RF Sniffer – G3JKX

Building and Using an RF sniffer.
A simple gadget every radio amateur should have.

Recently, Bob, MØRJS asked me a question about what the SWR of 1:1 actually meant. This is not very easy to answer without lots of maths, diagrams and what not. He is trying to get his HF aerials working really well. (Don’t we all?) So I said to him, ‘Why bother with getting the SWR down to an extremely low figure, when what you really want is the most RF output from your aerial ! So why not measure it? Adjust everything for maximum smoke!

What he needs, and I suspect you do too, is an RF ‘sniffer’ somewhere outside the shack. It consists of a small plastic or metal box, out of which sticks a small aerial. This is connected via (say) a 3 mH RF choke, to the outer connection of a coaxial socket which is mounted on the bottom of the box (to help keep the rain out of the connection). The coaxial socket inner is connected to the junction of the aerial and the RFC, via a small germanium diode. Nothing else.

Now, if you haven’t already realised it, your box contains a very crude crystal set. Any RF picked up by the aerial will produce a DC current and an audio signal if your rig puts out some Amplitude Modulation. By conveying these signals back to the shack via a coaxial cable (to keep RF from getting in) it is now possible to listen to the AM on headphones or measure the DC current, due to a carrier wave, with a suitable meter. This could be a self ranging digital
meter, or better, a micro-ammeter, with a suitable variable resistance in series to adjust the sensitivity, in case you are running a kilowatt and don’t want to fry the meter!

It may be necessary to wind the coaxial cable from the sniffer through a ferrite ring, positioned where it comes into the outside of the shack. This acts as an RF choke and prevents any of the RF from your transmissions getting back into the shack and also affecting the meter readings. Burying the cable inside a length of garden hose will make doubly sure.

It is important that there is a good water seal where the sniffer aerial exits the box. It might be a better idea to have the aerial coming out from UNDER the box, then no rain seal would be needed at all. A small hole must be provided in the bottom of the box, anyway, to let any condensation out. Not too big a hole mind or the local creepy-crawlies will make nests inside!

Now affix your box to a fence or on a stick outside somewhere. Wire up the coaxial cable to the meter and off you go. Start with low power carrier only, using AM or FM, adjusting the sensitivity potentiometer until you get a reading. Increase the power output and readjust the ‘pot’ to keep the current ‘visible’. Keep on doing this until you have Full Scale Deflection on the meter for the highest power output that you are ever going to use.

It will now be very easy to see, day to day, if your rig is losing output or something is wrong with the ATU, the coaxial cable to the aerial or indeed that the aerial itself has bits missing or has fallen down!

Earlier I mentioned that the RF choke has a value of 3mH; well it doesn’t have to be. Half a mH would do. What you don’t want is the RFC to be resonant on any band you are going to use, ‘cos then RF might damage your meter.

Remember that using FM will only produce a DC current from the sniffer. (FM is a frequency modulated carrier wave of course) and SSB will only produce a wildly variable DC which will be difficult to see. You can check if the peak output is looking normal of course. This is where using a very sensitive low current moving coil meter is a distinct advantage. It will respond so much better to the varying DC current.

A sniffer is a very simple gadget to make which will make checking that you are actually ‘getting out’ a piece of cake. The ability to listen to yourself, on AM, also means you can personally check on the audio quality and not have to
rely on the opinion of others whom you talk to over the air. You can also use it to check and adjust your audio compression and microphone gain settings. Just connect your gadget to some headphones or use a voice recorder and then play it back. You may just get a surprise.