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Author Topic: G5VR - What to do with excess feedline  (Read 7445 times)
KD4SBY
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« on: November 05, 2010, 11:05:30 AM »

I am planning to use a  G5VR (or Jr.) in my attic using a MFJ 927 remote antenna tuner. Do I understand it right that the feedline is part of the total antenna configuration? If so, what do I do with the excess line when the tuner is only a few feet of the antenna it self? Huh
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VE3FMC
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« Reply #1 on: November 05, 2010, 12:08:58 PM »

I am planning to use a  G5VR (or Jr.) in my attic using a MFJ 927 remote antenna tuner. Do I understand it right that the feedline is part of the total antenna configuration? If so, what do I do with the excess line when the tuner is only a few feet of the antenna it self? Huh

Try and suspend the twin lead from the rafters if possible. Get as much of as you can straight down from the legs of the antenna, then bring it back up to the rafters.

There is not much else you can do in your situation.
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KD4SBY
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« Reply #2 on: November 06, 2010, 12:55:47 PM »

OOPS! Noticed too late I made a booboo in the name of the antenna. Should be G5RV. Grin Sorry abt that!
Thanks for the answer. I was thinking along the same lines, but if everything cancels out in the feed line anyway, would a random zigzag pattern on the (ceiling) floor not be better? It would be further away from the actual ant. wire that way.
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WB6BYU
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« Reply #3 on: November 06, 2010, 02:02:22 PM »

The feedline helps to match the antenna impedance so the SWR isn't too bad on the coax.  It isn't
(or shouldn't be, at any rate) part of the radiator.

If you are using an autotuner that close to the antenna, you've already provided for matching the
antenna to the coax.  You can remove the parallel feedline and connect the tuner directly to the
feedpoint of the antenna, preferably using a good 1 : 1 balun.  In that case there is no magic to
the particular wire length, either. though efficiency drops when it is too short for a given band.

The reason for the G5RV is that the combination of wire and twinlead lengths gives a reasonable
impedance match (less than 4 : 1 or so) on 80, 40, 20, 15 and 12m.  Basically, the SWR is low
enough that feedline losses need not be too high, and the old tube rigs often could match it
with their built-in pi-network tuners.  If you're using an autotuner right at the feedpoint, you aren't
worried about losses is the coax (presuming the tuner has sufficient matching range) so there
is no advantage of using either specific length - you can use whatever works best for you.
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KD4SBY
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« Reply #4 on: November 07, 2010, 11:15:40 AM »

That certainly clears it up for me. Makes sense. Thanks.
While I am on the subject, what is the advantage of having a remote antenna tuner? If I am using no more than abt 15-20 ft of coax feed line, is a tuner on my desktop not just as good?
The price difference  between a MFJ 948 (desk) and the MFJ 927 (remote) is more than enough to pay for the antenna itself + a balun.
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WB6BYU
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« Reply #5 on: November 07, 2010, 12:45:41 PM »

Quote from: KD4SBY
...what is the advantage of having a remote antenna tuner? If I am using no more than abt 15-20 ft of coax feed line, is a tuner on my desktop not just as good?

It might be pretty close... or it might not.  It depends on the impedance at the antenna and the loss in the coax.

Here is an example:  I modeled a G5RV Jr. (based on Lou Varney's original design resonated at twice the frequency.)
Using the twinlead matching section the SWR on 20m is about 6.5 : 1 (it varies across the band.)  Feeding this
through 20' of RG-58 coax results in a loss in the coax of about 1.2dB.  That means you lose about 1/4 of your
power in the coax.  With a tuner right at the antenna the coax losses are only 0.4 dB because the SWR on the
coax is very low.  The losses in both cases will be lower if you use better coax - using RG-213 reduces the coax
losses to 0.5 dB and 0.1db, respectively.  Most hams are happy with those numbers (and many will tolerate
higher losses in exchange for convenience.)

With the same antenna on 15m the situation changes:  the SWR is 65 : 1 on the coax running from the rig
to the matching section and the losses in 20' of RG-58 will be about 6.2dB, so 3/4 of your power is lost in
the coax.  The feedline loss with the remote tuner is only 0.45 dB.  Again, using RG-213 cuts the losses
nearly in half.

(Note that these calculations ignore the losses in the twinlead matching section or in the tuner.)


On some bands where the SWR isn't too bad, the difference in losses between the remote tuner and one
in the shack isn't much.  On other bands where the SWR at the antenna is high it can make a big difference,
even in 20' of coax.  Trying to match the same antenna on 80m gives you about 92% of your power lost
in the coax with a tuner in the shack (though the losses in a remote tuner should also be considered.)  On
the other hand, if you extend the twinlead all the way to the shack instead of using coax, the losses
will be lower - though still significant on 80m.

VK1OD has an excellent analysis of losses in the full-sized G5RV that will apply to your antenna if you double
the frequency scale:

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


How important is 1dB or 3dB of loss?  Half your power (3dB) sure sounds like a lot!  But I usually operate at
5 watts output or so and make lots of contacts:  that's equivalent to 13dB of loss in your coax to have
the same signal strength.  (Not that it is always easy to make contacts, of course.)  So it is posisble to
get out even with 10dB or more of coax loss, but your signal will be weaker.  If you're trying to break a
pile-up on a rare DX station, that's not a good idea.  If you are happy having casual contacts with
relatively strong stations, it may be good enough for you.  That's a decision you have to make for
yourself.
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KD4SBY
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« Reply #6 on: November 08, 2010, 05:56:08 AM »

I looked at the material that you referred to in your link. Very interesting! From that and  your own explanation, I am amazed to see how quickly the losses add up when you start to deviate from the ideal situation. In my case, I might be able to afford to buy low-loss coax because of the short run involved instead of investing in an more expensive remote tuner. I have to run the feedline up the wall, through the ceiling, then a short run to the antenna, since it will be located almost right above me. I love to run twinlead up, but I don't think it will like having to go through a hole in the drywall ceiling which then I have to patch up all around it. That is why I am thinking to use coax.
BTW For practical reasons, mainly to do with the antenna situation, my main interest is 20M, and if possible, 40M. When propagation improves, I hope that I might be able to use the antenna for the higher bands.
In any case, I have to make some decisions as to what I have to do to get an antenna constructed to get on the air.
Thanks for the input, fellows.
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WB6BYU
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« Reply #7 on: November 08, 2010, 10:21:29 AM »

Quote from: KD4SBY
I love to run twinlead up, but I don't think it will like having to go through a hole in the drywall ceiling which then I have to patch up all around it.

I had a piece of 300 ohm receiving twinlead dropping through a slit in my ceiling down to the tuner - it
was no problem to spackle the hole when we left.  If you're using the larger window line or ladder line
just drill two holes in the ceiling with the proper spacing, pass the two leads through from one end
and splice on an extension on the other side of the drywall.  Much easier to patch than a hole for
coax.
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K2ID
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« Reply #8 on: March 14, 2011, 08:12:51 PM »

Throw it out with the rest of the antenna.  Grin Sorry just had to say it. Seriously, I know that this antenna has a large following due to its advertised multiband capability but every ham I personally know with one replaced theirs with a dipole and and tuned it for the various bands and got better results. That ladder line is more trouble than it is worth in my opinion. YMMV
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WX7G
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« Reply #9 on: March 17, 2011, 11:32:44 AM »

The "proper" length is there so that a lesser tuner can match it. With a good tuner (manual) the proper ladder line length is not needed.

But I see you have the MFJ-927 autotuner. This may or may not (probably not) tune the G5RV on all bands if the ladder line length is reduced. You can wrap the excess ladder line around a 5 gallon plastic bucket with each turn spaced a few inches.
« Last Edit: March 18, 2011, 09:08:33 AM by WX7G » Logged
KQ6Q
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« Reply #10 on: March 17, 2011, 12:12:51 PM »

If you trim an antenna to be resonant on a band, the losses in coax are insignificant. Run several resonant dipoles from the same coax feedpoint, and keep an inexpensive tuner at the rig (you can get by with a tuner built-into the rig). The G5RV setup is overhyped, and as pointed out earlier, actually quite lossy.
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KJ4VTH
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« Reply #11 on: March 23, 2011, 06:54:23 PM »

As posted on another thread, I'm a new ham and don't know what I am doing but I've logged QSOs on six continents (and heard the seventh) with a G5RV Jr.  Many of these contacts were made using low power, 5-10 watts.  I'd say hang that wire up in a tree or stretch it out in your attic and try it out!  You might be surprised.  It may not be the ideal antenna but it works for me.
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KD4SBY
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« Reply #12 on: March 29, 2011, 06:07:34 AM »

Thanks for the input fellows. Your comments were very helpful.
WX7G: No, I do not have a MFJ-927, nor a G5RV. Hindsight my original question was wrongly stated. I should have said "If I use a MFJ-927 and a G5RV". At the time, it was seriously my intention to buy them, until I got more information, and changed my mind. As a result I am holding off on the whole project. Also, my HOA gave me permission to have a vertical in the back yard, so I am trying that first. Am sure that will work better than any dipole in the attic! Grin
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