Small portable regenerative receiver during cold weather

The small portable regenerative receiver.
The Big portable regenerative receiver is a success. It is simple and has an extreme low current consumption. The regenerative circuit would be very suitable for a small portable receiver if there were not so many controls and switches. So for this small version, the number of knobs and controls are reduced.
It is a nice receiver to take with you on holidays, there is always some place in your luggage. Battery use is so low that you do not need to take a spare battery.

It is only used for CW/SSB reception of radio amateurs. Volume control can be done by increasing the regenerative feedback. LF volume control is only necessary for AM reception so it is deleted.

Instead of the big air variable capacitor, a small mica type (50 pF) is used with less comfortable coarse tuning performance as a result. To compensate this compromise a little we take a very big tuning knob. Tuning an SSB station will be possible by experienced amateurs but may be difficult for beginners.

Only the 40, 30 and 20m bands are covered so that we need only one coil.

The disadvance of a normal on/off switch is that the receiver can easily be switched on during transport. Can be avoided if switching on/off is done by plugging in the headphone plug.

Battery use is very low, no voltage stabilizer is used. So a small 9 volt battery is good enough for hundreds of hours of listening.

The two earpieces of the headphones are connected in series instead of in parallel for more audio level. The original plug is replaced, both earpieces are connected in series between the center pin and ground. The remaining second contact is connected to ground for the on/off function.

What is left:

Schematic diagram

Circuit diagram of the small simplified ultra portable version of the regenerative receiver
big diagram

RF preamplifier
T1 is the RF preamplifier and also a buffer between the regenerative circuit and antenna.

Regenerative circuit, detector and reflex circuit
T2 is the most active transistor in this circuit. Firstly it act as an RF amplifier. The amplified signal is detected by two diodes and the detected audio is also amplified by T2. T2 is thus not only an RF amplifier but also an AF amplifier.
But T2 does more! It is also a regenerative circuit. Part of the RF output level is fed back to the input by the regenerative control potentiometer. Gain and selectivity increase enorm due to this regenerative process. For SSB and CW it should just oscillate when maximum sensitivity is required. Increase the regeneration control to reduce the sensitivity.
Oscillation can be controlled very smoothly, due to the negative voltage from the detector to the base when oscillation starts. Gain of T2 will be reduced by this negative voltage.

Audio amplifier
T3 and T4 are BC547C transistors. Use the C type for more AF gain.

Tuning circuit
The band switches create a rather comfortable tuning range of each band. Fine tuning for SSB and CW can be done by varying the regenerative control a little or by moving your hand very close to the antenna.
Adjust the values of the inductances, capacitors and number of switches to your variable capacitor and desired bands, it is not so critical.
Of course you do not need to use toroides, all kinds of coils are suitable.
L1 is wound around the toroide, L3 is at the cold (ground) side of L1, L2a + L2b at center position of L1. If regeneration does not occur or if noisy oscillation only happens with the 10k regeneration control at maximum, reverse the connections of L3.

The receiver is very sensitive and you should expect that a lot of weak DX stations can be heard. But that is not the case. The receiver can only be used with short telescopic antennas, full size antenna's will cause a heavy overload of the receiver.

Interference by strong FM broadcast transmitters
On a few locations, interference was caused by strong FM broadcast transmitters. This could be solved by connecting a 10 pF capacitor between the base and emitter and another 10 pF capacitor between collector and emitter of T1, the RF amplifier transistor. And the 100 ohm resistor at the base of T1 is increased to 330 ohm.

Use BC547C transistors, the C types have the highest gain. The band switch has a center position for 20 meters whereby none of the contacts are connected. Both earpieces of the headphones are connected in series.

The receiver

Inside the receiver.

How to use the receiver
The receiver is only designed for SSB and CW reception, for AM reception you should add an AF potentiometer or at least a switched resistor in series with the 0.1 uF capacitor to the base of T3 (100k - 1M or so).
For weak SSB and CW signals it should just oscillate for maximum sensitivity. For stronger signals, increase the regeneration control (increase the amount of oscillation) to decrease the sensitivity.
Fine tuning can be done by varying the regenerative control a little.


Indeed, for AM broadcast stations the AF gain is too high. But SSB and CW signals from radio amateurs are not a problem at all! With the 30 centimeter telescopic antenna, whole Europe is already heard in SSB and CW on 40 and 30 meters with very comfortable signal levels. On 20 meters some USA stations are received, with good and easy readable signals, even in SSB. But do not compare it with a good receiver plus antenna, only stations of average strength can be heard. The receiver can be used indoors but the outdoors reception is much better.

The receiver during warm holidays

The small receiver sounds louder than the big one, perhaps because the earpieces are connected in series instead of in parallel or due to the higher battery voltage. I have not tested that yet.
Tuning is not so easy as with the big receiver. The big receiver has a very good variable capacitor that does 1.5 turn over its whole range, the small receiver 0.5 turn and it does not rotate so smoothly. Tuning of SSB signals will be very difficult for not so experienced amateurs, although fine tuning is possible by small changes of the regenerative control.
Conclusion: Except the somewhat difficult tuning (only for SSB, not for CW), the receiver is a very nice simple radio for SSB and CW with very low battery consumption. It is especially intended for portable use. For use in your shack or another fixed place, a bigger one with all the controls and a more comfortable tuning is advisable. If you want to make this small portable radio also suitable for AM reception, add an AF potentiometer.


Toroide replaced with coil on plastic piece of a potentiometer shaft.

The toroide problem
The frequency stability of the receiver was very sensitive for temperature. And that is strange because my first receiver with identical components is quite stable. The problem was a damaged toroide T50-6 (yellow one). As an experiment it was replaced by a coil on a plastic rod of 6 mm, a piece of the potentiometer shaft. Important is to keep a distance between the coil and the copper of at least 0.5 cm! L1 is about 25 to 30 wdg and L3 is 3 wdg. As an extra experiment, L2 is modified. L2a and L2b are replaced by one coil L2 of 6 wdg. The potentiometer and the 100 ohm resistor are connected together to one end of L2. The other end is connected to ground.
After replacement of the damaged toroide by the new coil, the frequency stability is much better. For best performance however, I would recommend the version with a good T50-6 toroide instead of the plastic rod. The coil with the toroide has a better Q-factor, needs less feedback for oscillation and is also less sensitive for metal parts. A good T50-6 toroide is not sensitive for temperature.