GIR for tuning wire antennas. Scheme, description

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GIR for tuning wire antennas. Encyclopedia of radio electronics and electrical engineering

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

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Simple to manufacture and operate, heterodyne resonance indicators are widely used by radio amateurs. They are used, in particular, when tuning antennas. However, the classical versions of GIR are focused on inductive coupling with the measured oscillatory circuit. Their small inductors in most cases do not allow sufficient coupling with antenna elements, for example, with a wire loop. As a result, the indication of the resonant frequency of the element becomes fuzzy, which leads to significant measurement errors.

The English shortwave Peter Dodd (G3LDO) solved this problem simply by making a simple specialized GIR to adjust the elements of his "double square". It differs from the classic versions of this device only in its design (Peter Dodd. Antennas. - RadCom, 2008, March, p. 66,67).

The circuit design of a heterodyne resonance indicator can be anything - a great many of them have been published in amateur radio literature. Peter Dodd used one of the simplest GIRs. Its scheme is shown in fig. 1.

GIR for tuning wire antennas
Fig. 1

Resonance is indicated in it by a change in the source current of the transistor VT1, and in order to make these changes more pronounced, a bias voltage is applied to the PA1 measuring device. It can be adjusted with a variable resistor R4, setting the instrument needle close to the end mark of its scale before starting measurements. The resonance frequency is recorded by a digital frequency meter. Of the domestic transistors in this GIR, for example, KP303V transistors can be used. The frequency meter is connected to connector XW1.

The constructive difference from the traditional versions of the GIR is that the author used a large coil, which made it possible to provide a noticeable connection with the antenna element, the resonant frequency of which must be measured (with a frame or a linear vibrator). The appearance of his device is shown in Fig. 2. Its base is a dielectric plate 150 mm wide and 15 mm thick. Its length is not critical - it depends on the size of the box in which the GIR elements are placed, and on the size of the frequency meter. The author used a factory-made frequency meter.

GIR for tuning wire antennas

At the top of this plate is wound a coil that contains five turns of wire 1 mm in diameter in insulation. Its inductance turned out to be about 3 μH, which ensured the overlap of the GIR with the KPI used from 12 to 22 MHz. By changing the number of turns, one can also obtain another frequency overlap required for tuning a particular antenna.

In the upper part of the plate there are two dielectric hooks (of those used for fixing electrical wiring), with which the device is suspended on the wire element of the antenna. This allows you to fix the relative position of the GIR coil and this element, which also increases the measurement accuracy. The part of the wire element of the antenna will be parallel to the long side of the rectangular turns of the coil. This, as the test showed, ensures a sufficiently strong connection of the GIR coil with the antenna element and reliable registration of its resonant frequency. So, when working with the "double square" frames, the change in the readings of the measuring device at resonance was approximately 40% of the entire scale.

Author: Peter Dodd

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