FM crystal radio receivers
Optimization of a radio receiver powered by the energy of radio waves

7/8/2013
Dante Bianconi
Vinci (Florence), ITALY

This report shows the results of some experiments carried out on the basis of Mr. Vladimir Polyakov (RA3AAE) researches. The original circuit was modified by the use of a more efficient detection circuit and by the use of a simple amplifier, self-biased by the radio itself. An efficient antenna (5 elements FM Yagi) was used also to permit to explore overall the weak signals close to the range between 98 - 103 MHz. Earlier it was proposed by Mr. Polyakov, to use a simple dipole as an antenna. According to the original circuit, the high frequency germanium transistor GT311A with Ft = 300 MHz is used there, it provides high impedance at its output (the transistor was grounded to provide the matching with the low impedance of the resonant tank), so it was possible only to use ear speakers with at least 600 ohm impedance to listening the radio.

A further analysis of the circuit came to the idea to use a simple amplifier based on a silicon transistor (BC109C with hFE = 700), it's finally allowed to get acceptable loudspeaking reception. With the use of a more efficient detection circuit, the voltage in unloaded condition reached 2.2 volt across the capacitor C8. With two loudspeakers (are both connected in parallel), the current in the high impedance circuit was also measured by a micro ammeter and sometimes the value of this current reaches more than 100 μA. The transistor of the simple amplifier was used in common emitter configuration to lower the impedance of the output of the high frequency transistor (AF239). The impedance transformer that gave the best results had a 14 kΩ in input and 4 Ω in output. An active low pass filter was used to reduce the noise coming from the first stage of RF amplifier to the low frequency stage.

With the directional antenna it was possible to receive three FM radio broadcast stations, two of them are located at 15 km, and the third - more than 30 km from the reception antenna. The current researches are focused on the circuit with a coaxial resonator, it would allow to reach high Q as well as the selectivity of the tuning.

Crystal radio receiver for FM band

Fig. 1. The circuit diagram of the FM crystal radio receiver

T: Zin 14 kΩ → Zout 4,8 Ω (K ≈ 60:1)
R1: 70+200 kΩ
R2: 30 Ω
Tr1: AF239
Tr2: BC109C
D1,D2: 1N82A
L1: 5 turns (silver wire 1 mm, coil with internal diameter of 8 mm)
L2: 7 turns (silver wire 1 mm, coil with internal diameter of 8 mm)
C1: 8.5 pF (ceramic NP0 type)
C2: 5-25 pF (KPV type)
C3, C4: 4n7 (ceramic type)
C5: 0.15 μF
C6: 3-28 pF (KPV type)
C7: 0.01 μF
C8: 1 μF
WA1: 5 elements YAGI antenna
LS1: 3.5 Ω loudspeaker (diameter 200 mm)
LS2: 3.5 Ω loudspeaker (diameter 100 mm)

FM crystal radio receiver

Fig. 2.

View to the side of the FM crystal radio receiver

Fig. 3.

Front view on the FM crystal radio receiver

Fig. 4.

The FM crystal radio, called BIDA 1: the dimensions of this design are only 80x35x80. In the circuit is used the variable capacitors with the silver plated coils, it allows to obtain a better quality factor Q and the high frequency germanium transistor AF239. The taps on both coils are visible on the Fig. 3, it allows to reach the better matching between the antenna impedance and the main resonant tank L1 and the second coil L2. The signal amplified by the transistor AF239. The detection circuit was composed by using a Villard voltage doubler with the use of two very sensitive germanium diodes 1N82A. This sensitive diodes has been used for radar applications after the Second World War.

Circuit board with the transistor mounted on it

Fig. 5.

Loudspeakers and the audio transformer

Fig. 6.

Circuit board and the audio transformer

Fig. 7.

5 Elements Yagi Antenna for FM band

Fig. 8.
5 Elements Yagi Antenna

5 Elements Yagi Antenna - view from the side

Fig. 9.
5 Elements Yagi Antenna - view from the side

It's clearly visible on the circuit diagram (see Fig. 1) that the HF radio amplifier stage is self biased by the main radio circuit. The output AF stage is loaded with the high impedance transformer. In the Fig 8 and 9 you can see the 5 elements Yagi antenna, the heart of the FM crystal set (the central frequency is 100 MHz). See the folded dipoles and the indicated dimensions of the RG8 cable. The impedance of this antenna is close to 52 Ω as well as the impedance of RG8 cable. The cable's length was optimized, so it is only 6 meters long.

While testing, the 5 elements Yagi antenna was pointed to the north-east direction (in the opposite site the field intensity is higher, but there is a medium voltage power line that probably interfered with receiving - unfortunately the most of the radio broadcasts are can be received in this south-east direction). The centre frequency of the Yagi antenna is 100 MHz, it allows to receive three radio stations:
Radio Lady --> 98.2 MHz (20 km from the receiving antenna);
Radio Sei Sei --> 101.5 MHz (20 km from receiving antenna);
RTL102.5 --> 101.2 MHz (35 km from receiving antenna).

References:
V. Polyakov, FM crystal radio receivers

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