[G0HZU_JMR]

The Haig probe circuit/design looks OK to me but it does have a few design limitations that could cause some confusion in certain circumstances. I hope it's OK to list these issues...

When it is in low gain mode I think that the J310 JFET will generate a significant amount of negative resistance at its gate. This would rarely cause problems but in theory at least it could become an oscillator when probing certain (inductive) loads. To null the negative resistance something like a series 82R resistor could be fitted inline with the 220pF cap at the input. This will degrade the input impedance a bit in both high and low gain modes but not by anything significant. I suppose that if you didn't bother with the 82R resistor it might be possible to exploit the negative resistance because you could use an inductor as a built in test load. If the probe is working it would oscillate (in low gain mode) with the required amount of inductance across the input.

So if the JFET and the diodes were OK then there would be a decent DC level seen at the detector output as it would detect its own self oscillation. So you wouldn't need an RF signal source to test the health of the whole RF probe/detector!

When it is in high gain mode, the input impedance will drop quite a bit by 4MHz. Much worse than a x10 scope probe in this respect. It could easily look like 15k ohm in parallel with 20pF by 4MHz. So it could cause significant circuit loading or pulling. By 10.7MHz the probe could look like 3k ohm in parallel with 20pF.

In high gain mode I'd expect the detector response to start rolling off by 5MHz. It could easily be -3dB by 20MHz. Not a problem if the upper limit of use is only 3 or 4MHz but it may cause disappointment to users hoping to try it out across the whole HF band up to 30MHz for example.

It's only going to have limited signal handling capability.

I'd have thought that a classic diode based RF probe would be more versatile than the Haig probe. Vastly improved bandwidth, lower circuit pulling/loading in many cases, cheaper to make, doesn't need a battery, easier to hold/handle and it can handle/detect much larger RF signals and it will easily work down to 50kHz.