From Johan, SM6LKM
Bob, ZL2CA, wrote:
Connecting the mains earth to the RF earth INCREASED THE RF
RESISTANCE OF THE ANTENNA. We re-checked several times,
and also checked that the voltage across the capacitor was low
(which it was). So there was no mistake in the experimental result.
In that case, it was not a good move to connect the mains earth.
This is interesting. I have had the same experience. My antenna is
an inverted L, about 80 - 90 meters long, height varies between 10
and 20 meters (sloping terrain) and the far end is the highest point.
The feedpoint resistance was about 75 ohms when mains protective
ground was used alone (including loading coil etc). When I tried the
6 inch steel tube in our deep drilled water source, the feedpoint
resistance fell to 36 ohms at resonance. The tube is 12 meters long,
11 of which of goes through soil and the remaining 1 meter is inserted
into the solid rock below.
When I connected the mains ground in parallel with the big tube,
feedpoint resistance increased to about 55 ohms.
I do not have a complete understanding of what happened but I think
that the efficiency of the antenna is better with the big tube ground
alone, regardless of feedpoint resistance changes. I will make field
strength measurements as soon as I find time to finish a PA...
(Thanks to Dick Rollema for a great article!)
The soil is quite"transparent" at 136 kHz and the penetration depth can
be several tens of meters (skin effect). With the deep tube, the average
electrical ground level is perhaps 6 meters *below* the surface so I
believe that the effective height is greater than what one might first expect.
Connecting the mains ground in parallel will probably reduce the effective
heigth.
Any comments?
73 de Johan, SM6LKM
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