Radio Direction Finder and PJ-80 Direction Finder Circuit

The radio direction finding movement originated in the 1960s. It was introduced into China from Europe in 1960. It is similar to the well-known hide and seek game, but not looking for people, but looking for the source of the transmission signal. Radio "hide and seek" is the combination of modern radio communication technology and traditional hide and seek game. The general process is to hide 3 to 10 signal sources (i.e. small transmitters) in advance in natural environments such as wilderness, mountain jungle or suburbs, parks, etc., and send out Morse code signals with different calls regularly. The participants hold a radio direction finder to measure the direction of the hidden radio station, and quickly and accurately find out these signal sources one by one by walking, so that they can find the specified number of radio stations within the specified time, and win if they use less time. Usually, we compare the signal source cleverly hidden in advance to a cunning fox, so this sport is also called radio "fox hunting".
At present, there are three main types of radio direction finding activities in China: first, short distance 80 meter short wave band direction finding suitable for popularization in middle schools, youth science and technology museums (stations), children's palaces, activity centers, etc; Second, direction finding in 160m medium wave band; The third is the direction finding of 80 meter long distance short wave band and 2 meter ultra short wave band in line with the international direction finding competition rules and suitable for college and middle school students.
Participating in this activity can not only learn the knowledge and technology of radio direction finding, but also learn the basic knowledge of relevant circuits, and master the direction finding machine and other electronic production skills. Because it is neither a purely scientific indoor production, nor a single run on a fixed site, it fully embodies the theory and practice. The combination of hands and brains, indoor and outdoor, physical and intellectual, is very beneficial to promote the comprehensive development of youth's morality, intelligence, physical, beauty and labor, and to enrich the content of the second classroom of the school.
The PJ-80 is a popular direct amplification 80 meter band direction finder with simple circuit, low price and easy installation. It is very suitable for radio direction finding and assembly of teenagers. The block diagram and circuit principle of the whole machine are shown in Figure 1 and Figure 2.
L1 is a magnetic rod antenna, A is a rod antenna, K1 is a single and two-way conversion switch, which is used to judge the direction of the radio station. BG1 and the peripheral circuit form a high-frequency amplifier, which amplifies the high-frequency Morse code signal received by the antenna and outputs it by coupling B1. BG3, C14, C15, D2, C16, C17 and C18 form an adjustable deformable capacitance three-point oscillator. Adjusting W2 can change the reverse bias voltage of the varactor diode D2, thereby changing the capacitance of the tube to change the oscillation frequency. The voltage stabilizing tube D3 is used to eliminate the unstable oscillation frequency caused by the drop of battery voltage. The oscillation signal is superimposed with the high-frequency signal output by B1, and then the beat signal is generated by diode D1 mixing. The audio signal generated after detection and low-pass filtering is amplified by the low-frequency power amplifier circuit composed of BG2 and LM386, where D1 plays a dual role of mixing and detection. K2 is the power switch controlled by the headphone socket. When using stereo headphones, K3 is closed.
debugging
1. Inspection of working point: the voltage stabilizing tube D3 can be 3.5~4.4V. When W1 is set to gain, the voltage at both ends of R3 is about 0.4~1V (Ic1 is about 0.4~1mV), the voltage at both ends of R9 is about 1.5~3V, and the voltage at both ends of R12 is about 2~2.5V.
2. Debugging of frequency coverage: W2 is placed at the central position, the high-frequency signal generator is placed at 3.5 ～ 3.6MHz, and the frequency knob of the high-frequency signal generator is turned to make the direction finder receive the signal of tone change, indicating that the beat oscillator of the machine has been working. The high-frequency signal generator is set at 3.55MHz, and the B2 magnetic core is adjusted to hear the signal; Rotate W2, the 3.5MHz and 3.6MHz equal amplitude signals output by the high-frequency signal generator can be heard, and there is a slight margin at both ends. If the high-end can't hear it, but the low-end has a large margin, the B2 magnetic core can be slightly turned outward; The opposite is true.
3. If the listening frequency is too wide, properly increase the R14 resistance; otherwise, reduce the resistance.
4. Adjustment of antenna circuit: the high-frequency signal generator outputs 3.53MHz signal, and adjust C1 and the coil position on the magnetic rod to make sound.
5. Adjustment of high voltage amplifier circuit: the high-frequency signal generator outputs 3.57MHz signal, and adjusts B1 magnetic core to make sound.

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