Antenna current indicator. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Antennas. Measurements, adjustment, coordination

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The proposed device will help when tuning the antennas of both stationary and portable radio stations in cases where this cannot be done with a familiar SWR meter.

This current indicator can be used on both HF amateur bands and CBS. The authors used it to debug the antennas of portable CB radio stations. The reliability and range of radio communications, including those in the CB band (27 MHz), depend on the efficiency of the antennas. It is no secret that the standard antennas of most portable radio stations have low efficiency, which limits the communication range, so the desire of radio amateurs to use antennas with improved characteristics is understandable.

True, there are more efficient antennas on sale, but they are not always suitable in one way or another. You have to make them yourself. And then there is the issue of configuration. SWR meters [1], designed to tune stationary antennas powered by cable, are usually unacceptable here, since there is no cable between the transmitter and the antenna in a portable radio. Connecting a whip or helical antenna through a cable to tune it will cause the antenna to be inconsistent and the coaxial cable braid will work as a counterweight. You need to configure the entire system - the antenna together with the body of the radio station.

Best results can be achieved using a field strength indicator. However, even here there are difficulties. For example, the design variant of the antenna current indicator described in [2] is of little use for tuning small-sized antennas of portable radio stations.

We offer a simple portable antenna current indicator (Fig. 1). It contains a current transformer T1, a rectifier on a diode VD1, a low-pass filter (capacitor C1), a sensitivity regulator (resistor R1), a microammeter RA1 and a protective diode VD2.

The magnetic circuit of the transformer T1 is a ferrite ring, which is put directly on the antenna of a portable radio station in its lowest part.

When the transmit (TX) key is pressed, the current flowing in the antenna induces an RF voltage in the transformer T1, which is rectified and fed to the PA1 microammeter. In this case, the greater the current, the more the arrow will deviate. The sensitivity of the indicator is set by the resistor R1. The antenna is tuned to the maximum current [3]. This is done either by changing the parameters of the antenna, for example, the length, inductance of the compensating coil, or by adjusting the matching device [2].

The sensitivity of the indicator is quite high, it works with radio stations with an output power of 100 mW or more.

The design of the device is shown in fig. 2. The ring on which the transformer is wound is chosen large enough (K32x16x8), and a hole of the corresponding diameter is made in the board. This allows you to put the transformer on antennas with a bayonet-type RF plug, for example, СР-50-74ФВ. The transformer and the microammeter are glued to the board, which can be used as non-foiled fiberglass or plexiglass. Resistor R1 is installed on a plexiglass corner or in a hole in the board. Mounting is carried out by a hinged method.

For the manufacture of transformer T1, it is recommended to use a magnetic core made of 50VCh ferrite with an outer diameter of 32 mm or more. Its winding contains 8 ... 12 turns of MGTF wire 0,2 mm 2. Diode VD1 - KD522B or similar, resistor R1 - SPO, SP4, capacitor C1 - KM, K10-17, microammeter PA1 - from household tape recorders with a total deviation current of 100 ... 200 μA.

Setting up the indicator comes down to selecting the VD2 diode in such a way that it protects the PA1 microammeter from overload and does not affect its readings. To do this, you need to measure the voltage on the microammeter with the full deflection of the needle. If it does not exceed 0,2 V, then a germanium detector diode or a diode with a Schottky barrier will do, and at a voltage of 0,2 to 0,4 V, a silicon low-power diode will do.

It should be noted that the indicator can be used to quickly check the health of the radio station's transmitter and evaluate its output power.

Literature

1. Efremov V. Universal SWR meter. - Radio amateur, 1994, No. 1, p. 58.
2. Vinogradov Yu. On the coordination of small antennas. - Radio, 1996, No. 4, p. 9.
3. Rothammel K. Antennas. - M.: Energy, 1979, p. 298.

Authors: Igor Nechaev (UA3WIA), Igor Berezutsky (RA3WNK)

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