Radio emission indicator in the 27 MHz band. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Field strength detectors

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The proposed device allows you to fix the operation of the transmitter in the range of 27 MHz within a radius of several hundred meters. This can be useful for monitoring the air in a security system, when radio conversations that have arisen near the protected object should be perceived as an alarm signal.

The sensor is a wideband receiver operating in the 27 MHz band. The 0,7 bandwidth is 3 MHz. With a supply voltage of 6 V, the current consumed in standby mode is -1,1 mA, in alarm mode - 3,2 mA. Any full-size antenna can be connected to the device, which is powered by a cable with a characteristic impedance of 50 ohms. The use of shortened antennas will lead to a decrease in "range". Experience has shown that a sensor equipped with a full-sized antenna ("half wave") detects the operation of a four-watt "portable" at a distance of 200m.

The scheme of the device is shown in fig. 1. The L2C2 circuit is tuned to the middle of the controlled frequency band. The radio frequency amplifier is made on transistors VT1 and VT2. The amplified signal is taken from the inductor L3 and fed to the detector - diode VD1. On transistors VT3 and VT4, a DC amplifier (DCA) is assembled, which forms a low level on the resistor R10 if the air is "clean", and high - if a working transmitter appears in the controlled area. An alarm signal generator is assembled on the DD1 chip.

The sensitivity of the radio receiver depends on the mode of operation of its detector and UPT, i.e., on the extent to which the cutoffs of the diode VD1 and transistor VT3 are compensated. The desired shift of the operating point VT3 is achieved by the divider R5R6, and the input of the VD1 diode into the microcurrent mode by the voltage drop across the resistor R4.

The device is assembled on a printed circuit board (Fig. 2) from double-sided foil fiberglass with a thickness of 1,5 mm. The foil on the side of the parts is used only as a common wire and screen. In places where conductors pass, circles with a diameter of 1,5 ... 2 mm are etched in it, and connections to it are shown by blackened squares. The squares with a bright dot in the center show the wire jumpers and the pins of the microcircuit and the tuning resistor "grounded" by soldering to the foil. The cases of transistors VT1 and VT2 must also be connected to a common wire.

Coils L1L2 are made on a frame with an MZ thread for a carbonyl trimmer (Fig. 3). Coil L2 contains 13 turns, it is wound in a series of wires PEV-2 0,41. Coil L1, containing three turns, is wound over the "cold" end of coil L2 with PEVSHS wire with a diameter of 0,15 to 0,25 mm. Although soldering "pull-in" of the L2 leads mechanically secures the frame itself, it is recommended to fix its position with glue introduced under the frame supports.

Resistor R2 - SPZ-38a, the rest - MLT-0,125. Capacitors: C2 - KG or KD, the rest - KM-6 or the like. Throttle L3 - D1 -0,1.

The adjustment begins with setting the DC mode. The antenna is turned off, the tuning resistor R2 is set to maximum resistance and, by moving the engine, they find its position at which the transistor VT4 starts to open. It is necessary to set the position of the engine so that the transistor VT4, while still closed, is near the opening threshold. In this mode, the voltage on its collector will be close to zero. When an RF signal with a level exceeding the threshold appears at the input of the receiver, transistors VT3 and VT4 open and the voltage at the VT4 collector becomes close to the supply voltage.

To tune the L2C2 loop to the middle of the controlled frequency range, you can use any suitable RF generator, for example, a CB radio station operating on a 50-ohm antenna equivalent (so as not to overload the receiver with too strong a signal). The resonance is set by a voltmeter or oscilloscope connected to the collector of the transistor VT3. As the natural frequency of the L2C2 circuit approaches the frequency of the generator, the constant voltage at the collector VT3 decreases, reaching a minimum when they coincide.

Author: Yu.Vinogradov, Moscow; Publication: cxem.net

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