Without more detail of your system, the cause is only a guess. Here are
some possibilities.
A single wire isn't much of a ground plane. It will make a vertical
directional. When you change from a single wire to a different ground
system you will change the antenna pattern. You should expect some
difference in noise performance.
Since a single wire is a poor ground system, if it is closer to a local
noise source than a different ground system, it could be picking up more
noise.
Since a single wire is not a good ground system, and if you don't have a
very good choke at the antenna feedpoint, the coax could be acting as
part of the antenna. Changing the ground system also changes the
effects of the coax.
A longer wire used for ground will have a lower resonant frequency and
may provide a lower impedance at the frequency of interest, providing
more decoupling for the coax.
Those are all just possibilities. Further experimentation may determine
the real answer. It's not likely that NEC will show the answer.
Solution: Change to a good ground system as suggested by some of the
other responders.
Since you have been playing with loops on the ground as receiving
antennas, you may be interested in reading the results of my experiments
here:
http://lists.contesting.com/archives/html/Topband/2018-08/msg00019.html
Jerry, K4SAV
On 5/4/2020 3:57 AM, Chris Moulding wrote:
I've developed a High Z Antenna Amplifier for 160m and other HF use as
previously mentioned on the list.
Usually these would be used with a ground rod and 5m vertical element.
With the lockdown it's not possible to nip to the shops for a ground
rod so I looked at supplying a 10m wire as a counterpoise.
At home and the workshop I have Loop on the Ground antennas from
previous antenna experiments so I also tried using these with both
ends of the loop connected to the amplifier ground terminal.
On testing this gave significantly better signal to noise ratios than
using a ground rod or a single wire counterpoise. Checking with a SDR
receiver I could see that the usual local VDSL internet hash had
disappeared.
Both loops on the ground are 3m or 10' square.
I've also tried it using a G7FEK vertical antenna at home with two 3m
or 10' square loops on the ground with similar results seeing much
reduced local noise compared with the ground radials I had before.
Topband Dx might be a possibility for me now.
I've never seen this mentioned in ham magazines before and I can't
find anything with an internet search. Usually I find that all my good
ideas have already been thought of 50 years ago.
I suspect that the RF voltage in the loop counterpoise is much reduced
over the voltage at the end of a radial wire reducing noise pickup in
the radial system.
I would like to model the loop on the ground counterpoise in a
modelling tool. I use 4NEC2 but only have access to NEC2 so wires on
the ground don't model correctly.
Is there any one out there with access to suitable software that could
model it for me?
73, Chris G4HYG
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