Sensitive electromagnetic field indicator. Scheme, description

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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
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Sensitive electromagnetic field indicator. 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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A sensitive field indicator can be useful not only on an amateur radio station. The scope of its application is much wider. This, as well as the successful design of the field indicator, is described in the proposed article.

The problem of detecting and detecting weak electromagnetic fields can be successfully solved by using inverted diodes in the detector. The current-voltage characteristic (CVC) of a reversed diode (OD) is shown in fig. 1. As you can see, its reverse branch starts from zero. This form of the reverse branch is determined by the fact that the OD at a zero bias p-n junction is in a state of electrical breakdown, or, in other words, the reverse breakdown voltage of such a diode is zero. This property of the p-n transition is achieved by a high concentration of dopants in the original semiconductor material.

Sensitive electromagnetic field indicator

The direct branch of the CVC OD corresponds to the characteristics of a conventional diode and for various semiconductors begins with a certain threshold voltage, about 0,3 V for germanium and 0,6 V for gallium arsenide. Thus, by "turning" the diode "upside down", we get an ideal detector with a CVC starting from zero and with a "reverse voltage" - fractions of a volt. On fig. Figure 2 shows a 180° I–V OD with near-perfect characteristics for detecting small signals - the best available on today's hardware.

Sensitive electromagnetic field indicator

However, taking a closer look at the CVC of the inverted diode, one can also notice a pronounced third-order nonlinearity. This makes it possible to successfully apply OD in mixers of highly sensitive radio receivers and converters. I'm not sure about the purity of the experiment, but during the years of my active work on the 432 MHz band (the end of the 70s, the collective VHF radio station of Moscow State University UK3ACF), a simple replacement of the diode in the converter mixer with a reverse one increased the volume of received radio stations by 2 ... 3 points. At the same time, the ether subjectively seemed "cleaner", since overloads from powerful stations almost completely disappeared.

Schematic diagram. So, we are building an electromagnetic field indicator with a detector on inverted diodes. The indicator circuit shown in fig. 3 corresponds to the circuit of a conventional detector receiver without an oscillating circuit.

Sensitive electromagnetic field indicator

The polarity of the head is indicated correctly - the diodes are reversed, the detection occurs on the reverse branch of the CVC. The sensitivity of the device is determined exclusively by the available pointer microammeter. When using a device with full deflection of the arrow at a current of 50 μA, the indicator detects cellular network transmitters from a distance of several hundred meters. Broadcasting transmitters of VHF and FM ranges provide deviation of the indicator pointer by 30...70% of the scale at a range of 1,5...2 km. In practice, moving around Moscow with this device, it is rarely possible to find a place where the arrow does not deviate. Particularly interesting effects can be observed in apartments on high floors. Sometimes in a completely unpredictable place in the room, the device shows an almost complete deviation of the arrow. A detailed study and inspection of nearby buildings, visible from the window, reveals the presence of transmitting antenna systems within the line of sight.

A slight refinement of the device with the introduction of an oscillatory circuit (Fig. 4) will further increase the sensitivity of the device and allow observing the spatial patterns of the fields of certain radio transmitters or broadcast radio stations. Now we get a resonant, i.e. selective indicator of the electromagnetic field. For reliable identification of one or another radio transmitter, it is advisable to supplement the device with high-resistance telephones (R > 2 kOhm) connected by alternating current in parallel to the pointer device. The parameters of the coil L1 and capacitor C1 are selected based on the desired frequency range. The tap to the antenna is made from 1/5, and to the detector from 1/3 of the total number of turns of the coil, counting from the lower output according to the output circuit (connected to a common wire).

Sensitive electromagnetic field indicator

The positive qualities of the device can also be attributed to the fact that when it is placed in fields with high intensity, the detected voltage never exceeds a fraction of a volt due to the specifics of the I–V characteristics of the inverted diode. This quality protects the microammeter from failure.

Details. Inverted diodes of the GI401 or AI402 series with any letter indices can be used in the indicator. Capacitors C1 (see Fig. 3), C2 and C3 (Fig. 4) can be of type K10-17-1 in or any other ceramic leadless for surface mounting, C4 (Fig. 4) - KM-6 or K10-28 , K10-47. Variable capacitor C1 (Fig. 4) - type KPV or any other, with an air dielectric.

Coil L1 for the indicator to work in the range of meter waves must be wound with a thick (0 > 1 mm), preferably silver-plated wire on a ceramic ribbed frame. In the decimeter range, the L1 coil can be frameless, made with silver-plated wire 0 > 2 mm. Just 1 ... 3 turns are enough. At higher frequencies, it is advisable to use stripline resonators.

The M4204 microammeter with a total needle deflection current of 50 μA and a loop resistance of 1600 Ohm is used as a pointer indicator in the device. These parameters are not critical, so any microammeter is suitable, preferably with a full deflection current of the arrow of no more than 100 μA.

Design. The device antenna (Fig. 5) is a broadband symmetrical vibrator with linear polarization.

Sensitive electromagnetic field indicator

It is made of one-sided foil fiberglass. All the elements of the indicator are located on it (Fig. 6) and a microammeter is attached to it with the help of its screw terminals. Using such an antenna, the polarization of the observed electromagnetic field is easily determined, and even, with some skill, the change in polarization when radio waves are reflected from the walls of reinforced concrete buildings and large metal objects.

Sensitive electromagnetic field indicator

This device can be very useful in radio circles and in educational institutions for a visual illustration of the spatial interference pattern of the total electromagnetic field that surrounds us every day.

When using an antenna with a more pronounced directivity (multi-element frames, wave channels, log-periodic antennas), the device can be used for the "indoor" game "Fox Hunting" in radio clubs. However, in this capacity it can be useful, for example, to search for hidden radio transmitters.

This indicator is not designed for operation in the short wave band (f < 30 MHz), which is due to the low efficiency of a strongly shortened vibrator - antenna.

The described device has been working properly for me since 1985. It has never failed and has not been repaired. At one time, being a developer of radio transmitters, I always kept this device on a working stand, ensuring control over the quality of shielding and the absence of radiation not only at my workplace, but also in the immediate environment. And I must say that he coped with this task better than the official radio control services. Thank him very much!

Author: S.Komarov (UA3ALW), Moscow

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