I think one problem is that your feeding it with open wire line. An off
set feed dipole is in no way a balanced load. This means the feedline is an
active part of the antenna and is picking up noise. Feed that way it's what
I guess they call a Carolina Windom where the feedline is used to give a bit
of vertical polarization to the antenna. In a G5RV the open wire line is
supposed to be a matching section on 20m and a bit of feedline on the other
bands and as its center feed the feedline is not supposed to be a active
part of the antenna.

--
John Passaneau W3JXP
State College Pa.



wrote in message
oups.com...
I just put up an offset fed dipole cut for 80 meters, up 40 feet,
though the ends bend down a bit. It is fed with 40 feet of 177 ohm
open wire twin lead, and a portion of the antenna goes over the top of
my three-story house. At the feed end of the twin lead is a Guanella
two-core 4:1 balun to sixty feet of RG-8U ( actually, the better
stuff--RG-216?) to the shack. The coax has a common-mode choke just
before the tuner. The antenna tunes well on all bands 80 and above.
The balun was tested to have less than 0.1 dB loss over HF and
impedances from 25 to 100 ohms on the 50 ohm side.

Last night was the Spartan Sprint, a QRP CW contest that provided many
weak signals suitable for evaluating the antenna. I tuned the contest
in on 40 meters, and found a few stations calling, but S/N on all
signals was never high enough to copy solid.

I then went to the ham radio room at our city building a block away,
and listened to their late-model Kenwood on a G5RV up 30 feet. On the
same frequency with an SSB bandwidth I received at least a dozen
signals, solid copy on each.

I took the Kenwood (I have permission to do this) to my shack and
hooked it up to my new antenna (without the tuner--the Kenwood has a
matcher inside) with results somewhat between what the city antenna
gave and what my own radio gave.

Now here's the rub: I'm willing to accept the idea that my antenna is
noisier and needs work, but an experiment I did puzzles me: when I
evaluated the Kenwood with my antenna I left the matcher on the
settings it had for the G5RV antenna. As mentioned, I could hear
stations better than with my radio, but not as well as the g5rv at the
city building. The noise level (as measured on the Kenwood) was about
s2 on the g5rv, and s5 at home. About five signals were copyable in
the SSB bandwidth I had selected.

I then tuned the Kenwood to the dipole. It tuned successfully, and
when I was done, the noise had gone to s9. This is not a surprise
since the matcher has an effect on signal and noise strength. The
surprise was that all signals instantly dropped below the noise, with
only one signal that could even be heard. The radio's performance at
this point matched my original evaluation with my ICOM and an external
tuner matched to the antenna.

The engineer in me says that though tuner settings can affect how well
signals and noise couple to the receiver, they should not affect
overall S/N as long as we are within the dynamic range of the radio,
but I definitely saw an effect. I finished the experiment by returning
to the city building and verifying the band was still active.

I think I know what might be happening and am designing an experiment
to verify this, but would appreciate any speculation antenna experts
may have. I doubt it is something like signal overload--rather, I
think the antenna was really the culprit. Has anyone else seen this
effect?

Thank you,
Glenn Dixon AC7ZN