G5RV problem/modification idea
pr...@ocvaxa.cc.oberlin.edu
17m, 15m and 10m; now that I'm using the autotuner of a Yaesu FL-7000
which has an impedance matching range of 15-150 ohms it will only
tune up on the middle of 80m, 40m, 17m and 10m--it refuses to tune up
on 20m or 15m now. I'm considering adding 12 feet of wire to each
end of the 102 foot flat top to convert it into an 80m dipole in
hopes that it might be tunable on 80, 20 and 15 again, and wonder if
anyone has lengthened a standard g5rv, and what happened as a result.
The additional length would need to go off at odd angles due to space
constraints. The g5rv is mounted as an inverted V with apex at 35
feet and ends up about 15 feet. Thanks. --Bill KB8USZ
DB Wilhelm
I'm surprised that you are having trouble with the G5RV on 20 mtrs.
That antenna IS a 20 mtr antenna - 3/2 wavelength long with a half
wavelength of parallel line feeder. The impedance at the center of
the antenna should repeat at the input end of this feedline.
I wonder if you really have a halfwave of feedline on 20 mtrs.
BTW, the G5RV antenna 'just happens' to work well on bands other
than 20 - it was not designed to do that - some things are like
that - there is no magic to it - just seems like it at times.
73,
Don
cmo...@sedona.intel.com
Grant H Youngman <us00...@interramp.com> wrote:
>Now a plain old flattop cut for your favorite part of 80 meters and fed
>with a good tuner (forget one of those lossy low power things -- even if
>you only run low power) and 450 ohm ladder line ( or even, heretically,
>with coax) will work just fine and load (with the tuner) just fine and
>radiate as well or better (better on 80 meters, of course .. and at least
>as well everywhere else) than your precisely cut, precisely fed G5RV.
Hi Grant, I agree with most you said. There's so many and's above
that I can't tell whether you said the 80m halfwave dipole will
load as well on 40m as a G5RV - it won't always. The SWR on even
multiples of full-wavelengths is as high as SWR ever gets. This
very high SWR can transform into unmatchable impedances depending
on the length of the transmission line. And the losses in the coax
in a G5RV tend to lower the SWR, often making matching easier than
even full wavelengths.
To avoid super high SWR's on all bands, an antenna length that is
not resonant on *any* band is better. In that respect, 100~105 ft
is a good non-resonant length. I use 88 ft myself (5/4 WL on 20m).
73, Cecil, KG7BK, OOTC (not speaking for my employer)
W8JI Tom
writes:
>
>To avoid super high SWR's on all bands, an antenna length that is
>not resonant on *any* band is better. In that respect, 100~105 ft
>is a good non-resonant length. I use 88 ft myself (5/4 WL on 20m).
>
>73, Cecil, KG7BK, OOTC (not speaking for my employer)
>
>
>
Hi Cecil,
Watt's cooking?
Actually I had to measure a G5RV a while ago to write a manual. The SWR
was very low on 80, 40, and 20. But the bandwidth was narrower than a
regular dipole on all bands. The thing was horrible on 15 and 10. I don't
remember the WARC bands off hand.
But they work ok with a good tuner.
73 Tom
Steve Ellington
strange urge to put up a 3/2 wave dipole. Really, I believe he just
wanted to name an antenna after his call sign. So he puts this junker up
there and used 31 feet of ladder line to half way match it to his coax on
20 meters. Period. End of sense. Thats it! To use this arrangement on
other bands makes as much sense as using your 40 meter dipole on 2 meters!
Will it work? Sure, anything will radiate if you can load it up. I've
loaded up window screens, rain gutters, aluminum siding, you name it.
Now if you want a low swr with that G5RV, just use a long piece of cheap
RG58 coax. The loss will flaten that swr righ out.
Finally, the G5RV is:
1: A 3/2 wave 20 meter dipole with a matching section to 50 ohm coax.
2: A 80 meter radiator with performance less that a full size dipole and
a lossy feedline which requires a tuner.
3. A 40 meter radiator that has a directional pattern slightly different from
a normal dipole and a lossy feed system.
4: A radiator that above 20 meters becomes quite directional and has a
lossy feedline if much coax is used plus can be quite difficult to
load.
FOLKS, IT'S JUST A WIRE WITH A CRUMMY FEED SYSTEM.
Don't fall for the hype.
--
Steve
n4...@iglou.com
cmo...@sedona.intel.com
Christopher Scibelli <scib...@ix.netcom.com> wrote:
>Ok, then what would you recommend to those of us who don't have the
>room for a full sized dipole on 80?
Hi Chris, I doubt that anyone listening to your signal on 75m could
tell the difference between a halfwave dipole at 45 ft and the G5RV
at 45 ft. At 45 ft., the major problem for both types of antennas on
75m is the lack of height. A G5RV with the same losses as a halfwave
resonant dipole will radiate the same amount of power. And remember,
the difference between radiating 100w and radiating 25w is only one
'S' unit (6dB).
You could improve the performance by using ladder-line all the way
to the shack and using some sort of matching there. But there's
basically nothing wrong with a 102 ft dipole. There's just better
ways of feeding it than the G5RV way (except maybe on 20m).
Christopher Scibelli
>You could improve the performance by using ladder-line all the way
>to the shack and using some sort of matching there. But there's
>basically nothing wrong with a 102 ft dipole. There's just better
>ways of feeding it than the G5RV way (except maybe on 20m).
>73, Cecil, KG7BK, OOTC (not speaking for my employer)
Steve and Cecil,
Ok, 2 votes for feeding the thing with ladder line all the way down.
Just one question: Am I gonna be more suspectible to TVI and computer
RFI than if I use coax to carry the signal out of the shack? I don't
wish to revisit those problems. BTW, I have one of those big Heathkit
tuners (SA-2060A) which has a 4 to 1 balun and is rated at 2KW PEP.
Did manage to work a VK6 on 80M SSB via the long path the other day so
the antenna is radiating. I appreciate the help.
Chris
----------------------
scib...@ix.netcom.com
sjs...@delphi.com
Christopher Scibelli
>Finally, the G5RV is:
>1: A 3/2 wave 20 meter dipole with a matching section to 50 ohm coax.
>2: A 80 meter radiator with performance less that a full size dipole and
> a lossy feedline which requires a tuner.
>3. A 40 meter radiator that has a directional pattern slightly different from
> a normal dipole and a lossy feed system.
>4: A radiator that above 20 meters becomes quite directional and has a
> lossy feedline if much coax is used plus can be quite difficult to
> load.
>FOLKS, IT'S JUST A WIRE WITH A CRUMMY FEED SYSTEM.
>Don't fall for the hype.
>--
> Steve
>n4...@iglou.com
Ok, then what would you recommend to those of us who don't have the
room for a full sized dipole on 80? I've got room for a 110 foot
antenna and can't go higher than 45 feet. A pool takes up most of my
backyard so I can't lay a radial system for a vertical. I'm
interested in 30/40/80 meter operation. Suggestions please.
Chris
NU1O
----------------------
scib...@ix.netcom.com
sjs...@delphi.com
Steve Ellington
: Ok, then what would you recommend to those of us who don't have the
: room for a full sized dipole on 80? I've got room for a 110 foot
: antenna and can't go higher than 45 feet. A pool takes up most of my
: backyard so I can't lay a radial system for a vertical. I'm
: interested in 30/40/80 meter operation. Suggestions please.
: Chris
: NU1O
The 110ft flat-top is fine. Just make a 110ft dipole. Feed it with either
300 or 450 ohm open line. Yes they make 300 open line and it is easier to
match because the swr excursions are not as great. You are tuning the
entire antenna system, not just the antenna. Use the heaviest balun you
can find or a tuner that has one rated for 1kw built in. Or you could buy
my old Johnson matchbox tuner. It doesn't need any balun and handles 275
watts very conservitily. That's all there is to it. I recommend a
grounding switch or relay outside because nearby lightning can really
play a tune on this type of antenna. Mount it as an inverted vee or
flat-top. At 45 feet, it will be onmidirectional on 80, slightly off the
ends on 40 and moreso on 30. This antenna has no one's callsign or name.
They have been used for over 60 years.
If you want to experiment, add another 110ft or so of wire and try to
form a loop. Feed it with the open wire line. Different feed points will
give different radiation patterns. It doesn't have to be level but works
better if it is. You will find this much easier to match with a tuner
than a dipole and since it is at dc ground potential, less noisey and
safer from lightning. My antenna here at N4LQ is a 560ft horizontal loop
at 65ft. It works great on all bands. 73 and have fun.
--
Steve
n4...@iglou.com
WB3U
>Ok, 2 votes for feeding the thing with ladder line all the way down.
>Just one question: Am I gonna be more suspectible to TVI and computer
>RFI than if I use coax to carry the signal out of the shack?
This is a distinct possibility if you use the Heathkit tuner with a
balun at the output. That balun doesn't want to see the impedances
this antenna is likely to present on bands other than 20M. Also, losses
in the balun on those bands may well exceed any losses incurred in the
coax due to SWR.
You should either take Cecil's advice and use the coax, or build a
"balun-first" tuner (which is what I did a few years ago).
BTW, tests in May 1995 QST indicate that neither of the two models of
Johnson Matchbox will tune *any* impedance (including 50 ohms) on the
30M band.
73,
Jack WB3U
Steve Ellington
SWR of 40 to 1 according to the antenna manual I just consulted. Go
figure how much signal would be lost in 75ft of RG-58.
--
Steve
n4...@iglou.com
Steve Ellington
the house with some heavy coax i.e. 9913 or better coax going through the
wall to the tuner. This will help keep down the RFI but keep the coax
short. The impedance of the coax doesn't matter since everything is
already screwed up. Even hardline would be good for this if you can find
the connectors. On second though, who needs connectors? SWR isn't a
concern anyway!
You will still probably have some RFI problems especially if you have a
computer tied to the rig. It will show up on certain unpredictable bands.
All you can do is try to work around it by changing feeder length and
using chokes on the computer's I/O cables etc. Again, the loop antenna is
a lot better about this kind of thing. 73
: Steve
: n4...@iglou.com
--
Steve
n4...@iglou.com
WB3U
>A G5RV used on 30 meters at the junction of the coax and open line
>has an SWR of 40 to 1 according to the antenna manual I just
>consulted. Go figure how much signal would be lost in 75ft of RG-58.
This was probably a rhetorical question, but here's the answer,
according to my transmission line program:
For 75' of RG-58A coax at 40:1 SWR -
SWR at Load: 40:1
SWR at Input: 6.9:1
Additional loss due to SWR: 6.7 dB
Total Line Loss: 7.7 dB
All numbers are approximate.
73,
Jack WB3U
Mark Monninger
>Ok, then what would you recommend to those of us who don't have the
>room for a full sized dipole on 80? I've got room for a 110 foot
>antenna and can't go higher than 45 feet. A pool takes up most of my
>backyard so I can't lay a radial system for a vertical. I'm
>interested in 30/40/80 meter operation. Suggestions please.
Put up a dipole 55 ft on each side, fed with twinlead or ladder line into a
good tuner. A balanced tuner is best, not one that uses a balun in the output.
A dipole like that at 45 ft will work fine. It doesn't have to be resonant.
Matter of fact, it's sometimes better if it isn't. That's what I have (mine's
a little shorter than 110 ft) and it loads on 80/40/30 with a tuner. It gets
out fine...not as well as a big beam on a 100 ft tower, but it's sure better
than nothing.
Any antenna is better than none. Do the best with what you have and don't
worry about the details. Try it, you'll like it. Don't worry too much about
what the books or computer simulations say. You'd be surprised what you can do
if you'd don't know it won't work. Get some wire up and get on the air.
73... Mark AA7TA
Steve Ellington
the antenna. Wrap a few turns of the coax around your coffee mug to keep
your coffee warm.
--
Steve
n4...@iglou.com
W8JI Tom
Monninger) writes:
>
>Any antenna is better than none. Do the best with what you have and don't
>worry about the details. Try it, you'll like it. Don't worry too much
about
>what the books or computer simulations say. You'd be surprised what you
can
>do
>if you'd don't know it won't work. Get some wire up and get on the air.
>
>73... Mark AA7TA
Amen.
73 Tom
David Rutter
quite well with the appropriate tuner (balanced).
Good luck on your project.
Dave
N8CLF
maurice norris
You gave me the answers I have been looking for.
Mo Norris KC7EXG