Topband: Loop on the ground as a counterpoise

[Lee STRAHAN]

Greetings All,
   It seems this discussion has morphed into a discussion on ground planes for regular vertical antennas where if I read the authors description correctly he is using a Hi-Z amplifier. This may mean a whole different ground system would be very satisfactory. I believe it was Sinisa YT1NT, VE3EA that mentioned there was a great deal more information needed to understand any noise changes due to using a loop antenna for a ground on a Hi-Z element. I agree with Sinisa. Frank W3LPL was right on when he mentioned the ground screen as a possibility. Two very important things about using a Hi-Z amplifier with a short vertical are the need for serious common mode rejection at the amplifier output and a very high input impedance consisting of a high resistance and a very low capacitance. Typically the amplifiers I am familiar with have a 43Kohm or more parallel resistance and about 10 to 15 pf parallel input capacitance. A typical shortened vertical an inch in diameter and 24 feet in length will exhibit an output source impedance that is very high. It consists of a very low series resistance of an ohm or two and a series capacitance of about 80+ pf. Looking at 85pf driving 10 to 15 pf amplifier input capacitance does not require a super counterpoise around the element for the ground terminal on the amp. A rather large capacitance or very low resistance to mother earth will both suffice. The only fly in the ointment is if the ground system for this Hi-Z element is resistive dominant and goes much above about 40 ohms resistance which then appears in series with the element driving the amp, the received signal begins to be phase shifted causing a possible difference from one element to another. Not good in an array of elements. 
   As far as the noise changing with a loop as a radial or counterpoise I have not seen anything that would make me believe the loop configuration was responsible. I would rather think it was lowered counterpoise resistance or lack of common mode isolation in the amp having lowered noise. This could also come from other close antenna installations radiating noise into the single wire more so than the loop due to the common angle of a radiating noise source like a large antenna radial, fence or power line. A good Hi-Z amplifier with lots of common mode isolation will act very much like a differential amplifier which means the noise could come from either direction, the ground terminal (loop) or the element. It is my experience that in very dry and or rocky soil 6 to 8 ground radials no longer than the length of the element in addition to a ground rod will work quite nicely.
   It will be most interesting as Chris continues on with these loops as a counterpoise and reports more of his observations from experimentation. Perhaps there is indeed something new...
Lee  K7TJR   OR

-----Original Message-----
From: Topband <topband-bounces+k7tjr=msn.com at contesting.com> On Behalf Of Chris Moulding
Sent: Monday, May 4, 2020 1:58 AM
To: topband at contesting.com
Subject: Topband: Loop on the ground as a counterpoise

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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