Transmitter for sports radio direction finding. Scheme, description

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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
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Transmitter for sports radio direction finding. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / Civil radio communications

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The transmitter offered to the attention of readers can be used not only in training, but also in official competitions in sports radio direction finding "Fox Hunting" in the range of 80 m. The parameters of its signals correspond to national and international standards for such devices. The output power of the transmitter is 10 mW, but if necessary, it can be increased using an additional power amplifier.

The transmitter circuit is shown in fig. 1. Its basis is the PIC16F84 (DD1) microcontroller. The clock generator of the microcontroller, stabilized by the ZQ1 quartz resonator, simultaneously serves as the transmitter master oscillator. Its frequency of 3,579 MHz lies within the 80-meter amateur band. The generator signal through the buffer (element DD2.1) is fed to one of the inputs of the element AND-NOT DD2.2, the second input of which is connected to pin 13 of the microcontroller. By changing the logic level at this output, the program turns the transmitter signal on and off.

Sports DF transmitter

The inverter DD2.3 between the output of the element DD2.2 and the gate of the transistor of the output stage VT1 is necessary so that at a low logic level at pin 13 of the microcontroller, which corresponds to the absence of an RF signal, the voltage at the gate of the transistor VT1 is also low, closing the latter.

The primary winding of the high-frequency transformer T1 is included in the drain circuit of the transistor VT1.

The secondary winding of the transformer forms an oscillatory circuit with capacitors C6 - C8 and the antenna's own capacitance, tuned to the operating frequency of the transmitter. Resistor R3 limits the drain current of transistor VT1, capacitor C5 is blocking.

The transmitter operating mode is determined by the control program of the microcontroller and is set by setting jumpers ("jumpers") between adjacent pins of the "even" and "odd" rows of contacts of the XP1 plug.

The callsign is determined by the first jumper. If she closes contacts 39 - 40, the call sign will be - MOE, if 37 - 38 - MOI, 35 - 36 - MOS, 33 - 34 - MON, 31 - 32 - M05. If there is no jumper in any of the listed positions, the call sign will be MO. When multiple jumpers are installed, the "highest" jumper is active, corresponding to the callsign ending with the most dots. The transmission speed is approximately 50 characters per minute.

Jumpers in positions from 29 - 30 to 21 - 22 are used to set the duration of the "fox" operation cycle (from 1 to 5 minutes) and the number of the active minute of this cycle. For example, if jumpers 21-22 and 27-28 are present, the callsign will be transmitted in the second minute of a five-minute cycle. If there is only one jumper in this group, the last minute of the cycle will be active. The absence of jumpers will result in a continuous transmission of the callsign without pauses.

The following jumpers (in positions from 19 - 20 to 9 - 10) set the duration of the pause in minutes between turning on the transmitter power and starting the on-air cycle. The values corresponding to each of them are added. Thus, if all six are set, the delay will be 63 minutes, if none, operation will begin immediately after power is turned on.

It should be noted that at the specified time, the transmitter for which the first minute of the cycle is active will go on the air. In another minute, the second transmission will begin, and so on.

The transmitter emits an unmodulated signal, however, by setting the jumper to position 7-8, you can make it tone. Modulation frequency - 1000 Hz. If a jumper is also placed in position 5 - 6, the intervals between the dots and dashes of the Morse code will be filled with an unmodulated carrier. Otherwise, the signal is turned off in these intervals.

Setting the jumper 3 - 4 disables the software formation of parcels, turning the transmitter into a conventional telegraph one. The manipulator (telegraph key) is connected to pins 1 and 2 of the XP1 plug or to pin 2 and a common wire.

All necessary jumpers must be in place when the transmitter is powered on. Their installation or removal during the operation of the last modes will not change. The microcontroller DD1 remembers the position of the jumpers in its internal non-volatile memory. Therefore, if you turn off the power, remove every single jumper and turn on the transmitter again, the modes that were in effect before turning off will be automatically restored. With one exception, the transmission will start immediately without a start pause.

The question arises, how to set a mode that does not require a single jumper (call sign - MO, transmission - continuous, without start delay)? To do this, it is enough to install jumper 1-2, which, in the absence of a jumper, serves as a sign for the program to analyze the state of the switching field, and not read the modes from memory.

LED HL1 and piezo BQ1 are used to control the operation of the transmitter. Immediately after switching on, they report every minute in Morse code the number of minutes remaining before the start of work on the air. There are no signals at the transmitter output at this time. During transmission, all emitted signals are duplicated by sound and LED lighting.

If there is no need for sound and light signaling, HL1 and BQ1 can be excluded or switches can be provided in their circuits.

The transmitter is assembled on a single-sided printed circuit board, shown in fig. 2. If it is possible to make a double-sided board, the jumper wires provided on it can also be printed.

Sports DF transmitter

Above the diodes VD1 - VD20, next to the XP1 plug, a label is placed (Fig. 3) with inscriptions explaining the purpose of the jumpers.

Sports DF transmitter

There are no special requirements for details. Resistors - C2-23, capacitors - ceramic KM, KD, KT, K10, oxide C3 - K50-35. Trimmer capacitor C6 - small ceramic imported. Transformer T1 is made of a unified choke DM-0,4 25 μH. Its winding is used as a secondary winding, and the primary winding - 10 turns of enameled wire with a diameter of 0,15 mm - is wound over it in bulk.

The DD2 chip can be replaced with an imported 74HC00. It is not recommended to install functional analogs from TTL or slower CMOS series instead. Microcontroller DD1 before installation on the board is programmed in accordance with the table.

(click to enlarge)

Pin plug type PLD-40 (2x20 pins) with 2,54 mm pitch.

The digital part of the transmitter usually does not require adjustment. If, when the device is turned on, there are no light and sound signals, which were described above, you should check the operability of the microcontroller clock generator. This is best done by connecting an oscilloscope to pin 11 of the DD2 chip. Stable generation is achieved by selecting the capacitances of capacitors C1 and C3. The output circuit of the transmitter is tuned with a tuning capacitor C6, always with the antenna and ground connected. The author used a whip antenna 1,5 m long. For the transmitter to work with other antennas, it may be necessary to select the capacitors C7 and C8 again, change the number of turns of the primary winding of the transformer T1.

The adjustment is carried out according to the maximum field strength created by the transmitter. The simplest field indicator in laboratory conditions can be an oscilloscope with an "antenna" connected to the input - a piece of any wire. To avoid low-frequency interference, the input can be shunted with a choke with an inductance of 50 or more microhenries. In the field, the indicator can be assembled from a wire loop with a diameter of 300 ... 500 mm, a germanium diode connected in series with it and a microammeter. It is useful to shunt the latter with a capacitor with a capacity of at least 1000 pF.

Author: A. Dolgiy, Moscow

See other articles Section Civil radio communications.

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