Super-regenerative Air Band Radio
Civil aviation still uses AM communications between 108MHz and 136MHz. It is fairly easy to build a receiver that operates on these frequencies, however, it will also be found easy to build a receiver that won’t tune high enough! The trick to getting the tuning range high enough is to use a high frequency transistor and VERY short leads. I used a BF199 transistor but a 2N2369 will probably work well too.
By far the most reliable construction method is ‘ugly’ construction on a 1 inch by 2 inch piece of copper clad board (I built this circuit on perforated board many years ago and it only just tuned high enough to receive my local airport signals). By using the copper foil as a ground plane we can mount components in mid air. This is more robust than it might seem because many components connect to decoupling capacitors that provide an anchor to the board. Keeping leads short reduces stray inductance and capacitance, and helps ensure stability and good tuning range. The circuit is based on an old one advertised in a hobby magazine many years ago, modified slightly.
The tuning coil ‘L’ is simply four turns of 0.8mm diameter wire with a diameter of 5mm. There is no former. The coil is compressed or expanded to adjust the tuning range. We need to arrange that we can just tune the upper end of the FM broadcast band when the tuning potentiometer is at minimum setting (wiper at the ground end). This will bring the Air Band into the main tuning range of the receiver. The antenna is a short telescopic one and couples with a ¾ turn link turn. Note that, if the coupling is too tight (the link too close to the main coil) then the regenerative stage will be too loaded and won’t oscillate.The varicap diode ‘D’ was an unmarked component in my receiver, however, the BV409 should work admirably here. All capacitors with the exception of the electrolytic are ceramic. The transistors are best mounted upside down with their leads bent sideways.
No switch was used in my receiver. Instead, a stereo 3.5mm socket was used and the third contact used to switch the supply. Note that this trick only works when a mono plug is inserted! I mounted the earphone socket such that it traps the battery in one corner of the case. I expect this apparently cool solution is going to come back to bite me when I drop the radio, and the socket gets smashed off by the momentum of the battery!
In operation the regeneration control is increased until a rushing sound is heard. This noise indicates that the circuit is oscillating but will diminish when a signal is received. At this time the tuning and regeneration potentiometers can be adjusted for best reception. The receiver will be found to be most sensitive when the regeneration is set so that the circuit is just oscillating.
Purists will add an RF gain stage prior to the regenerative section, as this minimises coupling of oscillations to the antenna as well as adding sensitivity. In practise the circuit is sensitive enough but one should be aware that RF radiation from the antenna can interfere with nearby equipment, in particular, televisions.If the circuit were operated on a plane it might well interfere with communications so on no account should this circuit be used near an actual plane. In any case, you would rightly be barred from bringing such items on a plane, let alone be permitted to use one!!!