m0xpd’s ‘Shack Nasties

RF Probe

Today I finally got round to building a nice little RF probe kit from Rex, w1rex, the Tuna King over at QRPme.

I took the well-executed PCB which – unusually for Rex – had through hole mounting for the components rather than the ‘Limerick’ construction that recently I’ve come to associate with him…

m0xpd's 'Shack Nasties

Following the excellent instructions, I whittled away the PCB at the business end, to make the probe easier to probe and poke into awkward places, then added the four components…Actually, I tested the whole assembly at this point – but let’s pretend I just went confidently ahead and sealed it all up.

I built up a few layers of heat shrink at the tip…. before covering the whole thing in a protective sheath, which prevented my fingers messing up sensitive measurements by touching the wrong (high impedance) place.It isn’t going to win a beauty prize – but then neither am I.

To check the new little toy in operation, I turned to another gift from Rex (kindly donated at last year’s FDIM) – a Dummy Load (which makes a nice addition to my collection)…
I threw some RF at the dummy load and probed about at the top of the resistors, having first connected the new RF probe to an ugly old digital multi-meter of unknown parentage.
As you see from the little inset photo of the ‘scope screen, I’d set the RF level to 1V peak. The RF probe was indicating a voltage of 0.656 on the multi-meter but, given the 1V peak on the resistors, it “should” have been reading 0.707106…. V.

The circuit of this RF probe is a shunt design of the form regularly appearing in ARRL handbooks (etc). My most recent copy is the 2009 edition, in which it appears as Fig 25.9 (D) on page 25.9.

The series combination of a 3.9 M and a 220 k resistor as used on this probe gives 4.12 M Ohms.

When this resistor drives a meter WITH A 10 MEG OHM INPUT IMPEDANCE, then the probe will scale the peak voltage detected on its diode to the correct RMS voltage for a SINUSOIDAL input waveform.

This is because of the potential divider action between the meter’s input impedance and the source impedance formed by the 4.12 M Ohm resistor:


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