SMALL PORTABLE REGENERATIVE RECEIVER
KLIK HIER VOOR DE NEDERLANDSE VERSIE
Small portable regenerative receiver during cold weather
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:
Circuit diagram of the small simplified ultra portable version of the regenerative receiver
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.
T3 and T4 are BC547C transistors. Use the C type for more AF gain.
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.
Inside the receiver.
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.
The receiver during warm holidays
TOROIDE REPLACED BY COIL ON PLASTIC ROD
Toroide replaced with coil on plastic piece of a potentiometer shaft.
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