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Date: Fri, 12 Mar 1999 21:10:31 +0100
To: rsgb_lf_group@blacksheep.org
From: "M. Bruno" <spin@inrete.it>
Subject: Re: LF: Ground systems ?...
In-reply-to: <26498@yo2kjo.ampr.org>
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At 19.56 11/03/99 +0000, YO2IS wrote:

> Have lately seen very incouraging info's regarding VLF antennas built on
>small surfaces, there was no mention about the ground systems. Was reading
>in a very old book about VLF antennas and the Braun's grounding praxis.
>To day have read a message from IK5ZPV describing his impresive ground sys.
>Do use myself the "ground" ofered by the urban heating network, sure its
>may be OK but not ideal, the RF curent measured in aerial and in the ground
>wire are more or less the same. Our backyard is only 12 x 8 m, good soil
>conductivity, will a small ground system improve the antenna eficiency over
>the actual "ground" ?.

Hello Szigy,
the ground offered by a large heating network may be not so good, if tubes
are thermally (and electrically) insulated.

> Would like to know, just for comparaison, what's the antenna curent for
>a VLF tx claimed for 25W output ? (about 75W input, class B).

If it was my system, about 0,9A (I have 5,1A with 800W, 31,5 Ohm total R
including the variometer and all losses)

My earth situation is very lucky, however. I had a pre-existing ground
system at this site, measuring about 27 Ohm at 137 kHz.
 
Calculation demonstrate that the ground loss is lower if the antenna
has a capacitive top hat. The calculated ERP is better for a short antenna
with heavy top load, comaparated with a vertical twice as long, due to
inferior losses. Try the programs of G4FGQ and do some simulations.


73 de IK1ODO Marco

Rivalta, ITALY JN35SA (N 45 01' 25.6", E 7 31' 09.4")