Ok, I am back on the receivers again. It seems that many QRP circuits are all usually transmitters but I believe that receivers are quite important too. In this project you can build a SIMPLE receiver for VHF NBFM and the only active component is a diode. Yes, it's a crystal set! But there is a little more to it than that.

This project was originally intended to be a NBFM transmitter monitor for my VHF station but one day I found that I could hear quite strong stations on it with very little effort. The cavity has such a high Q that NBFM signals will become Amplitude modulated if the cavity is just slightly off-tuned. A diode detector in the output loop demodulates this AM to give audio. No decoupling capacitor is required because the screened cable to the stereo does the job.

The drawing gives the mechanical construction of a cavity which may be built using household "garbage". Coffee tin's, Milk tin's or even an old galvanised dustbin have all be used sucesfully in this project. A 20dB commercial preamp in the antenna lead will increase the sensitivity, and the "shack" stereo amp forms the audio stage. Connect the output of the cavity to the PHONO input of the stereo amplifier. There is normally sufficient gain in the average stereo amplifier for this purpose. In the circuit RF is the antenna input. AF is the audio output to the stereo. D1 is a germanium signal diode (or better).

APPROXIMATE dimensions of the cavity are given below. Notice that 900 MHz is included so you can also listen to NMT (analogue celular) traffic if you live close to a base station.

Frequency (MHz) ----- 50 ----- 70 ----- 145 ----- 430 ----- 900 ---1296

Resonator length ------1364-----974 -----470 -----159 -----76 ------53

Resonator Dia. -------- 45 ------32 --------16 ------5.3 ------3 ------2

Tin dia. ----------------454-------325 -----157 ------53 ------25 -----18

Screw length -----------272 ------195 ------94 ------31 -------15 ----10

The above are given as a START for experimentation. The resonator length is quite critical and should be within +0 -10% quoted. All other dimensions may be varied by as much as +100 -50%. The input and output coupling loops may be varied. If the loop is too large then the Q will decrease. If it is too small then the output will decrease. A good compromise is 5% to 10% of the length of the resonator. I made my loops with flattened copper car brake-pipe.

The resonator is a length of plumbing pipe cut as per the table, and soldered to the lid of the tin as shown. A screw is inserted though a nut (soldered to the base of the tin), so that it protrudes into the copper tube. The screw must NOT be allowed to touch the copper tube resonator. In my cavities I used a bit of plastic tube from a felt-tipped pen to guide the screw.

The tin may be any old coffee tin etc that has suitable dimensions and can be soldered. You can extend tins in length by using a tin-opener (back to the kitchen again !!) to remove the top of one tin and the botom of another, then solder the two (or more?) tins together to form one big tin. If you only have a 15 watt soldering iron then go back to the kitchen (yet again!) and use the gas stove to get the cavity to about 220 degrees. The 15 watt iron then works well.

I have built about twenty or thirty of these receivers for a variety of bands with 100% success every time. I even took a steel dustbin down to 28 MHz, but the resonator was a bit different: 1.5meters of break pipe wound around a bit of plastic drain-pipe. The capacitor was a vaned variable type, NOT the screw type mentioned above. It got rid of the CBer from the 28 MHz band nicely !

This project may be used as a TX/RX filter by using 2 identical input/output loops, but you will need to experiment with the loop size. Internal silver- plating will also help reduce losses still further.

Do NOT forget to wait until your wife, (or mother) is out when you build this project, or you could end up in the same divorce court as all the other radio amateurs.

Have fun QRPing, de HARRY, SM0VPO, Upplands VasbySweden

I am very grateful to Harold, SM0VPO for allowing me to add his files to my web site. He can be contacted by email:

Frank, G3YCC

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