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Hello David,
The pa0rdt-Mini-Whip was designed as a low noise
reception antenna at LF, because at my location a loop antenna appeared to
pick up to much noise.
This type of antenna does work very well at low
frequencies, in fact it excels at VLF.
On the other end of the spectrum, reception
drops off above 20 MHz, though I have received signals up to 50
MHz.
The frequency response of the buffer amplifier is
flat to at least 100 MHz, so this is not the cause of degraded performance
versus frequency.
Hajo, DJ1ZB, has done some computer analyses,
but his work does not confirm a decrease in performance up to 100
MHz.
The capacitance of the probe and the input
capacitance of the FET form a voltage devider for rf.
Theoretical this should also give a flat frequency
response; which does not happen in practice.
This might be due to straight circuit
capacitances.
A mini-whip with leaded parts has a PCB antenna
measuring 30 x 40 mm.
Using SMD parts 22 x 26 mm of copper clad PCB
is neccessary for the same signal levels.
This is approximately 50% less.
One solution could be to increase the
capacitance of the probe and decrease the gate resistor of the FET.
Doing so the gate capacitance has less impact on
the frequency response as it is swamped by the gate resistor.
To be honest, I have not done much work in this
area yet, as my main interest is reception at LF.
I have tried some circuits with a very low input
capacitance, which yielded an even smaller probe.
Which was not much use in
practice.
A combination of a very low input capacitance and a
low gate resistor could do the trick.
Best regards,
Roelof Bakker, pa0rdt
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