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WD8QXP
02-18-2010, 10:59 PM
Hi,

I installed a low pass filter in my transmisson line. How effective would a rf choke be, in addition to the low pass filter, at the base of the antenna? What are some home made and comercial options as to rf choke?

Thanks, Helmut

WB2WIK
02-18-2010, 11:02 PM
What's the low pass filter for?

Coaxial choke baluns are very effective but over a fairly limited frequency range to be optimized.

What are you trying to do?

WD8QXP
02-18-2010, 11:10 PM
Hi,

I use the low pass for harmonic suppression. I want to use the choke to prevent the transmission line from radiating .


Thanks WD8QXP

W5DXP
02-18-2010, 11:20 PM
I want to use the choke to prevent the transmission line from radiating.

A choke with 10k ohms of choking action will probably do that.

WB2WIK
02-18-2010, 11:25 PM
Hi,

I use the low pass for harmonic suppression. I want to use the choke to prevent the transmission line from radiating .


Thanks WD8QXP

Those are the "typical applications," but can you be more specific? Such as, what harmonics are you trying to suppress? What kind of filter is it, exactly? What frequencies are you operating?

Ditto with the choke. What frequency (or frequencies)? Where did you intend to install it in the system?

This stuff determines how to go forward with various designs or components.

WD0GOF
02-18-2010, 11:46 PM
Hi,

I installed a low pass filter in my transmisson line. How effective would a rf choke be, in addition to the low pass filter, at the base of the antenna? What are some home made and comercial options as to rf choke?

Thanks, Helmut

Check out http://www.hamuniverse.com/balun.html This type of choke is not narrow banded like toroid wound chokes. If wound for 80 meters then all higher frequency currents in the shield will be blocked. I use the same setup with the tvi low pass at the rig and the coax choke at the antenna. KEY,, my antenna is resonant on 80,40,20,15 and 10 meters. Therefore no antenna tuner is needed. I have no RF in the shack on any band any power level. I run a Hallicrafters SR-2000 transceiver with full legal power into a GAP Titan DX.
73's Walt.

K0RGR
02-19-2010, 12:15 AM
Modern solid state radios rarely need a low pass filter, because they incorporate one in their output stages, anyway. It won't hurt anything to have it there, but it's probably not needed.

A coax choke balun is recommended by some antenna makers, like Cushcraft. It's something that's simple to do, and may actually do some good. However, if you search, you'll find a recent post here on QRZ from G3TXQ, where he measured the actual performance of coax choke baluns, and found that they were fairly limited.

These commercial units should provide somewhat better performance, but they are not in the 'cheap' category: http://www.dxengineering.com/Parts.asp?ID=1474&PLID=166&SecID=81&DeptID=9&PartNo=DXE-FCC050-H05-A

WB2WIK
02-19-2010, 01:29 AM
Check out http://www.hamuniverse.com/balun.html This type of choke is not narrow banded like toroid wound chokes. If wound for 80 meters then all higher frequency currents in the shield will be blocked.

I just read that article and a lot of it is pure unsliced balogna.

Not one of those RF choke baluns would work on 160m, for example.

Some had too many turns to work effectively on 10-12 meters, also.

One showed the balun so far away from the antenna that it wouldn't do anything to prevent radiation between the antenna and the choke, which was several feet.

People discussed results based on "SWR?" That has nothing whatever to do with whether the balun is working.

That's a silly article. RF choke baluns certainly work, but the one that covers 160 through 10 meters hasn't been invented.

WB2WIK
02-19-2010, 01:31 AM
Modern solid state radios rarely need a low pass filter, because they incorporate one in their output stages, anyway. It won't hurt anything to have it there, but it's probably not needed.



That's for sure, and the reason I asked what the real reason for the filter was.

I've seen people use "30 MHz low pass fllters" on 40 meters, as though the filter would actually perform any function at 7 MHz...where it will obviously pass the 2nd, 3rd and 4th harmonics right on through just fine.

It pays to think about why the filter is being used before using one.

WD8QXP
02-19-2010, 02:42 AM
Here is what I want to do. I want to work cw on the slow speed section of 40and 20 meters. It probably will be a 100 kc spread at the very most. I have a MFJ veritcal dipole in the attic. I have it set to and swr of 1.4 to 2.0 for most of the 40 meter cw portion for the advanced privelages. I just purchased a Drake TV-42-Lo 100 watt low pass filter. I will be running 20 or so watts. I live in a condo situation and want to do all I can to prevent problems.
I think I will use the drain pipe coil form for it has less loss as a coil form. What is the magic length in coax for what I am trying to do? Also, are there comercial chokes that are of known quality and performance with a reasnable cost available?
Sounds that it might be a good idea to insert a choke or wind one. I have the low pass filter connected to the tranceiver through a 90 degree connector and a coupling. I would place the choke right at the antenna uhf connector.


Where is the slow speed cw action on 20 and 40 meters?

Thanks Helmut

N0SYA
02-19-2010, 03:17 AM
Hi
I think you will have better results by employing ferrite cores on the coax.

W9PSK
02-19-2010, 04:50 AM
I looked into this type of choke when I put up my ten meter hamstick dipole up. There's a formula for figuring out how many feet of coax you need, but I don't remember what it is. I will see if I can find that thread and find the link that was given to me. I know mine was way less than the 18-21 feet the article calls for, but mine was just for 10 meters.

AE5JU
02-19-2010, 05:01 AM
A Low Pass filter has the knee above the range of the radio, and its purpose is to suppress harmonics... interfering with TV's, etc.

A choke balun is a High Pass filter set above the freq of the radio, and it's purpose is to suppress radiation from the coax shield.

Two different filters for two different purposes.

W9PSK
02-19-2010, 05:02 AM
Ok. I found it. This link should be very helpful.

http://www.k1ttt.net/technote/airbalun.html

WD0GOF
02-19-2010, 05:18 AM
Well, Helmut, by now you have probably guessed that every thing is wrong and every thing is right. But to start with get an MFJ-259A analyzer, then look at what the antenna makers recommend. And then experiment to find what is best for your site. Remember a lot of the replies you receive on a forum are from very well read folks who have never bent a wire. So look at everything keeping a grain of salt handy and then go bend some wire.
73's Walt

KM3F
02-19-2010, 07:14 AM
It appears the original poster is confused about what a low pass filter is and what a choke does.
Two different actions and purposes.
The low pass filter generally cuts down harmonic signals above 10 meters unless designed to cut off above 6 meters.
This filter won't do anything on 20 and 40 meters but introduce a small amount of loss.
.
A choke close to the antenna feed pont impedes outer shield current from influencing the the antenna pattern and allowing power to return to the transmitter on the outside of the shield due to changes in match as you move frequency away from the best match point.

If the antenna is close to your shack or other apartment units, you will have a hard time preventing any interference to others due to the closeness involved.
Interference to computer speakers, stero etc won't be helped by filters and may or may not by chokes in many cases. The remedies lay with filtering/bypassing/shielding at those devices to stop them from reacting to the RF field of your antenna..
.
It's difficult for another person to accept that their equipment is not good enough after you come along with a Ham operation legally and cause them interference they didn't have until you begin operating.
It's not your duty to clear there issues but sometimes it can be worked out if you know what to do and they allow you to try.
.
Some examples in my own operation:
At first I would get into the telephone line and TV by brut force with signal intensity off my 75 m dipole and return power especially on 40m.
Now with a tuner and chokes outside on the dipole at the remote antenna switch, a choke on the tri band beam right off the feed point up on the tower, I have no more RFI even at 1 kw power in anything in and around the house.
No filters involved except what is required 'in the radio equipment' to meet FCC original acceptance requirements for second and third harmonic content.

KA5S
02-19-2010, 07:17 AM
Here is what I want to do. I want to work cw on the slow speed section of 40and 20 meters. It probably will be a 100 kc spread at the very most. I have a MFJ veritcal dipole in the attic. I have it set to and swr of 1.4 to 2.0 for most of the 40 meter cw portion for the advanced privelages. I just purchased a Drake TV-42-Lo 100 watt low pass filter. I will be running 20 or so watts. I live in a condo situation and want to do all I can to prevent problems.
I think I will use the drain pipe coil form for it has less loss as a coil form. What is the magic length in coax for what I am trying to do? Also, are there comercial chokes that are of known quality and performance with a reasnable cost available?
Sounds that it might be a good idea to insert a choke or wind one. I have the low pass filter connected to the tranceiver through a 90 degree connector and a coupling. I would place the choke right at the antenna uhf connector.


Where is the slow speed cw action on 20 and 40 meters?

Thanks Helmut

Hi, Helmut.

The reason for choking off transmission line shield currents is that RF radiating into living areas from the coax can cause problems with electronics installed there. There are good 50 ohm 1:1 chokes that will do what you want, but some coax wound on the right material ferrite toroid (http://lists.contesting.com/_towertalk/2003-07/msg00296.html) is essentially the same thing. (In that same thread, Joe Reisert, W1JR, analyzes a solenoid Balun made with 50 feet of coax on a 4.5 inch PVC pipe and says it has NO choking action on 20. You'd want to use a smaller solenoid than THAT.)

Unfortunately, RF radiating in the attic can be a problem as well, if it couples to nearby electrical or telephone/TV wiring. That's one of the perils of apartment hamming.

Good luck!


Cortland
KA5S

W5DXP
02-19-2010, 01:10 PM
... In that same thread, Joe Reisert, W1JR, analyzes a solenoid Balun made with 50 feet of coax on a 4.5 inch PVC pipe and says it has NO choking action on 20.

That's understandable. Such a choke has ~42 turns which is more than double the number of optimum (maximum impedance) turns for 20m. The calculator at:

http://hamwaves.com/antennas/inductance.html

says that the self-resonant frequency for that 42 turn coil is ~9.3 MHz making it pretty much worthless as a choke on 20m, as W1JR indicates.

In fact, it appears that a reasonable rule of thumb for a single-band ugly balun on a 4.5" coil form would be approximately the same number of turns as the wavelength in meters, i.e. ~20 turns for 20m. Anyone want to wind one for 80m? :)

AB8XE
02-19-2010, 02:23 PM
snip
I just purchased a Drake TV-42-Lo 100 watt low pass filter. I will be running 20 or so watts. I live in a condo situation and want to do all I can to prevent problems.
snip

Thanks Helmut

It is quite admirable to be proactive in avoiding problems, however the feedline radiation at 7mhz/20 watts will probably be minuscule, and highly unlikely to be noticed. It has created a lively and informative discussion on filters and chokes.:p

AI3V
02-19-2010, 02:37 PM
It is quite admirable to be proactive in avoiding problems, however the feedline radiation at 7mhz/20 watts will probably be minuscule, and highly unlikely to be noticed. It has created a lively and informative discussion on filters and chokes.:p


Hear, hear (rises to feet clapping and whistling:))

A more sensible approach would be to actually MEASURE the feedline current, and determine if there is even any rf to choke!

Suppressing feedline radiation will sometimes make a difference in the antennas radiation pattern, But to hope for a change in TVI/RFI is a stretch.

Consider that you are talking about suppressing a few percentage points of power flowing out of a device -the antenna- that is DESIGNED to RADIATE!

*******************************

Want to know what REALLY works for preventing TVI/RFI?

Simple.

MOVE your antenna away (read get it up higher in the air) from whatever you are trying not to interfere with.:D

Winding a choke when you are in the antennas "near field"?:rolleyes:

Rege

AI3V
02-19-2010, 02:42 PM
A mini-rant:

Folks that claim that there station causes -no- interference whatsoever make me laugh.

I have repaired literally thousands of transmitters.

EVERY SINGLE ONE of them caused interference.

All you had to do is know where to look for it.

Rege

P.S. Sorry about the poor rant, it's friday, and that's all yinz are gettin.:D

Rege

G3TXQ
02-19-2010, 03:03 PM
Of course, the closer the antenna to the house/shack the more likely you are to get interference because of signals induced directly from the strong radiation fields. But if you feed the antenna with coax and don't choke it effectively you have inadvertently brought an antenna element within feet, maybe even inches, of susceptible equipment. I don't understand why you would ever risk doing that.

We are not talking about "a few percentage points" here. A dipole fed with an "unlucky" length of coax can easily show over 90% of the feedpoint current flowing back along the coax.

I recently published some measurements on the impedance of common-mode chokes:

http://www.karinya.net/g3txq/chokes/

You'll see that "coiled coax" air-cored chokes can achieve high impedances, but they are quite narrowband and you have to get the number of turns/diameter correct for the frequencies you are interested in. Ferrite-cored chokes generally cover a wider bandwidth.

73,
Steve G3TXQ

K0RGR
02-19-2010, 03:33 PM
Here's another commercial option - http://www.unadilla.com/w2dinline.htm

I've never torn one apart, but I believe this is composed of a series of ferrite beads on the outside of a section of coax. I think this would be similar to a couple of the ferrite units G3TXQ has so ably documented.

I've used W2AU's similar yagi baluns and I thought they worked great. But, I thought 8 turns of RG-213 on a 2 liter Pepsi bottle worked great too. G3TXQ's measurements show that it probably does work fairly well on the bands above 40 meters - which is really the only place I've used it.

WD8QXP
02-19-2010, 03:53 PM
Hi,

I think an ounce of prevention is worth a pound of cure. I just dont want to be on the radar screen. So I am thinking that whatever I can do that might help even in the slightest degree is worht it.
If a choke being in the field of the antenna is a problem and if the choke has to be as close to the antenna as posible should you shield the coke? Is there an optimum distance from the antenna for the choke?
What type of ferite core would you use for 40 and 20 meters choke if I were to wind my own choke?
Is there a chance that inserting a choke may cause more problems for me? Can you have more than one choke in the feed line?
Thanks for all the information.

Thanks Helmut

AI3V
02-19-2010, 03:55 PM
Of course, the closer the antenna to the house/shack the more likely you are to get interference because of signals induced directly from the strong radiation fields. But if you feed the antenna with coax and don't choke it effectively you have inadvertently brought an antenna element within feet, maybe even inches, of susceptible equipment. I don't understand why you would ever risk doing that.

We are not talking about "a few percentage points" here. A dipole fed with an "unlucky" length of coax can easily show over 90% of the feedpoint current flowing back along the coax.

I recently published some measurements on the impedance of common-mode chokes:

http://www.karinya.net/g3txq/chokes/

You'll see that "coiled coax" air-cored chokes can achieve high impedances, but they are quite narrowband and you have to get the number of turns/diameter correct for the frequencies you are interested in. Ferrite-cored chokes generally cover a wider bandwidth.

73,
Steve G3TXQ

Alright then, Fair enough,

Let's agree (for the sake of discussion:)) that you have 90% of your power coming back down your feedline.

That just reinforces my opinion that you need to get a proper antenna, and get it up higher.

The real world pattern of the typical compromise antenna (notice these posts NEVER start out "I have a 5 el monobander on a 100 ft tower") is the problem.

You have RFI because you are trying to run a random antenna inside or very near your station, Not because you left out 6 turns of coax on your monobander.

Rege

KD0CAC
02-19-2010, 04:31 PM
This is an example of why I like this site , hashing out solutions to issues .
One theme that I pickup on , from being in the auto repair bis. [ diagnostics ] is that have a , good diagnostic way of thinking .
Get all the details up front , what is being worked on , then a good description of the issue .
Do not through parts at the problem , find the issue , fix , repair or replace .
Using good diagnostic principles , then test equipment to confirm , both before and after repair to see that what you did , acutely addressed the original problem .

G3TXQ
02-19-2010, 04:46 PM
notice these posts NEVER start out "I have a 5 el monobander on a 100 ft tower"
Perhaps that's because the fellow who has purchased a 5-el monobander and a 100ft tower can also afford the extra 6ft of coax to wind an air-cored choke or the $8 needed for a ferrite core.

Having the antenna up in the clear doesn't guarantee immunity from common-mode problems. If the common-mode path is low/medium impedance compared to the impedance into the antenna, you can be sure a significant proportion of the feedpoint current will flow that route.

Steve G3TXQ

G3TXQ
02-19-2010, 05:02 PM
Hi,

I think an ounce of prevention is worth a pound of cure. I just dont want to be on the radar screen. So I am thinking that whatever I can do that might help even in the slightest degree is worht it.
If a choke being in the field of the antenna is a problem and if the choke has to be as close to the antenna as posible should you shield the coke? Is there an optimum distance from the antenna for the choke?
What type of ferite core would you use for 40 and 20 meters choke if I were to wind my own choke?
Is there a chance that inserting a choke may cause more problems for me? Can you have more than one choke in the feed line?
Thanks for all the information.

Thanks Helmut
Helmut,

Common-mode current gets onto the coax braid by two mechanisms - conduction and induction.

The conducted current happens because current flowing along the inside surface of the braid reaches the feedpoint and "sees" a low impedance path back along the outer surface of the braid. A choke as close as you can get it to the feedpoint should stop conducted current.

The induced currents are caused because the feedline - not the choke - is close to the RF field radiated by the antenna; the coax braid is acting like another antenna. If the feedline were absolutely symmetrical with respect to the two sides of the antenna there would be no induced current because the field from the two sides would cancel; but in most installations there will likely be some assymetry, and so induced currents can flow. To stop it, try installing another choke close to the shack, or part way along the feedline - you may need to experiment.

What sort of ferrite core should you use? Did you look at my web page:

http://www.karinya.net/g3txq/chokes/

Steve G3TXQ

WB2WIK
02-19-2010, 06:40 PM
My impression is you should just go ahead and operate and see what happens. If there are problems, try to find solutions. Being proactive to avoid interference is admirable but likely unnecessary.

On 20m and 40m, with an indoor antenna you're far more likely to interfere with things like telephones, telephone modems (if you have any), alarm systems, smoke detectors and things of that nature. No amount of "filtering" will prevent or reduce any of these things -- impossible.

If you have feedline radiation issues, a good choke system can surely help; but you may not have any problems at all. The MFJ verticals already have a feedline decoupling system designed into the antenna, and may not need anything to supplement this, especially if you route the transmission line away from the feedpoint in such a way that it's unlikely to couple to the antenna. That just takes a bit of planning and maybe some tape or tie-wraps.

"TVI" from a 20W transmitter on 20m or 40m is extremely unlikely.

You asked about the slow-code areas of these bands and I don't think anyone answered this: I think you'll find a lot of slow code on 40m above about 7050 kHz, including the old "Novice" subband, which is legal for all amateur license classes today, between 7100 and 7125 kHz. The new "Advanced/Extra" phone band starts at 7125, so I wouldn't go above that (although it's certainly legal to). I hear a lot of "slow code" on 40m in the late afternoons and evenings around 7100-7125.

On 20m, I think there's "less" of the slow code stuff, but you might focus above 14050 kHz. That's fairly wide open space unless there's a contest going on -- then it's very crowded.

WB2WIK/6

OH2WW
02-19-2010, 06:45 PM
Helmut,

... Did you look at my web page:

http://www.karinya.net/g3txq/chokes/

Steve G3TXQ

Hi Steve. I'm looking to add an air cored choke to a multiband vertical mobile antenna, and I'm using Aircell 7 which is the same diameter of RG58 but has less loss than the RG213.

Would the recipes for air cored chokes on your site using either RG58 or RG213 be close enough for the Aircell 7, and if so, which ones? Or would measurements and specific recipies be needed for the Aircell 7 coax?

G3TXQ
02-19-2010, 07:03 PM
Think of it as a solenoid inductor comprising just the braid and its jacket - what matters is the wire (braid) diameter and the jacket diameter, which will determine the turns spacing. I don't know Aircell 7 but I believe it falls somewhere between RG58 and RG213 for braid and jacket diameters; that means the performance should lie somewhere between my results for RG213 and RG58.

If this is a multiband antenna, note the relative narrowband performance of air-cored chokes. Also, be sure that you have the specified bend radius for that cable. Finally, I don't know what the dielectric is, but watch out for "migration" of the centre conductor.

73,
Steve G3TXQ

WD8QXP
02-20-2010, 04:01 AM
Hi,

I see that in the table of home made chokes that some of the combinations use certain coax with ferite cores. What, if any effect, does the switching of coax types in the feedline have on the feedline performance. Also, I have not soldered any of my connectors to the braid. Is that a big deal?
I never considered fire alarms or phone lines, always thinking about radios and televisions.
I am thinking that staying with one type of coax, making an air coil or by winding the coax around a ferite core would have an advantage.
Also. My ground wire is at least 50 feet long, number six copper. Are there rf considerations with such a long grounding lead, and if so what would be a fix. I am bonded to the service grounding system.
How good an rf path is a uhf connector junction? Would going to another style of connector,such as an N be a better way to go?
Also, I have always wondered if having an antenna in the attic would be a lightning risk.
Thanks for the slow speed cw spots.

Thanks WD8QXP

G3TXQ
02-20-2010, 10:12 AM
Hi,

I see that in the table of home made chokes that some of the combinations use certain coax with ferite cores. What, if any effect, does the switching of coax types in the feedline have on the feedline performance.

It has no effect, providing it has the same characteristic impedance as your main feedline and can handle the power. If you are worried about that, or making connections, you can use a "bead balun" which comprises ferrite cylinders slipped over the feedline.


My ground wire is at least 50 feet long, number six copper.

Then it isn't an effective ground - it's potentially another radiating element. Using a dipole you shouldn't need an RF ground anyway! If grounding the radio makes a difference when using a dipole, you have a common-mode issue.

Using an antenna in the attic is always going to be tough from an RFI perspective. The best you can do is to ensure that your transmitter power really is being radiated by the dipole in the attic, and not by the feedline - that's why you need the feedline choke. If you still get RFI problems, it's likely because of the proximity of equipment/wiring to the antenna - then it's a case of trying to choke the leads of individual equipments.

73,
Steve G3TXQ

KA5S
02-20-2010, 10:36 AM
Hi,

I see that in the table of home made chokes that some of the combinations use certain coax with ferite cores. What, if any effect, does the switching of coax types in the feedline have on the feedline performance. Also, I have not soldered any of my connectors to the braid. Is that a big deal?...


Just use 50 ohm coax large enough to handle your transmit power. The choice is often driven by the need for a high power coax small enough to put sufficient turns on a core, but even miniature coax like RG-142 will handle the power you are talking about. Not a good idea for the whole run, due to loss -- unless you need to hide it. It is easier and more common to find BNC connectors on 142.



The braid MUST make good connection to connector shells. The usual solder-type PL-259 has access holes to allow soldering the braid, and absent a clamped braid connection, you should do so. There are other kinds of braid termination; some (N and BNC) connectors are crimped (not a bad idea) and some cheap connectors trap the braid between the shell and the center insulator. Some really cheap ones (avoid these except for fix-now repair later) just screw onto the braid and depend on pressure exerted by the coax cable insulation to make contact. I like crimp types, but don't have many right now.

Type N is usually either crimped (you should use the right crimping tool) or have a soldered-on pin and a clamped/trapped braid. They work well. However, PL-259's are satisfactory at these frequencies. BNC and N connector clamping action uses the mechanical advantage of a screw thread to trap the rain between two metal surfaces, far better than The (Radio) Shack cheap PL-259's catching braid between metal and a plastic insert. Those do work, but fail a lot more often.

Cheers,


Cortland
KA5S

M0WAN
02-20-2010, 02:00 PM
Some of us liive in places that dictate compromise antennas. And as for a coax choke to work from 160 to 10, I'll bet it is possible, and it would take the form of a large coil wound in unequal sections much as you'd do with a plate choke. Losses at higher frequencies due to the length of coax used in the choke may be a problem though.

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