VHF/UHF Cavity Receiver

	...
	VHF / UHF Cavity Receiver
	...

Coffee tins, Milk tins or even an old galvanized dustbin have all be used successfully in this project. A 20dB commercial preamplifier in the antenna lead will greatly increase the sensitivity & the shack stereo amp forms the audio stage. Connect the output of the 'modified' cavity to the 'PHONO' input of the stereo amplifier. There is normally sufficient AF 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 listen to NMT (analogue) cellular traffic if you live close to a base station. All dimensions are given in mm.

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 is a START for experimentation. The resonator length is very 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 large then the Q will decrease. If it is 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, then soldered to the lid of the tin as shown. A screw is inserted though a nut ( soldered to the tin base ), so that it protrudes into the copper tube. The screw must NOT be allowed to touch the resonator. In all 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 & the bottom of another then solder the two (or more) tins together to form one big tin. If you have only got a small 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.