[Elecraft] Elecraft RF Probe

[Ron D'Eau Claire]

Geoff, GM4ESD, wrote:

Quite right, but there is a way to overcome this problem which is to apply 
an offset DC bias to the diode(s) which cancels the diode's natural 'bias'. 
The diode(s) will then conduct with low applied 'signal' voltage. Requires a

stable and 'quiet'  DC source e.g. a battery, and stable resistors in the DC

'level adjust' circuit, plus care when setting up. Perhaps it is not worth 
doing this unless there is a REAL need for a probe that can measure low RF 
levels.

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Some years ago Wes Hayward, W7ZOI, published a nice design for such a
low-level detector in "Solid State Design for the Radio Amateur". It uses a
simple op-amp and a second diode to set up the proper bias for the detector
diode so that ordinary small-signal silicon diodes (e.g. 1N914) could be
used to measure very small RF voltages. I used the circuit myself for a
number of years. 

Ron AC7AC