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W1GVT
08-07-2010, 10:02 AM
Newbie here. I'm looking to put together an inexpensive antenna for a FT-101B.

I've got a 25' length of RG59. Would that be suitable for a feed-through to the antenna from my basement shack? Thanks.

W8JI
08-07-2010, 11:57 AM
Newbie here. I'm looking to put together an inexpensive antenna for a FT-101B.

I've got a 25' length of RG59. Would that be suitable for a feed-through to the antenna from my basement shack? Thanks.

Depends on the antenna and the frequency. It will work fine in a lot of cases.

W1GVT
08-08-2010, 01:28 PM
Well, I'm probably going to use something like a GR5V to cover 10-80 meters. If it helps, I'll be going from the FT101B to a MFJ-941 then out to the antenna so I'm looking for suggestions. This is my first base setup so I'm not really stuck on anything.

I have a good length of the RG59 so that is why I asked about using it as a feedline but if you can suggest something better for an all purpose setup I'm happy to listen. Thanks!

KD0CAC
08-08-2010, 02:00 PM
I am posting to read answers , and share what little info I have gained so far .
If I am remembering correctly , the RG-59 is 75 ohm , most modern rigs are built to see 50 ohm , but you have an older tube rig , so may be less of a issue , they take a wider spectrum of input , as I understand .
The tuner maybe on the narrow range of of what it will tune , but again not sure of the spec.s .
But putting a station together starts with a plan of things that will hopefully work together , then testing to see if the plan worked out , in this case getting everything that you mentioned put up as close as possible to the design / factory recommendations , antenna height above ground etc. then putting some test equipment in place , like an antenna analyzer to see what may need tuning / adjustment .
If things do not work the 1st time , that were being a ham and seeing what it takes to make things to work well is 1/2 the fun , as I see it .
If things do not work out , hopefully you will not get disappointed and drop it , but move on to the next try .

W1GVT
08-08-2010, 02:05 PM
Ah, you're right about the RG59 being 75ohms. I missed that. (I'm new at this and lacking time to get anything done.) Rather than scrounge I'll cough up the money to do it right!

KD0CAC
08-08-2010, 02:22 PM
That was not what I meant , the 75 ohm coax may not be as much of an issue with you FT-101 , as it could be with a solid state rig .
Its the impedance / resistance that the radio was built to work with , the older tube rigs had a wider range of what they could work with , the newer solid state rigs have a narrower range of what they can handle coming into them through the feed line .
So you have more range to work with .
Some have used short pieces of 75 ohm coax to make matching networks , what those are , are as you build your antenna , feed-line and into your rig , there should be a set range of impedance that will make for efficient power transfer from transmitter to antenna and out to who ever you want to send the signal to .
Again with state rigs built to have 50 ohm feed-line .
I do not remember what the impedance of the G5RV is , but lets say it is 300 ohms , then it is attached to a 450 ohm latter-line , now you want to bring that into your shack , but have metal window frames , so you want to use coax , and like most you pick 50 ohm for solid state rig , now you have a mismatch between the coax & latter-line , this is where you would use a balun to get the impedance of the antenna & latter-line to match that of the coax , to go into your rig .
At least that is how I understand it , so I wait for others to confirm or correct .

AE5TE
08-08-2010, 05:13 PM
It seems like there is a lot of free RG-59 and RG-6 cable laying around and with your MFJ tuner (I have the same model) I wouldn't be afraid to use it, especially if the run you need is not too long.

One the other hand, the G5RV is something you'll need to live and learn. You say you want to use it on 80-10 and okay, you can tune it and work all bands, but really they only work "well" on 20M and 80M - they are not as magical as is often promoted. But it is a cheap and easy antenna to build and install and it will get you started.

KA5S
08-08-2010, 05:29 PM
Newbie here. I'm looking to put together an inexpensive antenna for a FT-101B.

I've got a 25' length of RG59. Would that be suitable for a feed-through to the antenna from my basement shack? Thanks.

Yes.

As to the G5RV, read what the inventor said abouy using a balun with one:

Recent experiments by the author to determine the importance or otherwise of "unbalance" effects caused by the direct connection of a coaxial feeder to the base of the matching section had a rather surprising result. They proved that, in fact, the hf currents measured at the junction of the inner conductor or the coaxial cable with one side of the (balanced) matching section and at the junction of the outer coaxial conductor (the shield) with the other side of this section are virtually identical on all bands up to 28mhz, where a slight but inconsequential difference in these currents has been observed. There is, therefore, no need to provide an unbalanced-to-balanced device at this junction when using coaxial feeder. However, the use of an unbalanced-to-unbalanced type of antenna tuner between the coaxial output of a modern transmitter (or transceiver) and the coaxial feeder is essential because of the reactive condition presented at the station end of this feeder which, on all but the 14mhz band, will have a fairly high to high vswr on it. This vswr, however, will result in insignificant losses on a good-quality coaxial feeder of reasonable length; say, up to about 70ft (21.3m). Because it will, inevitably, have standing waves on it, the actual characteristic impedance of the coaxial cable is unimportant, so that either 50 ohm or 80 ohm type can be used.
http://www.dxzone.com/cgi-bin/dir/jump2.cgi?ID=4238
emphasis added!


Cortland
KA5S

G3TXQ
08-08-2010, 06:53 PM
What a pity Louis is not still around today so that we could quiz him on some of his pronouncements:


..... on all but the 14mhz band, will have a fairly high to high vswr on it. This vswr, however, will result in insignificant losses on a good-quality coaxial feeder of reasonable length; say, up to about 70ft (21.3m)

On 10m the impedance at the base of the ladderline section is around 3000-j490 ohms. That results in just under 8dB of loss in 70ft of RG213. Louis must have a different understanding from me about what is an "insignificant loss"!


They proved that, in fact, the hf currents measured at the junction of the inner conductor or the coaxial cable with one side of the (balanced) matching section and at the junction of the outer coaxial conductor (the shield) with the other side of this section are virtually identical on all bands up to 28mhz, where a slight but inconsequential difference in these currents has been observed. There is, therefore, no need to provide an unbalanced-to-balanced device at this junction when using coaxial feeder.

I'm not sure how Louis could have measured that those ladderline currents were balanced; but even if they were, the result would have applied only for his particular situation. The balance is very dependent on the coax length, routing, and grounding. Just because Louis measured balanced currents doesn't mean someone else will. I show an example on my web site where the ladderline currents can be very unbalanced - Section C, here:
http://www.karinya.net/g3txq/g5rv/


Plus, of course, Louis contradicted himself in other writings on the G5RV:


Under certain conditions, either due to the inherent "unbalanced-to-balanced" effect caused by the direct connection of a coaxial feeder to the base of the (balanced) matching section, or to pick-up of energy radiated by the antenna, a current may flow on the outside of the coaxial outer conductor.

If there's a conducted current flowing on the outside of the coax braid you may be sure the ladderline currents are not balanced.

Steve G3TXQ

K9ZMD
08-09-2010, 01:53 AM
Greg,

After hanging around the zed for a decade or so, I've learned one important truism:

You will catch very little flak about plans to use 75 ohm coax as transmission line, and you will receive some helpful information about why it doesn't matter too much. It will work just fine for you.

On the other hand, you will catch all kinds of flak about plans to use a G5RV antenna, and maybe some helpful information about why it matters a whole bunch. :D

Gary, K9ZMD/6
Palmdale, CA



--- Friends don't let friends drive G5RV's ---

W1GVT
08-09-2010, 10:29 AM
Gary, and others, thanks for the feedback.

I will continue my search for an antenna!

N8LWF
08-09-2010, 01:54 PM
Gary, and others, thanks for the feedback.

I will continue my search for an antenna!

The best advice I can give WRT antennas is to simply try different antennas/cables/etc and make some tests - see what happens in your situation. There are countless people using G5RVs with good results for them. So, feel free to draw your own conclusions.

WA9SVD
08-09-2010, 02:40 PM
The best advice I can give WRT antennas is to simply try different antennas/cables/etc and make some tests - see what happens in your situation. There are countless people using G5RVs with good results for them. So, feel free to draw your own conclusions.

True, but "good results" is a relative term, and most ops using a G6RV do NOT have a reference point, and as is often stated, "ANY antenna is better than none at all..."
The G5RV was originally designed as a 20 Meter antenna, and subsequently found to "work" on other bands, WITH a suitable tuner. Even so, the original design (particularly on bands other than 20 Meters) also relied on radiation FROM the ladderline feedline portion of the design, which may or may not be a benefit.
It DOES provide some gain over a simple half-wave dipole on 20 Meters, but is still a compromise on most other bands. If it IS all that an operator can use, then it's a "spectacular" performer."

AI3V
08-09-2010, 03:25 PM
Newbie here. I'm looking to put together an inexpensive antenna for a FT-101B.

I've got a 25' length of RG59. Would that be suitable for a feed-through to the antenna from my basement shack? Thanks.


Well, I'm probably going to use something like a GR5V to cover 10-80 meters. If it helps, I'll be going from the FT101B to a MFJ-941 then out to the antenna so I'm looking for suggestions. This is my first base setup so I'm not really stuck on anything.

I have a good length of the RG59 so that is why I asked about using it as a feedline but if you can suggest something better for an all purpose setup I'm happy to listen. Thanks!


Ah, you're right about the RG59 being 75ohms. I missed that. (I'm new at this and lacking time to get anything done.) Rather than scrounge I'll cough up the money to do it right!


You do not understand the concept of charicteristic impedance.

You also do not understand that your transmitter will work absolutly fine in a 75Ω system- indistiguishable from a 50Ω set-up.

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

If I was you, I would forget about the tuner and G5RV.

Install a simple, 1/2 wavelength dipole for each band you are interested in (40M is a great start).

Feed the dipole directly with 75Ω coax, You will not be able to tell the difference between using a balun, and not using a balun.

Adjust the plate current of your transmitter for rated current at the resonance dip- the variable components in the transmitter do the EXACT SAME THING as the variable components in the "tuner"

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

Take the $$ you save on 50Ω coax, a balun, a "tuner" and a G5RV and buy yourself a couple books about antennas instead.

Rege

K8JD
08-10-2010, 12:25 AM
.
I do not remember what the impedance of the G5RV is , but lets say it is 300 ohms , then it is attached to a 450 ohm latter-line , now you want to bring that into your shack , but have metal window frames , so you want to use coax , At least that is how I understand it , so I wait for others to confirm or correct .

The impedance of a G5RV Along with the exact length of ladder line connected to coax to the shack is importaint because the specific length of 450 ohm line is ;chosen to transform the very high Z of the 3/2 wave dipole part of the antenna to somewhere near 50 ohms and that ONLY works on 20M.
The Z and X values seen on any other amateur band is not something the original designer ever considered.
Using a tuner with a G5RV is essensial, on the other bands, with a solid state rig and advisable with a tube rig because of the strange Z and X values on most bands. The FT101 may be difficult to tune when there are wide variations of Z and X from band to band.

W8JI
08-10-2010, 02:09 AM
The impedance of a G5RV Along with the exact length of ladder line connected to coax to the shack is importaint because the specific length of 450 ohm line is ;chosen to transform the very high Z of the 3/2 wave dipole part of the antenna to somewhere near 50 ohms and that ONLY works on 20M.
The Z and X values seen on any other amateur band is not something the original designer ever considered.
Using a tuner with a G5RV is essensial, on the other bands, with a solid state rig and advisable with a tube rig because of the strange Z and X values on most bands. The FT101 may be difficult to tune when there are wide variations of Z and X from band to band.

The feed impedance of the G5RV on 20 meters is a much closer match to 75 ohm cable than it is to 50 ohm cable. It is up around 90 ohms on 14 MHz, so 75 ohm cable is a good match.

It also has reasonable SWR on 80,40,20, and 12 meters.

http://vk1od.net/antenna/G5RV/index.htm

http://www.w8ji.com/g5rv_facts.htm

73 Tom

G3TXQ
08-10-2010, 07:48 AM
The impedance of a G5RV Along with the exact length of ladder line connected to coax to the shack is importaint because the specific length of 450 ohm line is ;chosen to transform the very high Z of the 3/2 wave dipole part of the antenna to somewhere near 50 ohms and that ONLY works on 20M.
There are a number of errors here:

A 3/2 wavelength centre-fed dipole is not "very high Z" - mine measures about 120 ohms.

On 20m the ladderline section does not transform the feedpoint impedance. On that band the section is an electrical half-wavelength and simply "repeats" the impedance at the feedpoint.

20m is not the only band where the dipole/ladderline combination results in a moderate impedance. Mine exhibits a much better VSWR on 12m than on 20m. In fact 12m is the only band where a tuner would not be needed:

http://www.karinya.net/g3txq/g5rv/

Steve G3TXQ

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