This Middle Wave (500...1500 kHz) Tuned radio frequency receiver uses a coil of wire as a loop antenna. The resonant tank consists of the coil L1 and the variable capacitor C1 (see the Figure 1.). The coil is tapped at the 1/10 of its windings. The RF signal goes from the tapping to the base of the transistor T1. From the load of this stage (resistor R2) the RF signal goes through the capacitor C3 to the next stage (the transistor T2). From the load (resistor R4) the RF signal goes to the envelope detector, based on diodes D1, D2, this is the Greinacher voltage doubler circuit. The envelope detector is loaded with R5, connected in parallel with the smoothing capacitor C5.
Circuit diagram of the Radio receiver with the loop antenna.
T1-T3 - KT315B;
D1, D2 - D9A (Germanium diodes);
L1 - 35+4 turns (350 μH), see the text;
R1, R3 - 51k; R2, R4 - 1k; R5 - 10k;
R6 - 2k; R7 - 510k;
C1 - 1..180pF; C2, C3, C4, C5, C7 - 4.7nF;
C6 - 4.7μF x 6V;
T1-T3 - can be replaced with BC547B, BC147 or a similar transistor, but it takes to match values of R1, R3 to about 500 kΩ.
D1, D2 can be replaced with 1N34.
From the envelope detector the demodulated signal is applied to the audio stage based on the transistor T3. This stage is loaded with an earpiece TM-2 (its resistance is 60 Ω). The earpiece is connected to the headphone jack J1. A modern headphones can be used instead if the earpiece TM-2 if connect both loudspeaker in series. Match the value of the capacitor C7 to provide a desirable timbre.
The radio receiver is powered by a 1.5V battery of AA type. The power is applied through the switch S1 to the receiver.
To make the loop antenna, wound enameled cooper wire of 0.15 mm in diameter (AWG #34) around a former of 56 mm by 56 mm in dimension with smoothed edges. The coil has 39 turns, it is tapped at the 4th turn. The inductance of the coil is around 350 μH. To correct the resonant tank frequency, reduce number of turns of L1 or connect a small capacitor C' in parallel with the coil L1 as it shown in the circuit diagram.
How to adjust the circuit. Connect headphones to the jack J1, turn on the power with the switch S1. Try to tune to a radiostation by turning the knob of the variable capacitor C1 and the same time rotate the loop antenna in vertical and horizontal planes. Match values of R1, R3 and R7 - replace them with potentiometers for a while, make adjustments, then replace potentiometers with resistors of corresponding values.
This design was described in the December 1981 issue of "Radio" magazine by G. Shulgin.