On 3/30/2023 7:47 PM, Frank W3LPL wrote:
Hi Dave,
Anyone who has experimented with a high impedance antenna such
as an end fed half wave has experienced the extreme environmental
influence on the feedpoint impedance of a high impedance feed.
The required spacing to trees and buildings is very difficult to predict
with any confidence. The impedance of a high impedance element
-- and hence the amount of voltag if feeds into the preamp --
is heavily influenced by its immediate environment.
If all of the high impedance verticals in an array do not produce the
same voltages from the signals received by the array, the pattern
of the array and especially its nulls are significantly degraded.
73
Frank
W3LPL
Allow me to quibble a little with this analogy. The 1/2 wave dipole
is a tuned circuit with a Q in the low double digits. This leverages
any capacitance change. The voltage probe antenna is just a plain
non-resonant capacitance. Still sensitive to nearby conductors, but
probably not to the extent of the EFHW antenna. I would be more
worried about circuit capacitance in the hi-Z preamp. In general,
the capacitance of JFET's (or any FET's) is very loosely specified.
The transconductance is also very loosely specified, and can
act as a modulator of input capacitance due to the Miller effect.
As if that isn't bad enough, the DXE preamp comes with tank circuits
that you can optionally activate with jumpers. This adds another
random variable to the input capacitance. Ideally, there would be
a "factory select" padding capacitor to make the capacitance the
same on all copies of the preamp. AFAIK, no preamps have this
feature. And finally, the CB whip moves around wildly in the wind.
If there are nearby objects, this adds a time variability.
73
Rick N6RK
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