In the June 1934 issue of QST, George Grammer described a simple receiver consisting of a regenerative detector and an audio amplifier. A similar receiver was described by Guy Grossin in the RadioREF of January 1935. Today, such a receiver remains relevant. To build it, we will use a bipolar transistor for the detector and an integrated audio circuit for the audio amplifier. An untuned RF amplifier transistor is added between the antenna and the detector stage to avoid the frequency variation that occurs when changing the antenna coupling. The significant improvement in components over the past 90 years has made it possible to obtain astonishing results.
Recall that a regenerative detector is actually a variable gain oscillator. By gradually increasing the gain, we obtain an increase in amplification. At the point of oscillation, the stage begins to oscillate and the gain is theoretically infinite. This explains the extraordinary results of this type of receiver. For listening to a SSB or CW station, it is necessary to be slightly beyond this point to demodulate the signal. For amplitude modulation, it is necessary to stay just below this threshold to avoid whistling interference with the carrier of the emission.
With such a receiver, it has been possible to hear from France a QSO between New Zealand and Panama with a 2 x 10m Levy antenna. Here are the three secrets of this receiver that allow such a result:
Use of high-quality components. The detector transistor will have a transition frequency close to 1 GHz. The audio amplifier must have low noise. The fixed capacitors in the RF circuit will be of the NPO or COG type.
Use of a tuned circuit with a very low L/C ratio. In practice, a tuning capacitance of more than 1000 pF is required. Even at 24 MHz, this value can be respected. It allows good frequency stability. It is possible to listen to an SSB station on 24 MHz for 30 minutes without retuning. The hand effect is almost canceled. Finally, the higher the value of the tuning capacitor, the better the selectivity, especially for eliminating out-of-band broadcast stations.
Battery power. This avoids most of the noise from the AC power supply, and in particular that caused by the modulation of the oscillation of the detector locked by the AC power supply (tunable hum).
A particular difficulty is the spread of the bands without using a mechanical device. Three variable capacitors are therefore necessary (coarse tuning, fine tuning, and ultra-fine tuning). The use of 4-gang capacitors makes it possible to use large values for bands below 15 MHz and a much lower value above 15 MHz.
All connections of the tuned circuit must be very short (less than 7 cm). In practice, interchangeable circuits on DIN plugs will be used. This avoids lengthening the connections when using switches. Mechanical construction is also greatly facilitated. Between 14 and 24 megahertz, the coils have less than one turn, so they have the shape of an inverted U.
Three switches (S1, S2, S3) allow adding capacitors in parallel with the resonant circuit, which facilitates their construction at high frequencies (> 15MHz) and makes it possible to cover the entire 40m and 80m amateur bands.
The coupling of the antenna to the oscillating circuit must be finely adjusted. An ON-OFF-ON switch allows choosing three values of coupling capacitance. A potentiometer completes the adjustment of the coupling.
A 100 nF capacitor (NPO or COG) for coupling the oscillating circuit to the base of the detector transistor is essential to short-circuit to ground the 50 Hz or 60 Hz induced by the AC power supply. Such a capacitor only exists in SMD. However, it is a large SMD on which it is easy to solder two wires to use it as a classic component.
On 80 meters in the evening, you can hear whistling interference between the 2nd harmonic of the receiver and the broadcast stations of the 41 m band. A switchable trap circuit located in the antenna circuit can eliminate this whistling.
The main drawback of this receiver is synchronization on powerful stations. SSB becomes incomprehensible. It is then necessary to reduce the antenna coupling, which changes the frequency. The use of an untuned RF amplifier is essential so that the frequency variation does not exceed a few hundred hertz during this adjustment.
The receiver is made in a wooden box (23x15 cm). The internal walls are covered with 5 cm wide copper adhesive strips to create the ground plane. The components are soldered directly to each other. When necessary, 10 Mohm resistors serve as connection points. Bare 24 AWG wires soldered between 2 10 Mohm resistors are used for long connections.
The resonant circuits are made on male DIN plugs (4 pins) of which only the base has been retained. The wire used for the coils is 2.5 mm˛ (20A), except for 80 m where 24 AWG single-strand wire is used. The given values of tuning capacitance are indicative. It is essential to put several capacitors in parallel to obtain the desired frequency. From 14 MHz and above, do not exceed a value of 470 pF per capacitor or 220 pF for 24 MHz. For inverted U-shaped coils, the capacitors must not be placed more than 1.5 cm from the base of the DIN plug. Sometimes 8 capacitors in parallel are needed to obtain the desired frequency. The result is obtained by successive excesses. This is the most difficult part of building the receiver. It is essential to buy assortments of multilayer NPO or COG ceramic capacitors to build these resonant circuits. These circuits determine the results of the receiver.
The 443df variable capacitors are simply glued with cyanolite superglue. Before gluing, a 2.5x25 screw is fixed in the central axis in order to use conventional knobs. These will be large (A05 knob).
Rediscover the amateur bands with this receiver. Hearing stations from the other side of the world with equipment that you have built entirely is a marvel.
80m / 5 turns, 22mm diameter / 1141 pF
40m / 2 turns, 22mm diameter / 3322 pF
20m / Reversed U, 55 x 8 mm / 2533 pF
17m / Reversed U, 50 x 8 mm / 1510 pF
15m / Reversed U, 50 x 8 mm / 1000 pF
12m / Reversed U, 40 x 8 mm / 1100 pF
Wire: 2.5mm˛ (20A) except for 80m (24 AWG single strand)
Note: The value of C (capacitance) must be determined experimentally. Up to 8 capacitors may be needed to obtain the correct value.
key words
multilayer ceramic capacitor npo assortment
443df capacitor
A05 knob
copper adhesive foil
single strand 24 AWG
F5LVG
Olivier ERNST
11-2024