I’m using the simple 1979 “RF Sniffer” circuit and I’ve found that the minus line of the meter does add a lot of signal when you plug in a test lead and use it as a radial / ground plane.

One really great use is to see how much radiation emanates from an amplifier at various power levels, and where the “drop off” is in your operating position. The SPE Expert 1.3K-FA does seem to be shielded well, the drop off is before my body and head – and of course is very low on the MID and LOW power settings. In the HIGH setting, with the sensitivity all the way up on the FSM and with a ground plane I am reading some RF at my head and face.

The other interesting thing is that 160M has a low reading on the FSM, but 40M offers a very good range. This makes sense since its a wide band receiver of sorts – in fact, its similar to a “cat whisker” radio we all built when we were kids with a 1N34A diode and a coil and wire antenna.

It works very, very well with a hand held 2M/440 HT, and also works on all the bands on 40M up. I did not test 80M, but see that the antenna length and ground plane do effect the sensitivity. In fact, it seems as though I am getting a much bigger dose of RF from the HT than the amplifier, which is interesting.

I wired two banana plugs across the meter so I can use it with my oscilloscope or DVM, and I am getting readings on both pieces of test equipment, but I really need to dig into the Tutorial on oscilloscopes that I have. The DVM showed .1 volt DC being picked up when I transmitted using the amp on MAX power. The HT was a whopping 3.2 volts – and the FSM meter was pegged. I did not register anything from my cell phone – which seems a bit strange. I figure this FSM acts as an RF probe, and even a way to check RF radiation as it might affect your health – so that’s very useful and fun.

Its interesting, but I’m finding even little circuits that you an build get you thinking and asking questions. Here are a few:

  1. What other variations of this circuit might I try? I’m guessing that the other circuit that I have will work better on 160M because it front ends what I have now with a tuned circuit
  2. How might I use my oscilloscope with this FSM, and what can I measure?

The uses so far that really make this a great little box:

  1. Check RF fall off from an amplifier and where there is the most radiation
  2. Check to see if a transmitter is working and an antenna is radiating
  3. Plot actual patterns off antennas and check interaction between my two vertical arrays – and see how the closer verticals interact

There will be more, but for such a simple and inexpensive toy – its turning out to give me more than I expected and is a fun thing to play with and learn.

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