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To All from PA0SE
To measure the total resistance of my aerial system I can use a
Wayne Kerr admittance bridge type B.601. To do that the
bridge must be connected to a generator and detector.
A method involving less work is using my homemade impedance
bridge with built-in noise source and battery and using the 137kHz receiver as
detector. This gadget measures R up to 270 ohms and X as a capacitance
between -150pF and +150pF in parallel with the R-control. On 137kHz
the X-control range is too small and I leave it at the zero position.
The null is found by varying the R-control of the bridge and the vacuum
capacitor tuning the aerial system. The null is very sharp when the receiver is
switched to a narrow passband.
The calibration marks on the R-control are at multiples of 10
ohm. But the R-value can be read with much better
accuracy by afterwards measuring the resistance of the R-control pot
with a digital multimeter. There is no need to open the box of the bridge for
this; the resistance can be measured at the port for the
unknown.
After subtracting the loss resistance of the loading coil I find
for the earth resistance:
July 1999 :
28 ohm
February 2000: 33 ohm
February 2001: 32 ohm
Summer 2000 I have measured even lower values but I did not put
them on paper.
It amazes me that the resistance is lower in summer than in winter.
I had expected the opposite.
There are no trees near the aerial.
When it rains or in fog the resistance rises considirable, as
indicates by the aerial current dropping by up to 25%.
Could that be due to water particles being heated in the
field around the aerial? Or is it only caused by increased leakage via the
aerial insulators and the feed-in?
73, Dick, PA0SE
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