Field strength indicator. Scheme, description

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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
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Field strength indicator. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / Field strength detectors

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A field strength indicator may be required when setting up a radio station or transmitter, if you need to determine the level of radio smog and find its source, or when searching for and detecting hidden transmitters ("spy radio microphones").

Field strength indicator. Schematic diagram of the field strength indicator

The peculiarity of the indicator is in the way of displaying the level of tension - on a five-level LED scale. The indicator can monitor field strengths with a frequency of up to 1000 MHz. The frequency response of the indicator was not measured, since its function is not to measure the level of the HF field in absolute terms, but to demonstrate its level and the change in this level in conventional units. However, if you have the necessary equipment, you can make the appropriate tables. In any case, it confidently responds both to the signal of a CB radio station operating in the 27 MHz band, and to a cell phone signal operating at much higher frequencies. The RF signal is induced in the W1 antenna and fed to the amplifying stage on VT1. The relatively low-frequency transistor KT3102 works here, perhaps using a transistor of the type KT368, KT381, you can improve the operation of the indicator at high frequencies. A detector on germanium diodes VD1 and VD2 is switched on at the output of the amplifying stage. A constant voltage is allocated on the capacitor C3, the value of which is proportional to the strength of the RF field. This voltage is measured by a dial indicator on the VA6137 polycomparator IC, designed to work in level indicators. The field strength level is evaluated on a linear scale of five LEDs HL1-HL5.

The indicator is powered by a source of two galvanic cells connected in series. The role of the case is played by a plastic case for a toothbrush. It contains two batteries (one after the other) and indicator parts. LEDs are glued into the drilled holes, forming a linear scale. The outputs of the LEDs also serve as reference points for mounting the A1 chip. The role of the antenna is played by a folding telescopic antenna (with a swivel joint) of the radio or radio. The hinge is attached to the side of the body so that in the folded position the antenna is parallel to the body. To work, it is turned 180 ° (or another angle) and pulled to the desired length. The sensitivity can be adjusted by changing the length of the antenna.

When setting up the transmitter, the indicator is placed at a certain distance from its antenna, the value of which depends on the power, and the change in its radiation power is evaluated with an LED scale. If necessary, the indicator is removed or brought closer to the transmitter antenna. It is advisable to use the indicator when establishing transmitters with a power of not more than 0.5W, otherwise it turns out to be too sensitive even with the antenna folded and it has to be taken far away. In the event that it is necessary to indicate a significant radiation power, a switch can be provided that turns off the power from the UHF on the transistor VT1. Instead of an antenna, you can connect a bulk coil with a diameter of about 100 mm from 3-4 turns of thick winding wire. One end of the coil is connected instead of W1, and the other end is connected to the common minus power supply. The option with interchangeable tunable contours, for different frequency sections (you will get a wavemeter) is not excluded.

Based on materials from Radioconstructor magazine.

Publication: cxem.net

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