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")