Dear OM,
Walter, G3JKV, wrote:
If you measured the bandwidth of an entire antenna system, (earth-tuning
coil-antenna), then measured the bandwidth of the coil itself alone, could
this be used to get some idea of earth losses? Could it be assumed the
ohmic losses in the system are negligible compared with earth?
Yes, that can be done, Actually that was how I measured the earth resistance
at my QTH as a function of frequency. I first measured the total resistance
of aerial + coil. Then the resistance of the coil was measured separately
and this value was subtracted from the first reading. That gave the earth
resistance.
The resistance of the aerial wires and the radiation resistance were
neglected because they are a very small part of the total resistance in the
system.
I reported my findings on the reflector and I now repeat part of the text of
that message.
"I used a Wayne Kerr admittance bridge that shows the admittance as a
conductance in millimhos (now called millisiemens) in parallel with a
capacitance in pF.
I left the C-control at zero because the aerial resistance was measured at
resonance. Jim, M0BMU, has clearly explained why this is the better way.
As source and detector I used a Wandel & Goltermann signal generator SP-12
and selective level meter SPM-12. They can be interconnected so the SPM-12
also controls the frequency of the SP-12. Because in this way source and
detector are always tuned to exactly the same frequency I could use the
SPM-12 at its 25Hz bandwidth position which helped to avoid strong signals
and noise received by the aerial polluting the measurements.
The coil has taps after every ten turns. I shortcircuited increasing parts
of the coil using those taps. (I also tried leaving the unused part open.
But as the used part became smaller and the unused part larger the voltage
at the top end of the coil increased more and more, rendering the system
extremely sensitive to hand capacitance effect).
At each tap the frequency for resonance was sought and the conductance read
from the bridge.
The resistance of the coil at each tap was found by replacing the aerial by
a variable vacuum capacitor. The capacitor was adjusted until resonance was
obtained. The high Q of the coil made tuning extremely critical; in fact
hand capacitance made it impossible to tune precisely to the resonance
frequency. Fortunately the frequency of the PS-12/SPM-12 combination can be
adjusted in very small steps and this was used to tune exactly to the
resonance frequency. "
73, Dick, PA0SE
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