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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To establish the antenna-feeder paths of amateur radio stations, an indicator of the intensity of the high-frequency electric field is required. The device proposed in this article differs from those commonly used in high sensitivity and a wide band of operating frequencies.

Traditionally, a field strength indicator is an antenna (most often in the form of a short pin), an amplitude detector (RF voltage rectifier) and a pointer meter (usually a microammeter). To increase the sensitivity, the indicator is made active by supplying it with an RF or DC amplifier.

(click to enlarge)

The proposed indicator (Fig. 1) does not have a conventional amplitude detector, since its functions are performed by the K174PS4 microcircuit, a signal multiplier widely used by radio amateurs in radio receiver mixers, converters, etc.

How does the microcircuit work in our case? The input signal (usually sinusoidal or close to it), received by the WA1 antenna, is fed to two inputs of the microcircuit - pins 8 and 11 (the other two - pins 7, 13 - are connected by alternating current to a common wire), and it multiplies the signal "by himself". If U_vh=usinwt, then at the output of the microcircuit there will be a signal Uout = KU²without²wt, where U is the amplitude of the input signal, w - its circular frequency, K - the transmission coefficient of the microcircuit. This expression can be transformed; U_O=KU²without²wt = KU²/2 - (KU²cos2wt)/2.

Thus, the output signal of the microcircuit contains a constant component and an alternating component of twice the frequency. The constant component is proportional to the square of the input voltage, so the readings of the PA1 microammeter connected to the output of the microcircuit will be proportional to the signal power emitted by the antenna. The variable component is easy to suppress by installing a capacitor C7 of sufficient capacity. Diodes VD1, VD2 are used to protect the input circuits of the microcircuit from powerful signals.

The device is powered by a 9 V battery ("Krona", "Korund", "Nika") and consumes a current of approximately 1,5 mA. Operation is maintained when the supply voltage is reduced to 6 V. The maximum current through the RA1 microammeter is limited by resistors R1, R2.

Field strength indicator

Almost any small-sized pointer indicator with a full deflection current of the pointer from 50 to 150 μA can be used in the device. At a frequency of 28 MHz, the sensitivity of the device (the minimum recorded signal) was 2 ... 3 mV, and the dependence of the readings on the input voltage had a quadratic character (Fig. .2). Thanks to the atom, the device is more sensitive to changes in the field strength, which allows you to more accurately tune the antenna-feeder paths. So, for example, when the voltage at the input of the device changes by 1,4 times (3 dB), the indicator readings double.

Instead of the K174PS4 indicated on the diagram, it is permissible to use the K174PS1, K174PS2 microcircuits. In addition to the KD510A diodes, KD522B, KD503B are suitable, Capacitors - KLS, KD, K10-17, KM, resistors - MLT, S2-33, Switch - any small-sized, preferably sliding in two positions.

Field strength indicator

Most of the parts are placed on a printed circuit board (Fig. 3) made of one-sided foil fiberglass. The board must be placed closer to the antenna inside a metal case of suitable dimensions. Next to the board, you can strengthen the power supply. A microammeter and a switch are installed on the front wall of the case. Antenna - telescopic from small-sized transistor receivers. It must be fully retractable into the body. By changing the length of the extended part of the antenna, it is possible, within certain limits, to adjust the sensitivity of the device to the strength of the electromagnetic field.

It does not require setting up the device, but if another microcircuit is used, then you will have to select resistors (they must be of the same rating) so that the microcircuit terminals have a voltage approximately equal to half the voltage of the power source. If necessary, balancing the device (zero readings of the PA1 microammeter in the absence of a signal at the input of the device) can be done by selecting resistor R1 or resistor R2.

Compared to a passive indicator, this device has a much higher sensitivity, which allows you to tune antennas at a lower power level, as well as detect the location of eavesdropping devices - "bugs". The frequency response of the indicator is determined by the parameters of the applied microcircuit. In the author's version, its sensitivity at a frequency of 145 MHz decreased by a factor of three.

If desired, the indicator can be made selective by installing a tunable LC circuit at its input.

Author: Igor Nechaev (UA3WIA); Publication: cxem.net

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