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1-10 of 12 messages
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G5RV construction
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by K3UOD on September 18, 2000
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I want to build a G5RV for 80-10. I've seen diagrams showing dimensions but no construction tips on connecting the coax to the twinlead. Any good tips? Any good links to ant construction sites?
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RE: G5RV construction
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by WB6BYU on September 18, 2000
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If I were to build a G5RV, I'd run the twinlead all the way to the tuner
instead of splicing it to the coax. (You'll probably need a tuner anyway
to operate all bands.) If you have to go through a wall, use two pieces
of coax in parallel, with the balanced line connected to the center
conductors and the shields grounded at the tuner - this maintains the
balanced line.
If you want to try the coax feed approach, you will want some sort of
line-decoupler (such as a current-mode 1 : 1 balun) at the junction.
This could be as simple as winding the coax into a coil of about 6 turns.
Otherwise, some of the RF will flow back along the coax shield, which
may unbalance the feedline.
Of course, you could just attach the center of the coax to one side of the
twinlead and the braid to the other, and see how it works. It might work
well enough that you don't have to try anything more elaborate. Be sure
to waterproof the joint if you are going to leave it out in the weather.
Good luck! - Dale WB6BYU
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RE: G5RV construction
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by K3UOD on September 18, 2000
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Dale,
TNX for your post; but, I believe that if you eliminated the coax, the antenna would no longer be a G5RV. If I could run twin lead all the way I would to take advantage of the low loss. But, in my situation, I have to use coax.
73, Jerry
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RE: G5RV construction
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by WB6BYU on September 19, 2000
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Actually, Lou Varney, G5RV himself, recommended the use of twinlead
(or open wire line) as the preferred method where practical.
I used a similar technique a few times, connecting coax directly into
balanced line. Sure, it freaks out those folks who insist on following
convention, but that is part of the fun. The unbalance can result in RF
flowing along the outside of the cable, and, if this is laying on the ground,
it will be dissipated, resulting in lost power. I have not been impressed
by the performance of G5RVs that folks have set up for Field Day, and
have seen other studies where they were 1 to 2 "S" units down from a
dipole. I suspect this is due to using a long feedline laying on the ground
without some sort of decoupling at the feedline junction. With the use of
a 1 : 1 current balun (or decoupling coil), you should have good results
if you keep the feedline a reasonable length and use a good tuner.
Good luck! - Dale WB6BYU
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RE: G5RV construction
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by K3UOD on September 19, 2000
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Interesting Dale,
All of the diagrams of G5RVs I've seen show a doublet shorter than 1/2 wave (usually 102 ft) fed by some short length of twin lead, connected to a random length of coax. I've never read Mr. Varney's original article, though I've tried web searches for it several times. Do you have a link or a copy that you can e-mail me?
From your comment I infer that what makes a G5RV a G5RV is the length of the horizontal part and not how it is fed. Or am I mis-reading you? I thought that the reason for the twin lead was to compensate for the foreshortened doublet.
In a similar vein, the first Windom I ever saw was a half wave horizontal wire fed of-center with a single wire feed. Now I keep seeing off-center fed dipoles refered to as "Windoms". It seems that definitions change. Trouble is, how do you know which is the real thing?
73, Jerry
k3uod@earthlink.net
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RE: G5RV construction
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by WB6BYU on September 21, 2000
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Lou Varney has authored a number of articles in "Radio Communication"
(aka "RadCom"), the journal of the RSGB. There is a summary in a current
RSGB book with a purple cover, which I believe is on HF antennas, but
I don't remember the exact title. (It is NOT the one titled "HF Antennas
for All Locations" by G6XN, though that is quite thought provoking also.)
The original G5RV was made 103' because it was 1 1/2 waves on 20m,
so has a reasonable feedpoint impedance on that band (around 90 ohms).
The half wave of balanced line won't change this, but helps bring the
SWR down on some other bands. With the old tube rigs, you could often
use the antenna on all (or most) bands without an external tuner because
the pi-net output stages had sufficient tuning range. Since then some
others have reported attempts to find combinations of feeder and antenna
length which would give a better match on other bands - these have
been reported in the Technical Topics column of RadCom, and also
appear in the various volumes of "Technical Topics scrapbooks"
(published by the RSGB, of course.) (They used to publish a book titled
"Amateur Radio Techniques", which was a compendium of many years
of columns, but now just reprint the columns in 5 year increments.)
By the way, the "purple book" I refered to above includes an antenna
comparison that one club did, which reported the G5RV to be 1 to 2
S-units down from dipole on most bands.
Happy antennas!
- Dale WB6BYU
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RE: G5RV construction
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by KB8UUZ on September 22, 2000
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I have had a few versions of G5RV's and all have worked very well. Some commercially made and some home made. The best method I have found is using 102 ft of #14 copper braided wire for the top section (51 Ft each side), attach this to one of the "Ladder Grabbers". Then using 34 Ft of 450 Ohm #16 ladder line. The other end of the ladder line goes to a 4:1 balun, then a minimum of 70Ft of coax into the shack - I use Mini 8X coax. Don't let the coax fold over itself - it will act as another balun in the line. Keep the ladder line vertical - this is actually part of your antenna in a true G5RV. This means you have to be up at least 40 feet or so. I use two large trees and am up at about 55 Ft. I have found with using the 4:1 balun, on 20 meters I do not need a tuner - it's flat through the entire 20 meter phone band and no RF in the shack at all. The MFJ analyzer says all is pretty good. The tuner takes care of everything else from 80 to 10 meters.
Feeding a the horizontal portion with just ladder line all the way to the tuner is the same as a dipole, that is not a G5RV.
I have also used 300 Ohm twin lead for the vertical section through an air core balun then to coax, but have found using ladder line and the 4:1 balun much better.
Get the whole thing up as high as you can - and WORK THE WORLD!!!!!!
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RE: G5RV construction
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by K3UOD on September 22, 2000
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Thanks. This is all good info. I see that there are several ways to skin this cat.
However, my original question is, how do you make a mechanically sound, weatherproof, connection between the ladder line (or 300 ohm twin lead) and the coax?
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RE: G5RV construction
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by WB6BYU on September 26, 2000
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Bind the coax and twinlead together with a tie wrap. (For open wire line,
it may have to go around a spreader. For 450 ohm line, punch a couple
holes in the insulation to pass the tie wrap through.) This provides the
strain relief.
Make your connections with stranded jumper wires. Coat entire joint
with RTV sealant. Cover with heat shrink tubing if desired.
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RE: G5RV construction
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by W2EJG on October 6, 2000
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The G5 is by definition a non-resonant antenna.
A wire fed at the center with open-wire line or twinlead is called a "Center-fed-Zepp". You must use a parallel line impeadance matching device.
The G5RV is best done by following the directions. Join the coax and twinlead with one of the center insulators sold at hamfests for $ 3. They have eyehooks at the sides and end at a SO239 connector at the bottom.
I have used the G5 for years and it works. I buy mine from VanGorden... .
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