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Date: Thu, 06 Jun 2002 18:08:31 +0100
To: rsgb_lf_group@blacksheep.org
From: "James Moritz" <j.r.moritz@herts.ac.uk>
Subject: Re: LF: Counterpoise Experiment
In-reply-to: <Pine.LNX.3.95.1020606225757.270A-100000@fitec.omskcity.com >
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At 23:09 06/06/2002 +0000, you wrote:
>What coil resistance You have? Or 37 ohm it is whithout coil resistance?

The resistance values quoted are those due to the antenna alone - I have 
already subtracted the coil resistance which is about 5 ohms.

>It seems to be much better if You will not use ground, conterpoise only.
>To reduce loss all the currend should return through conterpoise.
>In this case it is good to use ferrite  transformer betwin TX and
>antenna to avoid HF voltage on TX box.
>

It seems to me that in this case, where most of the antenna field (and 
therefore the displacement current) is going to the ground rather than the 
counterpoise,  most of the return current will still be flowing in the 
ground, even if the counterpoise is insulated from ground. Rather than 
returning directly to the grounded terminal of the TX output, this current 
would have to flow through the capacitance between the counterpoise and 
ground. This means there must be a voltage between the counterpoise and 
ground - if the  ground-counterpoise capacitance is roughly 20 times that 
of the antenna-ground capacitance (7nF and 340pF respectively - which is 
what I estimate in this case), the counterpoise voltage would be roughly 
1/20 of the antenna voltage. In my antenna system, this would put an RF 
voltage of about 1kV on the counterpoise when running at full power - so 
the counterpoise, antenna tuner and isolating transformer between TX output 
and tuner would all have to be insulated to withstand this voltage, making 
things quite complicated again... The advantage would be that the 
resistance of the ground path should be reduced, since the ground 
"connection" is effectively the whole area of the counterpoise, decreasing 
the loss due to this source. However, many experiments have shown that the 
resistance of the ground connection is only a minor factor in the overall 
losses in a small antenna like this one (3ohms is probably not that 
unrealistic), so I would expect little improvement overall.

Cheers, Jim Moritz
73 de M0BMU