Dear Lawrence,
remembering the few results on ground resistance measurements published on this
server I would regard your 120 ohms somewhat above average for 136 kHz. My own
value is in the order of 80 ohms when using the house ground, and also when
using an insulated radial system consisting of 28 wires of 10 m to 30 m in
length, depending on the space in the relevant direction. Someone who had used
his 160 meters ground system with several long buried radials also for LF had
reported a value of 50 ohms at 136 kHz.
I have just heard of two reports of house grounds of about 25 ohms; in this
cases a tube 12 meters deep into the ground is employed, or an extended wire
ground system also deep in the ground.
Paralleling different grounds may be a special problem. Direct paralleling often
results in an INCREASE in ground resistance. Therefore some stations use link or
transformer coupling between feeder cable (connected to the house ground) and
the antenna ground, to avoid the bad effect of direct paralleling.
But if both ground systems are tuned against each other the resulting resistance
may be the same as one ground system alone, or luckily somewhat lower, but never
the value that is expected due to the paralleling of two resistances.
From next spring onward I will do further experiments with bare radials buried
flat into the ground, before settling on a final solution.
73 Ha-Jo, DJ1ZB
LAWRENCE MAYHEAD schrieb:
I have just measured my TX ant impedance(R term) and it is 120 ohm! much
higher than I originally thought.It is of course all ground loss,I am using
3 ground rods close to the TX and the house water/mains system.I wonder
wether anyone can offer me comparitive figs for impedance and details of
ground systems?
To improve I will have to put in an awful lot of radials! or consider
counterpoises.
73s Laurie.
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