Hi John,
Am 03.01.2012 21:51, schrieb John Rabson:
The earth system consists of three wires radiating from the shack with angular
spacings of about 60°. They run underground through plastic conduit for a
distance of about 10 m and the remote ends are terminated in metal rods of the type
used here to provide safety earth connections for mains installations. The three
wires are connected together at the transmitter to provide the earth.
That sounds not bad.
The local geology is a mixture of granite and limestone with intrusions of
calcite.
Not optimal.
What about the house mains earth? Is it connected in parallel? I would
do so. If you use the TX antenna as the RX antenna too, it may be an
idea to disconnect the mains earth automatically by a relay to prevent
local QRM coming in to much...
At present the resistance at 137 kHz at the feed point is about 150 ohms (plus
of course some reactance which it is not difficult to cancel).
Are there many trees sourrounding the antenna? A phot would help us here.
At a frequency of 980 Hz, I get a resistance of about 25 ohms, depending on
whether there has been recent recent rain.
Where did you connect that frequency generator. One pole is the earth,
but what is the other pole?
I think it is only useful to measure the losses at the desired frequency.
Reasoning that the much higher resistance at 137 kHz might result from the
return current flowing through the ground rather than through the earth spikes,
I tried strapping the remote ends of the earth wires. The additional wire ran
almost below and effectively parallel to the antenna wires.
This strapping made practically no difference to the 137 kHz resistance, so I
had further thoughts:
1) laying down an earth mat or something like chicken netting, or the kind of
metal mesh which is used for fencing. Unfortunately, such things are expensive
here (I estimate the cost would run into three figures in Euros), or
2) making the mat out of hookup wire or something similar. I have plenty of
such wire. Would I need to join the wires at each crossing, and what spacing
should I use?
Any suggestions, please?
Hmm, first i would switch the mains earth in parallel.
Then, if the goal is to optimise the antenna efficiency it appears to me
it is rather useful to invest in a larger pole to increase the effective
height of the antenna wire. There are 18m poles available, not to
expensive. With 2 or 3 x 3 supports there should be no problem.
The metal rods you use are excellent!
Maybe make some experiments (on 137.0 kHz, with a resonated antenna),
measuring the antenna current and power. First use 1 rod (exclusively),
then 2, then 3. You will see if it is worth to spend another 3 rods or so.
Which antenna C do you have?
73, Stefan/DK7FC
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