Dear James,
I think the role of the ground with active whips is often neglected -
the output of the whip preamp is the voltage differential between the
whip element and the circuit ground, so the ground connection is just as
much part of the antenna as the whip element itself is. Clearly, an
electrically quiet ground is needed for the active whip, whether this is
provided by the coax feeder or a seperate local ground connection.
It seems that the requirements for a ground connection are quite relaxed.
I have tested a completely isolated system with the coax running
horizontal tied to a non conduction mast and shoved out of an upstairs
window. Signal strength received with the same system and the mast
vertical mounted in the garden to put the antenna at the same location in
space, was within one dB for a vertical polarised signal.
I have also build a system with CAT5 cable as a balanced feeder. One
twisted pair was used for RF, the other for power feed. At both ends of
the RF pair rf-isolating transformers were used. This resulted in a very
low local noise pick up.
Using a common mode choke at the antenna end does not change anything at
all either. My guess is that the antenna element is so small, that even a
small (capacitive) coupling is good ebnough for a ground.
Best regards,
Roelof Bakker, pa0rdt
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