough to be dangerous, and the RARE guy who knows a lot (probably because he's also a ham)...Between these three types of characters, sometimes someone will stumble upon some odd phenomenon, like the effect of a 1/2 wave length of coax, and have zero understanding of it, which is common knowledge for hams, but will be dumfounded by the results. Remember, they are tinkering too. So, they put the antenna up, and it works, (duh, of course it works, but no better than it would otherwise) and thus a myth is born!
Heck, it must be so, cuz them thar SWRz don't lie, right!

CBers are OBSESSED with SWR(z).

[ve2nsm]

Quote[/b] (ab0wr @ Feb. 28 2007,08:49)]Quote[/b] (ve2nsm @ Feb. 28 2007,05:31)]Quote[/b] (KL1ZB @ Feb. 28 2007,05:43)]I will admit I am only a tech and some of us techs are stupid. But..

Doesn't everything(including coax) resonate? When dealing with a fixed frequency I always thought coax length could affect SWR, in theory?
In simple words, yes.
IF the load (antenna) is not matched with the feedline (coax), then the feedline *may* act as a transformer and change the apparent SWR.
Usually if the feedine is kept as a multiple of 1/2 wavelength, then no transformation occurs.
There is no "may" about it. The impedance seen at the end of a piece of coax connected to a mismatched load *is* dependent on the length of the coax.


tim ab0wr
I said *may* because if the length is a multiple of 1/2 wave, the coax will not act as a transformer (even if in reality it's two 1/4 WL transformers back to back, I know) an will not change the apparent SWR.

So in practice, a length of coax that is not matched with the feedline may (or may not) act as a transformer and change the apparent SWR.

[ab0wr]

Quote[/b] (ve2nsm @ Mar. 01 2007,19:34)]Quote[/b] (ab0wr @ Feb. 28 2007,08:49)]Quote[/b] (ve2nsm @ Feb. 28 2007,05:31)]Quote[/b] (KL1ZB @ Feb. 28 2007,05:43)]I will admit I am only a tech and some of us techs are stupid. But..

Doesn't everything(including coax) resonate? When dealing with a fixed frequency I always thought coax length could affect SWR, in theory?
In simple words, yes.
IF the load (antenna) is not matched with the feedline (coax), then the feedline *may* act as a transformer and change the apparent SWR.
Usually if the feedine is kept as a multiple of 1/2 wavelength, then no transformation occurs.
There is no "may" about it. The impedance seen at the end of a piece of coax connected to a mismatched load *is* dependent on the length of the coax.


tim ab0wr
I said *may* because if the length is a multiple of 1/2 wave, the coax will not act as a transformer (even if in reality it's two 1/4 WL transformers back to back, I know) an will not change the apparent SWR.

So in practice, a length of coax that is not matched with the feedline may (or may not) act as a transformer and change the apparent SWR.
The length of the feedline won't change the SWR. That is why every book I have ever read that understands the theory says you have to measure SWR at the junction of the feedline and the antenna. Too many people think of antenna SWR and a matched condition as being the same. They aren't.

All it will do is change the impedance seen at the transmitter end of the feedline because of the mismatched feedline and antenna.

If that happens to be the impedance the transmitter wants to see, fine. If not, you need a tuner to make it so.

Remember, not all feedline is coax. Not all feedlines are 50 ohm impedance. Not all antenna's are 50 ohm impedance. Not all transmitters need a 50 ohm load to tune correctly.

As someone else pointed out, you have to consider it all as a system. If the system works then everything is copacetic. If the system doesn't work then a tuner can help but so can changing the length of the feedline as well as changing the antenna to modify the feedpoint impedance.

The CB folks get misled a lot by only working with a single piece of spectrum with a single antenna. In this instance the length of your feedline can be used to match the transmitter to the rest of the system. That just doesn't work when you are trying to span all of the ham bands. No one length of feedline is going to provide the proper match because the antenna feedpoint impedance is going to be quite different at different points in the spectrum.

tim ab0wr

[ve2nsm]

ab0wr, we're saying the same thing but it seem we don't speak the same language.
I say everywhere "apparent SWR"

No problem http://www.qrz.com/iB_html/non-cgi/emoticons/wink.gif

[K4KWH]

Quote[/b] (KE5FRF @ Mar. 01 2007,19:16)]I think the problem with the CB myths is that a lot of CBers are tinkerers just like hams, but they lack the resources that hams have. In other words, in the ham community, if I have a question about antennas, I know at least a dozen local guys with engineering, physics, or electronics degrees who I can ask a question. I am a licensed Journeyman in an electronics trade myself, so I can offer insight on certain things. We are a body of knowledge. In the CB community, you have a bunch of doods who don't know their butt from a hole in the ground, with the OCCASIONAL guy who knows just enough to be dangerous, and the RARE guy who knows a lot (probably because he's also a ham)...Between these three types of characters, sometimes someone will stumble upon some odd phenomenon, like the effect of a 1/2 wave length of coax, and have zero understanding of it, which is common knowledge for hams, but will be dumfounded by the results. Remember, they are tinkering too. So, they put the antenna up, and it works, (duh, of course it works, but no better than it would otherwise) and thus a myth is born!
Heck, it must be so, cuz them thar SWRz don't lie, right!

CBers are OBSESSED with SWR(z).
You know, 40 years ago, I was a green 18 y/o kid. #I had NO idea how to set up an antenna at ALL! #I shorted out stuff, smoked transmitters (and they weren't CB, either), and my buddies just went "tsk, tsk" and shook their heads. "Will he EVER learn"?http://www.qrz.com/iB_html/non-cgi/emoticons/laugh.gif
But in all that time while my ham buddies patiently worked with me (I was in CAP in those days and was not yet a ham, but we had a lot of HF stuff--mostly WWII and homebrew stuff), and showed me a lot about how to make an antenna work. And I did slowly learn a little about it. But at NO time did I ever hear anything about "coax length". ZIP! NADA! NYET! #So I proceeded to install HF radios like Lettine 240's, Texas Bugcatchers (homemade and Master Mobile), and Webster Bandspanners. #Then I heard about "Yuh gotta have 18 feet of coax fer yer reddio to not have them thar SWR'zzzzzzzzzz thar" and I'm like, "SAY #WHAAAAAAAA----" #And, of course, the more I installed radios (and became a ham), I got better and better at doin' it, and better and better at having a pretty darned good mobile signal. #But I did it WITHOUT-EVER-EVEN-FIGURING out a certain length of coax. I just put in the antenna (even pre-antenna analyzer) and got the job done. #And, of course, the art has advanced to the point that the screwdriver is probably THE most popular mobile HF antenna. #Since those days of the 60's, 70's and 80's, tubes, it has changed in spades.
I STILL don't pay any attention to a "certain" length of coax, instead, just measuring to see how much it will take to get from the radio to the antenna. Then I prune/tune/resonate (whatever) the antenna to best SWR and feedpoint match.

I've had plenty of those conversations--mostly with CBers who quote formulas (492 x VF/frequency, or something like that) and make a huge deal out of "YOU JUST GOTTA HAVE A 'CERTAIN' LENGTH", and if you contradict them, there's a huge poochy lip and a righteous you-just-don't-know-what-your-talking-about argument. #I say they don't know what they are talking about, and then I lay the multi-band antenna thing on 'em. #Usually they don't answer. #It's because they CAN'T! #Actually, it's pretty amusing to think about them sitting there going "hmmmmm"!
http://www.qrz.com/iB_html/non-cgi/emoticons/laugh.gif

73

[ab0wr]

Quote[/b] (ve2nsm @ Mar. 01 2007,20:16)]ab0wr, we're saying the same thing but it seem we don't speak the same language.
I say everywhere "apparent SWR"

No problem http://www.qrz.com/iB_html/non-cgi/emoticons/wink.gif
I don't think we are saying the same thing.

Apparent SWR doesn't exist. SWR exists. Impedance exists. Transmitter impedance to feedline impedance exists.

A "SWR meter" used at the junction of the transmitter and the feedline is not an SWR meter at all. They should be labeled "Matching meter". That is all they are good for -- except for those exact frequencies where the feedline is 1/2 wavelength long.

Let's say you have 450ohm feedline hooked to an antenna with a feedpoint impedance of 75ohm. Let's say your transmitter needs a 300ohm impedance for matching (an old tube transmitter perhaps). There will exist some length of feedline that will present a 300ohm impedance to that transmitter and you will have a matched condition.

But that isn't "apparent SWR". It is my opinion, and you can take it for what you will, that you should teach those using that term that it is just another CB myth. It's why antenna tuners aren't called "apparent SWR tuners". Its why the old Johnson Matchbox wasn't called the SWRbox.

SWR is SWR. Matching transmitter output impedance to the antenna/feedline system is just that. It isn't "apparent SWR" -- again, in my opinion.

tim ab0wr

[ve2nsm]

Whatever you say.

[WB2WIK]

Youngsters probably don't realize that until the Monimatch circuit was published in the 1950s, the average ham didn't have an SWR meter or bridge of any kind.

Amazing: Hams built highly efficient transmitting stations all over the world, and communicated with each other all over the world, for more than 30 years without having any idea what their SWR was!

Horrors.

WB2WIK/6

[K9FV]

This has all be verying interesting, but let's put this in simple layman's terms. Let's assume normal 50 ohm radios at 28mhz (or 7mhz) and coax. IF I take a 50 ohm resistor as the load and check SWR, I will have a 1:1 SWR right at the load. I can check SWR with a 10 ft peice of coax, a 20 ft peice of coax, or a 50 ft peice of coax and the SWR will still be around 1:1 - correct?

Now (still at 28 mhz), IF a 200 ohm resistor is used, the SWR should be about 4:1 right at the load- correct?

Now, as I check with different lengths of coax (1/10 wl, 1/8 WL, 14 WL, 1/2 WL, etc and keeping freq constant), should the SWR not show a change? Even at some point reading very close to 1:1 SWR?

73 de Ken H>

[KC0NBW]

if you really want to drive a cb'er off the deep end, hook up a mfj antenna analyzer to their equipment and with a bit of devious adjustment(on the analyzer,not the antenna) you can make their antenna do anything you want it to ! http://www.qrz.com/iB_html/non-cgi/emoticons/tounge.gif http://www.qrz.com/iB_html/non-cgi/emoticons/laugh.gif

after all, test equipment can never lie to you,can they?

[ve2nsm]

Quote[/b] (K9FV @ Mar. 02 2007,12:35)]This has all be verying interesting, but let's put this in simple layman's terms. Let's assume normal 50 ohm radios at 28mhz (or 7mhz) and coax. IF I take a 50 ohm resistor as the load and check SWR, I will have a 1:1 SWR right at the load. I can check SWR with a 10 ft peice of coax, a 20 ft peice of coax, or a 50 ft peice of coax and the SWR will still be around 1:1 - correct?

Now (still at 28 mhz), IF a 200 ohm resistor is used, the SWR should be about 4:1 right at the load- correct?

Now, as I check with different lengths of coax (1/10 wl, 1/8 WL, 14 WL, 1/2 WL, etc and keeping freq constant), should the SWR not show a change? Even at some point reading very close to 1:1 SWR?

73 de Ken H>
Well, I'm gonna risk another simple explanation, but maybe somebody will come and say that I should not do it.

If your SWR bridge is 50 ohms, your coax is 50 ohms and your load is 50 ohms 0j, meaning purely resistive, then the length of coax won't have ANY effect.

If the load presents capacitive or inductive reactance, or is anything else than 50 ohms j0 then the length of coax will affect your SWR reading, except if kept in multiples of half a wavelength.

Now bash me because I did not mention smith charts or used x and y vectors to answer a simple question.



edited for spelling

[n6yg]

Quote[/b] (WB2WIK @ Mar. 02 2007,09:33)]Amazing: Hams built highly efficient transmitting stations all over the world, and communicated with each other all over the world, for more than 30 years without having any idea what their SWR was!

Horrors.

WB2WIK/6
Probably true: But come on, they didn't have to worry about TVI, or RFI. Its not like the next door neighbor's had a TV in every room or telephones for that mater. Weren't they still using quill pens and lighting their home with oil lamps. http://www.qrz.com/iB_html/non-cgi/emoticons/tounge.gif http://www.qrz.com/iB_html/non-cgi/emoticons/tounge.gif http://www.qrz.com/iB_html/non-cgi/emoticons/tounge.gif

[ab0wr]

Quote[/b] (K9FV @ Mar. 02 2007,09:35)]This has all be verying interesting, but let's put this in simple layman's terms. #Let's assume normal 50 ohm radios at 28mhz (or 7mhz) and coax. #IF I take a 50 ohm resistor as the load and check SWR, I will have a 1:1 SWR right at the load. #I can check SWR with a 10 ft peice of coax, a 20 ft peice of coax, or a 50 ft peice of coax and the SWR will still be around 1:1 - correct?

Now (still at 28 mhz), IF a 200 ohm resistor is used, the SWR should be about 4:1 right at the load- correct?

Now, as I check with different lengths of coax (1/10 wl, 1/8 WL, 14 WL, 1/2 WL, etc and keeping freq constant), should the SWR not show a change? #Even at some point reading very close to 1:1 SWR?

73 de Ken H>
Your first statement is correct. If you have a matched load and feedline the SWR will be 1:1 no matter where you look on any length of feedline.

Your second statement concerning what you will read for SWR is incorrect. With a 200 ohm load and a 50 ohm feedline what you will read anyplace except at the antenna or at 1/2 wavelengths multiples is an indication of how well the impedance presented by the feedline matches what your transmitter system needs. You will not, however, be able to tell anything about your antenna system SWR. Calling what you read at the end of a feedline connected to a mismatched antenna system the "SWR" is just propagating a CB myth.

In fact, if you are using a monimatch type SWR meter that Steve, WIK mentioned, you have to be careful about what the meter is indicating at all since they are designed for a characteristic line impedance. If you are running a tube transmitter that needs 88 ohms to match and the impedance at the end of the coax is 42 ohms, I'm not sure exactly *WHAT* you will read on such a meter since neither will be the 50ohm the meter is probably designed for!

It is not in any way an indication of the SWR of your feedline/antenna system. Your antenna system SWR will be 4:1 whereever you are on the feedline. No matter what your meter says.

tim ab0wr

[vk4xjb]

Quote[/b] (K9FV @ Mar. 03 2007,02:35)]Now (still at 28 mhz), IF a 200 ohm resistor is used, the SWR should be about 4:1 right at the load- correct?

Now, as I check with different lengths of coax (1/10 wl, 1/8 WL, 14 WL, 1/2 WL, etc and keeping freq constant), should the SWR not show a change? #Even at some point reading very close to 1:1 SWR?
The swr reading should stay the same. What will change is the impedance seen at the other end of the coax.

load 200ohm, coax 50 ohm, assuming lossless coax

0wl coax, Z = 200 + j0, swr = 4:1
1/10wl coax, Z = 16 - j25.35, swr = 4:1
1/8wl coax, Z = 13 -j9.7, swr = 4:1
1/4wl coax, Z = 12.5 + j0, swr = 4:1
1/2wl coax, Z = 200 + j0, swr = 4:1

With no loss the swr stays the same while the impedance changes as you go along the coax. With loss the swr measured at the radio end of the line will slowly converge on 1:1 and that is because of losses not because of some magic length. If the swr changes when you change coax lengths you have some other problems.

The longer the coax the less power makes it to the load and less of the reflected power makes it back to the radio. Giving you a lower reading if you measure at the radio end of the line. Which is one reason why people say measure swr at the antenna. If you used 200 feet of rg58 with your example the swr at the radio end of the line should be just under 1.5:1. Use 500 feet of rg58 at 28MHz and with no load you will still have a good dummy load.



If you know your coax characteristics you can do the measurements inside where it is comfy and calculate what the readings are at the antenna. http://www.qrz.com/iB_html/non-cgi/emoticons/smile.gif

[kd7msc]

It is not how long your coax is, it is how you use it. http://www.qrz.com/iB_html/non-cgi/emoticons/laugh.gif http://www.qrz.com/iB_html/non-cgi/emoticons/tounge.gif

[K9FV]

Quote[/b] (ab0wr @ Mar. 02 2007,15:58)]Quote[/b] (K9FV @ Mar. 02 2007,09:35)]

Now, as I check with different lengths of coax (1/10 wl, 1/8 WL, 14 WL, 1/2 WL, etc and keeping freq constant), should the SWR not show a change? #Even at some point reading very close to 1:1 SWR?

73 de Ken H>

It is not in any way an indication of the SWR of your feedline/antenna system. Your antenna system SWR will be 4:1 whereever you are on the feedline. No matter what your meter says.

tim ab0wr
That was the question, We all know the antenna did not change, the question was "What will the SWR meter show?" that and nothing more, just what does the SWR meter show. #Isn't that what concerns the radio?

It's my understanding, the radio does not care what the antenna is, only what the radio sees at the radio end of the coax.

My understanding this was why some antennas say "do not change coax lenght" was because the coax lenght is chosen to allow the radio to "see" the reasonable SWR and work....

Tim, I know you are MUCH more tech than I am, so understand, I am not bashing you, just trying to be sure we are talking the same thing.

73 de Ken H.

[W0LPQ]

6PYF says ... "But come on, they didn't have to worry about TVI, or RFI. Its not like the next door neighbor's had a TV in every room or telephones for that mater"..

Surely you jest ... TVI was a big thing then and it did matter .! 6M would tear up a TV, 15M would tear up a TV bad, since the IF usually was around 21 Mc.

BCI was equally bad ... The reason TVI is not such an issue now, most do not use an outdoor antenna as they did in the 40's - 60's. A lot of people used a preamp just to get rid of some snow on black and white TV's. Premps and Amateur Radio were not mutually in agreement.

TVI/BCI was real ... many had to change their hours of operation, due to FCC intervention.

Steve is much correct, until the Monimatch came on the scene ... almost no one knew (or cared) what their VSWR was.

Bill, W0LPQ/9

[ab0wr]

Quote[/b] (K9FV @ Mar. 02 2007,18:31)]Quote[/b] (ab0wr @ Mar. 02 2007,15:58)]Quote[/b] (K9FV @ Mar. 02 2007,09:35)]

Now, as I check with different lengths of coax (1/10 wl, 1/8 WL, 14 WL, 1/2 WL, etc and keeping freq constant), should the SWR not show a change? #Even at some point reading very close to 1:1 SWR?

73 de Ken H>

It is not in any way an indication of the SWR of your feedline/antenna system. Your antenna system SWR will be 4:1 whereever you are on the feedline. No matter what your meter says.

tim ab0wr
That was the question, We all know the antenna did not change, the question was "What will the SWR meter show?" that and nothing more, just what does the SWR meter show. #Isn't that what concerns the radio?

It's my understanding, the radio does not care what the antenna is, only what the radio sees at the radio end of the coax.

My understanding this was why some antennas say "do not change coax lenght" was because the coax lenght is chosen to allow the radio to "see" the reasonable SWR and work....

Tim, I know you are MUCH more tech than I am, so understand, I am not bashing you, just trying to be sure we are talking the same thing.

73 de Ken H.
if you are using an "SWR" meter that is a Monimatch type it is designed for a specific impedance.

If the impedance of the transmitter connected to the meter doesn't match the characteristic impedance of the SWR meter then what is the meter indicating? If the impedance of the radio end of the coax doesn't match the characteristic impedance of the SWR meter then what is the meter indicating?

In both cases the meter readings will have at least some relationship to the power flowing in the circuit in the forward and reverse directions. If you don't know what these power levels actually are then you can't figure reflection coefficients which means you can't figure "SWR" either but it is my opinion that you wouldn't be calculating SWR anyway!

If you are sure that your transmitter is a 50ohm impedance then you can do the following. Get a good 50ohm dummy load and hook it to the antenna side of the meter. Hook your transmitter up and while adjusting it to different power levels, calibrate your meter reading in the forward direction (if nothing else an oscilloscope can be used to read the RF voltage and the power level calculated from that). Then turn your meter around and calibrate the meter in the reverse direction. You may need a good 50ohm attenuator to bring your power level down.

Then the meter can be used in conjunction with a tuner to bring the feedline/antenna system to a 50ohm level. Just maximize forward power and minimize reverse power. If they don't happen at the same setting of the tuner then you know something isn't 50ohms in your system on one side or the other.

You still won't know what the SWR in your system is but you'll know the tuner is providing a match between your transmitter and your antenna system.

tim ab0wr

[K9FV]

I see that no one knows the answer to my question either.... I guess I need to take my MFJ-259B (or a standard SWR meter), various lenghts of coax, and a 200 ohm resistor and just do some checking to see what the "swr" indicated would be under these conditions - at a set freq.... say 28 mhz and 7 mhz just to see what it will read.

Ken

[ve2nsm]

Quote[/b] (K9FV @ Mar. 02 2007,23:41)]I see that no one knows the answer to my question either.... I guess I need to take my MFJ-259B (or a standard SWR meter), various lenghts of coax, and a 200 ohm resistor and just do some checking to see what the "swr" indicated would be under these conditions - at a set freq.... say 28 mhz and 7 mhz just to see what it will read.

Ken
with half a wavelength of the operating frequency (remember to compensate for the velocity factor of the coax) you will have 200ohms j0 so it will read 4:1

If you have 1/4 wavelength, then you have 12.5 ohms j0 at the end of the line, you will still read 4:1

Any value that is not 90 or 180º out of phase will create a strong reactive component and then you SWR meter can read whatever it feels like, it's not designed for that... but you will still have 4:1 SWR

[K4KWH]

Quote[/b] (KF6PYF @ Mar. 01 2007,18:05)]80 meter loops ??

How do you guys run 80 meters when your supposed to have 133 feet of coax between your radio and tuner. Oh my what happens if you add a linear now you have 266 feet of coax in the shack. I guess you could coil it up under the desk, http://www.qrz.com/iB_html/non-cgi/emoticons/biggrin.gif
No. #You wrap it around your house a couple times! http://www.qrz.com/iB_html/non-cgi/emoticons/biggrin.gif #http://www.qrz.com/iB_html/non-cgi/emoticons/tounge.gif (thar)

73

[n6yg]

Quote[/b] (W0LPQ @ Mar. 02 2007,19:39)]Surely you jest ... TVI was a big thing then and it did matter .! 6M would tear up a TV, 15M would tear up a TV bad, since the IF usually was around 21 Mc.

Bill, W0LPQ/9
Hi Bill:

You caught me! I admit it, I was being facetious. maybe even fishing a bit. http://www.qrz.com/iB_html/non-cgi/emoticons/biggrin.gif http://www.qrz.com/iB_html/non-cgi/emoticons/laugh.gif

[n6yg]

Quote[/b] (K4KWH @ Mar. 02 2007,21:11)]Quote[/b] (KF6PYF @ Mar. 01 2007,18:05)]80 meter loops ??

How do you guys run 80 meters when your supposed to have 133 feet of coax between your radio and tuner. Oh my what happens if you add a linear now you have 266 feet of coax in the shack. I guess you could coil it up under the desk, http://www.qrz.com/iB_html/non-cgi/emoticons/biggrin.gif
No. You wrap it around your house a couple times! http://www.qrz.com/iB_html/non-cgi/emoticons/biggrin.gif http://www.qrz.com/iB_html/non-cgi/emoticons/tounge.gif (thar)

73
What happens if you live in a trailer? On second thought never mind they're probably CBer's anyways http://www.qrz.com/iB_html/non-cgi/emoticons/mad.gif http://www.qrz.com/iB_html/non-cgi/emoticons/mad.gif

Disclaimer: no offense intended towards fine fellow trailer bound Hams http://www.qrz.com/iB_html/non-cgi/emoticons/smile.gif

[K4KWH]

73[/quote]
What happens if you live in a trailer? On second thought never mind they're probably CBer's anyways :

Disclaimer: no offense intended towards fine fellow trailer bound Hams # #http://www.qrz.com/iB_html/non-cgi/emoticons/smile.gif[/quote]

It just takes more trips around the trailer! #http://www.qrz.com/iB_html/non-cgi/emoticons/tounge.gif #(since a "trailer" isn't usually as big as a house)
Heck there ain't nothing wrong with a trailer if it is well maintained. #Makes a bang-up "mobile" station if it's metal http://www.qrz.com/iB_html/non-cgi/emoticons/cool.gif

73

[ky5u]

New antenna design....

[kn4ds]

whew... good thing I'm not an NCG, and I don't need a loop antenna.

[ab0wr]

Quote[/b] (ve2nsm @ Mar. 02 2007,20:59)]Quote[/b] (K9FV @ Mar. 02 2007,23:41)]I see that no one knows the answer to my question either.... #I guess I need to take my MFJ-259B (or a standard SWR meter), various lenghts of coax, and a 200 ohm resistor and just do some checking to see what the "swr" indicated would be under these conditions - at a set freq.... say 28 mhz and 7 mhz just to see what it will read.

Ken
with half a wavelength of the operating frequency (remember #to compensate for the velocity factor of the coax) you will have 200ohms j0 so it will read 4:1

If you have 1/4 wavelength, then you have 12.5 ohms j0 at the end of the line, you will still read 4:1

Any value that is not 90 or 180º out of phase will create a strong reactive component and then you SWR meter can read whatever it feels like, it's not designed for that... but you will still have 4:1 SWR
Absolutely correct!!

No one *can* know what it will read!

It depends upon the impedance of your transmitter at that frequency, the characteristic impedances of each end of your meter (i.e. how well is it designed for 50ohm operation), and the impedance at the radio end of your coax.

The only way to know for sure what it will read is to see what it reads!

tim ab0wr

[K4KWH]

Quote[/b] (kd7msc @ Mar. 02 2007,15:07)]It is not how long your coax is, it is how you use it. #http://www.qrz.com/iB_html/non-cgi/emoticons/laugh.gif # http://www.qrz.com/iB_html/non-cgi/emoticons/tounge.gif
So do we apply Enzyte to it? # http://www.qrz.com/iB_html/non-cgi/emoticons/tounge.gif #http://www.qrz.com/iB_html/non-cgi/emoticons/biggrin.gif #http://www.qrz.com/iB_html/non-cgi/emoticons/laugh.gif

73

[n6yg]

Quote[/b] (K4KWH @ Mar. 03 2007,20:34)]Quote[/b] (kd7msc @ Mar. 02 2007,15:07)]It is not how long your coax is, it is how you use it. http://www.qrz.com/iB_html/non-cgi/emoticons/laugh.gif http://www.qrz.com/iB_html/non-cgi/emoticons/tounge.gif
So do we apply Enzyte to it? http://www.qrz.com/iB_html/non-cgi/emoticons/tounge.gif http://www.qrz.com/iB_html/non-cgi/emoticons/biggrin.gif http://www.qrz.com/iB_html/non-cgi/emoticons/laugh.gif

73
With all this talk about "how you use it" and Enzyte"
Does anyone know if you can Get LMR-400 in white.
I need to replace some RG8 on our sailboat and my wife does not like the black coax. Now I know I can get RG8 in white, but I have over a 100 foot run and would rather use LMR-400 or other lower loss coax at 150mhz

[N4AUD]

Quote[/b] (KF6PYF @ Mar. 04 2007,01:41)]Quote[/b] (K4KWH @ Mar. 03 2007,20:34)]Quote[/b] (kd7msc @ Mar. 02 2007,15:07)]It is not how long your coax is, it is how you use it. http://www.qrz.com/iB_html/non-cgi/emoticons/laugh.gif http://www.qrz.com/iB_html/non-cgi/emoticons/tounge.gif
So do we apply Enzyte to it? http://www.qrz.com/iB_html/non-cgi/emoticons/tounge.gif http://www.qrz.com/iB_html/non-cgi/emoticons/biggrin.gif http://www.qrz.com/iB_html/non-cgi/emoticons/laugh.gif

73
With all this talk about "how you use it" and Enzyte"
Does anyone know if you can Get LMR-400 in white.
I need to replace some RG8 on our sailboat and my wife does not like the black coax. Now I know I can get RG8 in white, but I have over a 100 foot run and would rather use LMR-400 or other lower loss coax at 150mhz
Krylon.

[W8FAX]

The only time i ever worried about coax length was when phasing a vertical system I was building. For HF, take ANY length of coax you want, hook a 50 ohm resistor on one end, check the SWR at the other, waddya think it will be ......

[ne6ao]

Quote[/b] (K4KWH @ Mar. 01 2007,20:45)]Quote[/b] (KE5FRF @ Mar. 01 2007,19:16)]I think the problem with the CB myths is that a lot of CBers are tinkerers just like hams, but they lack the resources that hams have. In other words, in the ham community, if I have a question about antennas, I know at least a dozen local guys with engineering, physics, or electronics degrees who I can ask a question. I am a licensed Journeyman in an electronics trade myself, so I can offer insight on certain things. We are a body of knowledge. In the CB community, you have a bunch of doods who don't know their butt from a hole in the ground, with the OCCASIONAL guy who knows just enough to be dangerous, and the RARE guy who knows a lot (probably because he's also a ham)...Between these three types of characters, sometimes someone will stumble upon some odd phenomenon, like the effect of a 1/2 wave length of coax, and have zero understanding of it, which is common knowledge for hams, but will be dumfounded by the results. Remember, they are tinkering too. So, they put the antenna up, and it works, (duh, of course it works, but no better than it would otherwise) and thus a myth is born!
Heck, it must be so, cuz them thar SWRz don't lie, right!

CBers are OBSESSED with SWR(z).
You know, 40 years ago, I was a green 18 y/o kid. #I had NO idea how to set up an antenna at ALL! #I shorted out stuff, smoked transmitters (and they weren't CB, either), and my buddies just went "tsk, tsk" and shook their heads. "Will he EVER learn"?http://www.qrz.com/iB_html/non-cgi/emoticons/laugh.gif
But in all that time while my ham buddies patiently worked with me (I was in CAP in those days and was not yet a ham, but we had a lot of HF stuff--mostly WWII and homebrew stuff), and showed me a lot about how to make an antenna work. And I did slowly learn a little about it. But at NO time did I ever hear anything about "coax length". ZIP! NADA! NYET! #So I proceeded to install HF radios like Lettine 240's, Texas Bugcatchers (homemade and Master Mobile), and Webster Bandspanners. #Then I heard about "Yuh gotta have 18 feet of coax fer yer reddio to not have them thar SWR'zzzzzzzzzz thar" and I'm like, "SAY #WHAAAAAAAA----" #And, of course, the more I installed radios (and became a ham), I got better and better at doin' it, and better and better at having a pretty darned good mobile signal. #But I did it WITHOUT-EVER-EVEN-FIGURING out a certain length of coax. I just put in the antenna (even pre-antenna analyzer) and got the job done. #And, of course, the art has advanced to the point that the screwdriver is probably THE most popular mobile HF antenna. #Since those days of the 60's, 70's and 80's, tubes, it has changed in spades.
I STILL don't pay any attention to a "certain" length of coax, instead, just measuring to see how much it will take to get from the radio to the antenna. Then I prune/tune/resonate (whatever) the antenna to best SWR and feedpoint match.

I've had plenty of those conversations--mostly with CBers who quote formulas (492 x VF/frequency, or something like that) and make a huge deal out of "YOU JUST GOTTA HAVE A 'CERTAIN' LENGTH", and if you contradict them, there's a huge poochy lip and a righteous you-just-don't-know-what-your-talking-about argument. #I say they don't know what they are talking about, and then I lay the multi-band antenna thing on 'em. #Usually they don't answer. #It's because they CAN'T! #Actually, it's pretty amusing to think about them sitting there going "hmmmmm"!
http://www.qrz.com/iB_html/non-cgi/emoticons/laugh.gif

73
I went to the same school as this guy.I started in 68 Iwas 16.My friends were hams that also used cb to talk to us civilians.Was a tech but got delayed till I was 44 for my ham ticket.BUT I built many antennas for cb and used basic radio theory and antenna books to make experiments.I never ran power but learned the real way was to get the antenna to apprzx 50 ohms +- 5.The same people here who miss the obvious are also have cb guys who didnt like me to talk theory abt a conjugate match and other hard to pronounce words.I was taught that for all practible use the coax will be needed to be 50.For most radios sold,the ant needs to be 50 ohm load.PERIOD.So My job was to tune the antenna to appear as a 50 ohm load.Then it didnt matter how long or short the coax was until we got to 6mtrs or 440.So Its self evident that if you want to be respected for your effort it will only come when the principals of how and why are learned.Get the ant book .The cb guys just plain most of the time didnt want to learn,they wanted someone like me to fix their ant and so it goes,they also got pissed when they came to me and had paid some cb tech150$ to fix or tweak their rig,and Id put a watt meter on and a true 50 ohm resisitive load and say your getting abt 6 or 7 watts aout.When told something else ,and then Id get the final amp transistor part number and have them look at the spec and it would show that the transistor could only handle 5 watts continuously and were told by the "tech they were sending anywhere from 25-50wts out.Each standard instalation was a typical gnd plane base or mobile
ant .So a standard gnd plane is close to 70 theoretically and bend the radials down a little to get the feed point to appx a 50 ohm load.This is in every book on antennas.

[ne6ao]

Quote[/b] (AG4YO @ Mar. 02 2007,22:08)]New antenna design....
now repeat after me ,the impedance of a full wave loop is appx 100 ohms.You feed it witha piece of rg59 calculated to be a 1/4 transformer,then the end connects to anylength of standard 50 ohm coax.But this is very important to you ncg's your head needs to be extricated from your bootom and inserted in the loop to check for resonance by keyin your rig and feeling around for a corona discharge then you know your signals getting out.

[n6yg]

Quote[/b] (n4aud @ Mar. 04 2007,06:00)]Quote[/b] (KF6PYF @ Mar. 04 2007,01:41)]Quote[/b] (K4KWH @ Mar. 03 2007,20:34)]Quote[/b] (kd7msc @ Mar. 02 2007,15:07)]It is not how long your coax is, it is how you use it. http://www.qrz.com/iB_html/non-cgi/emoticons/laugh.gif http://www.qrz.com/iB_html/non-cgi/emoticons/tounge.gif
So do we apply Enzyte to it? http://www.qrz.com/iB_html/non-cgi/emoticons/tounge.gif http://www.qrz.com/iB_html/non-cgi/emoticons/biggrin.gif http://www.qrz.com/iB_html/non-cgi/emoticons/laugh.gif

73
With all this talk about "how you use it" and Enzyte"
Does anyone know if you can Get LMR-400 in white.
I need to replace some RG8 on our sailboat and my wife does not like the black coax. Now I know I can get RG8 in white, but I have over a 100 foot run and would rather use LMR-400 or other lower loss coax at 150mhz
Krylon.
I think paint would flake off as the coax flexed. I guess I could cover exposed coax with white heat shrink.

[KC0NBW]

Quote[/b] (KF6PYF @ Mar. 04 2007,17:27)]Quote[/b] (n4aud @ Mar. 04 2007,06:00)]Quote[/b] (KF6PYF @ Mar. 04 2007,01:41)]Quote[/b] (K4KWH @ Mar. 03 2007,20:34)]Quote[/b] (kd7msc @ Mar. 02 2007,15:07)]It is not how long your coax is, it is how you use it. #http://www.qrz.com/iB_html/non-cgi/emoticons/laugh.gif # http://www.qrz.com/iB_html/non-cgi/emoticons/tounge.gif
So do we apply Enzyte to it? # http://www.qrz.com/iB_html/non-cgi/emoticons/tounge.gif #http://www.qrz.com/iB_html/non-cgi/emoticons/biggrin.gif #http://www.qrz.com/iB_html/non-cgi/emoticons/laugh.gif

73
With all this talk about "how you use it" and Enzyte"
Does anyone know if you can Get LMR-400 in white.
I need to replace some RG8 on our sailboat and my wife does not like the black coax. Now I know I can get RG8 in white, but I have over a 100 foot run and would rather use LMR-400 or other lower loss coax at 150mhz
Krylon.
I think paint would flake off as the coax flexed. I guess I could cover exposed coax with white heat shrink.
just run it through white or light colored garden hose or pvc pipe ! http://www.qrz.com/iB_html/non-cgi/emoticons/cool.gif

[N5LRZ]

OVER 100 feet of coax?http://www.qrz.com/iB_html/non-cgi/emoticons/rock.gif

You need to look at more than transmission line if you have to run over 100 feet of coax anywhere on any band. May I suggest that you move either your antenna or your radio so that they are much much closer to each other and require half the coax at most--50 feet.

Doing so will cut your loss in transmission line by half automaticly.

Say it cant be done, I say hockey pucks. FIND a way, move things, rearrange. Move the antenna closer. Lower it a couple of feet if you must--a mere couple of feet is NOT going to cause that much loss of radio horizon/radio horizon in miles is the square root of twice the height of the antenna or 400 feet for every 20 miles of distnace to radio horizon. MOVE the radio closer to the anenna.

And buy an amp to compensate for loss in transmission line. Think on this. Say you are crusing down the highway in overdrive gear and want to increase your speed a bit to pass a vehicle. Do you stop and install a new overdrive. No you accelerate to increase engine RPM to pass. So just increase your radio RPM by increasing the power output to compensate. IF you want say a full 50 watts at the anenna (the output of the radio) and you need to apply 100 watts at the input then get and run a 100 watt or higher amp. Compensate, compensate compensate.

N5LRZ

[n6yg]

Quote[/b] (N5LRZ @ Mar. 05 2007,06:20)]OVER 100 feet of coax?http://www.qrz.com/iB_html/non-cgi/emoticons/rock.gif

You need to look at more than transmission line if you have to run over 100 feet of coax anywhere on any band. May I suggest that you move either your antenna or your radio so that they are much much closer to each other and require half the coax at most--50 feet.

Doing so will cut your loss in transmission line by half automaticly.

Say it cant be done, I say hockey pucks. FIND a way, move things, rearrange. Move the antenna closer. Lower it a couple of feet if you must--a mere couple of feet is NOT going to cause that much loss of radio horizon/radio horizon in miles is the square root of twice the height of the antenna or 400 feet for every 20 miles of distnace to radio horizon. MOVE the radio closer to the anenna.

And buy an amp to compensate for loss in transmission line. Think on this. Say you are crusing down the highway in overdrive gear and want to increase your speed a bit to pass a vehicle. Do you stop and install a new overdrive. No you accelerate to increase engine RPM to pass. So just increase your radio RPM by increasing the power output to compensate. IF you want say a full 50 watts at the anenna (the output of the radio) and you need to apply 100 watts at the input then get and run a 100 watt or higher amp. Compensate, compensate compensate.

N5LRZ
The antenna is at the top of the main mast on my sailboat. The mast is 65+ feet. By the time the coax runs through the wire conduit to the navstation its close to an 80 or 90 foot run. Unless I mount the antenna to a spreader or a railing there's not much I can do about it.

Using a low gain antenna at the top of the mast works far better then using a high gain antenna mounted just above deck. I have both types of antennas and on VHF I get far better results with the mast mounted low gain antenna. The high gain antenna suffers badly from QSB.

Even with all the coax loss when I switch to the mast mounted low gain antenna the broader pattern eliminates the QSB and I get far better range. Remember this is on a seagoing vessel that is subjected to a lot of motion. Furthermore the mast gets the antenna above the tops of the swells (most of the time http://www.qrz.com/iB_html/non-cgi/emoticons/tounge.gif ) I guess I could mount the high gain antenna at the top of the mast. If I did that I fear most of the time my signals would be destined for far away galaxy's http://www.qrz.com/iB_html/non-cgi/emoticons/biggrin.gif

[K9STH]

I wouldn't use LMR-400 on your mast. LMR-400 has a solid center conductor (see http://www.m2inc.com/main%20html/lmr400_a.pdf for specifications) and it WILL break very soon due to the fact that the mast will "flex".

RG213/U, which has a stranded center conductor, will hold up MUCH better in your application. Using a "super flex" type of LMR (or Heliax, etc.) will work, but the cost is going to be considerably higher and the additional benefits are going to be marginal, especially when the cost is figured into the equation.

Glen, K9STH

[n6yg]

Quote[/b] (K9STH @ Mar. 05 2007,19:59)]I wouldn't use LMR-400 on your mast. LMR-400 has a solid center conductor (see http://www.m2inc.com/main%20html/lmr400_a.pdf for specifications) and it WILL break very soon due to the fact that the mast will "flex".

RG213/U, which has a stranded center conductor, will hold up MUCH better in your application. Using a "super flex" type of LMR (or Heliax, etc.) will work, but the cost is going to be considerably higher and the additional benefits are going to be marginal, especially when the cost is figured into the equation.

Glen, K9STH
Thanks for pointing that out. I did mean to say LMR-400 flex