Here is a little circuit I put together one evening so I could use a high-impedance E-field probe antenna with my commercial general coverage receiver. I wanted something small and simple which I could use mobile, and which would run on its own batteries so that noise from the car DC supply would not be introduced.
The concept is simple: build an amplifier with the highest possible input impedance and connect a preselector to the input. I chose the AD712JN Bi-FET op-amp because I had a few hundred of them lying around, and they had the specs I needed: 3 MHz bandwidth and an astronomically high input impedance of 3 x 10^12 ohms.
Construction is not critical; the only sensitive part is the LC tuned circuit. Since the voltage gain is 1, there is little danger of oscillation even if "messy" wiring is used. Keep in mind that the capacitance of the LC circuit includes the input capacitance of the op-amp (5.5 pF) and of the probe and its feedline. It is not a good idea to use a long coaxial feedline for this type of antenna. It is better to mount the preamp at the whip and tune it remotely with varactor diodes or a motor. Of course, in mobile operation this is not a problem, since you are within a few feet of the probe anyway.
When using a loop antenna, just leave out the 4.7 mH inductor and select a capacitance that resonates the loop at the desired frequency.
You can also use the more common CA3140 or CA3140A op-amp, with the neccessary pin number changes. Inputs on 2 (-) and 3 (+), output on 6, power on 4 (-) and 7 (+). Thanks to Steve Olney VK2ZTO who first tried the CA3140 version.
If you try this circuit or modify it, I would be interested to hear your results. Have fun!
Pierre Thomson
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