[G0HZU_JMR]

I know IsquaredR only wants to use this probe up to 3 or 4MHz but if anyone else wants to use the Haigh probe as described in the Haigh article then I'd advise caution if you want to avoid disappointment. The second paragraph in the Haigh article implies that this probe was designed for use (in high gain mode) in comms receivers with a high first IF. The article implies it could be used to probe the response of the first IF filter. If this first IF is typically up at 45MHz and has an impedance of a few hundred ohms then I'd argue that the Haigh probe isn't fit for purpose in this respect because it will load the circuit under test quite a bit. It would be just about OK to use in a 50R circuit at 45MHz but I wouldn't recommend it.

I get the impression that Haigh simply assumes or hopes that the input impedance of the JFET will be high at VHF when in high gain mode. But any kind of basic analysis (or measurement) will prove otherwise. It isn't as simple as assuming that JFETs are high impedance devices so the input loading can be modelled by 1Meg in parallel with a few pF.

The J310 JFET has 2pF of D-G capacitance, a Gm of maybe -0.01 and the drain load is 1000 ohm. So there will be Miller Effect at play here. The Miller capacitance could typically be 22pF. With a Gm of -0.01 and drain load of 1000 ohm the Rs of this capacitance could be around 91 ohms based on this ultra crude model.

The parallel equivalent of (91R in series with 22pF) at 45MHz is about 375R in parallel with about 17pF and this is roughly what will be seen at the input of the JFET in the Haigh circuit at 45MHz in high gain mode. This is a very crude and nasty model but it will get pretty close to reality at 45MHz I think. However, in reality, there will be additional parasitic resistances and capacitances within the JFET so the impedance will be slightly lower than this but this basic analysis will be close enough I think. So I would argue that the probe will detune and load filter circuits quite badly at 45MHz unless the probe is used at very low impedance points in the circuit.

This Haigh circuit seems to be the latest in a series of JFET buffered diode detectors that have appeared on the forum in recent years. All the ones I've seen have been flawed in various ways and my advice would be to be very wary of what is promised in magazine based projects like this.