On the matching of a half-wave antenna. Encyclopedia of radio electronics and electrical engineering

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

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The half-wave vertical antenna, distinguished by its simplicity (conductor length -λ / 2) and efficiency, has become widespread in CBS. But excited from the end - in the antinode of the voltage, it has a high and difficult to predict impedance Za, which depends, in particular, on its design features.

It must have a matching (transforming) device that lowers the high active resistance Ra (Za turns into it after tuning the antenna) to the wave impedance of the feeder 50 Ohm, usually used in communication technology. Coordination can be carried out by a transformer, autotransformer, P-loop, any other converter that has a voltage transformation ratio k - √50/Ra and, of course, not introducing any significant losses.

Often, "half-waves" are matched with a 50-ohm feeder, a circuit tuned to operating frequencies, the inductance of which is an air coil, a feeder is connected to part of the turns, and a half-wave vibrator is connected to the entire circuit. There is usually no explicit capacitance in this circuit. Its function is performed by the coil's own capacitance, which is sometimes increased by a metal "flag" connected to the base of the vibrator. The tuning of the antenna circuit will, of course, be affected by the reactive component of the vibrator itself, if its length is l ≠ λ/2.

However, the calculations possible here do not pretend to be accurate, and the fine-tuning of a self-made "half-wave" in a rather laborious experiment is inevitable. Having climbed onto the roof, the antenna is "knocked down", either the point of connection of the feeder to the coil is changed (often shifting it only by a fraction of a turn), or the length of the vibrator (step no more than 1-2 cm), or slightly shifted-pushed coil turns. Having done one, the other or the third - separately, in pairs or all at once - and returning the antenna to its working position, the resulting SWR is measured. And so - many times, until an SWR < 1,5-2 is obtained in the desired frequency range. Or ... a "branded antenna" will be bought, remarkable only for the fact that almost all this work has already been done in it by someone.

When they say that at SWR = 1,5, the radiation power decreases by 4%, and at SWR = 2 - by 11% (both are still considered tolerable), then the reason for this is not the loss in the communication line, but the inconsistency of the transmitter with load. If we consider it a generator with an open-circuit voltage U and an internal resistance Rin = 50 Ohm *, then the dependence of the power dissipated on the load Рн on the load resistance Rн has the form:

Рн = U²RH/(50+Rn)².

*) But if this is a generator with a very small Rin, the mode of which is simply designed for a 50-ohm load, then the losses with the same mismatches will increase sharply, even with SWR \u7,5d 25 they can be 30-150%. In addition, the response of the generator to a mismatch with the load will no longer be "symmetrical". So, if at Rn \u3d 16,7 Ohm (SWR XNUMX) the radiation power only decreases sharply, then at Rn \uXNUMXd XNUMX Ohm (same SWR), output transistors can be burned.

But with SWR > 1, losses occur in the feeder itself. With a small linear attenuation of the feeder or its small total length, these losses in it can be neglected for almost any SWR. In this case, the half-wave matching device "may be as shown in Fig. 1.

As an external matcher, only in the first approximation of the leading Ra of the antenna to the wave impedance of the feeder, an autotransformer T1 is used, made on a ferrite ring magnetic core 400НН 22x12x5 mm.

Winding n2 - 20 turns of MSHP-0,07 wire (single-core mounting section 0 07 mm²) and on top of it - the winding n1 connected in phase with it - 5 turns of MSHP-0,2 wire (Fig. 2). Since the leakage inductance in such a transformer is small, then its transformation ratio k = n1 / (n1 + n2) = 0,2.

By loading the output winding with the number of turns (n1 + n2) of the autotransformer with a resistor RH = 1250 Ohm, and connecting the input winding (n1) through an SWR meter to the antenna output of a 4-watt CBS radio station (note that Rн = 1250 Ohm, converted to the antenna the output of the radio station, turns into the optimal for it Rn = 1250 k2 = 50 Ohm), it was possible to evaluate its operation at frequencies of the 11-meter range. The power delivered to the resistor was 3,4 ... 3,5 W, and the SWR did not rise above 1,2.

This autotransformer would already be a good enough matcher for a tuned "half wave" having Ra = 1250 ohms. But even if the active resistance of the antenna is different, it will bring this resistance to a value that is no longer so sharply different from the wave resistance of the feeder and thereby reduce the losses in it to negligible.

The adjustable stage of the matching device located at the other end of the feeder is an L1C1 circuit, to the coil of which a radio station and a feeder are connected by an autotransformer.

Coil L1 is wound with PEV-2-1,4 wire on a frame with a diameter of 6,5 mm. Number of turns - 12, winding length - 22 mm. Withdrawal to the station - from the 7th (counting from the "ground") turn; taps to the feeder - from the 4th, 5th, 6th, 7th, 8th, 9th, 10th or 11th turn, which lead to the SA1 type PG2 switch for 12 positions, installed near the coil (its connections to the coil make extremely short conductors).

Capacitor C1 - type KSO-1. Perhaps its capacity needs to be clarified.

The L1C1 circuit is adjusted with an M4x10 mm carbonyl core, enclosed in a thick plastic tube, which must enter the coil with some resistance. The circuit can also be adjusted with a variable capacitor with a capacity of 30 ... 40 pF, connected in parallel with capacitor C1.

Since, after only transformer matching of the vibrator with the feeder, the traveling wave mode will most likely not occur in it, it is recommended to use a feeder not of arbitrary length, but of a multiple of λ/2 (in a coaxial cable with solid polyethylene insulation, this is 0.66λ/2). A feeder of this length (it is called a high-frequency repeater) will only transmit unchanged Zа' - the antenna impedance converted into T1 - to an adjustable matcher, where the reactive component of this complex resistance will be compensated by the detuning of the L1C1 circuit, and the active component by switching taps to L1 is transformed into a resistance close to to 50 ohms.

The device described was used to match a wire "half wave" having a capacitive "expander" at the other end of the vibrator. The antenna was tuned and matched only in the C frequency grid. However, at other CB frequencies, the SWR turned out to be no higher than 1,2. Apparently, this is due to the more broadband resistance conversion in T1 than is the case in conventional overhead transformers. After all, a "half-wave" in itself, especially with a capacitive "expander", should not be narrower than any other half-wave vibrator.

Author: Yu.Vinogradov, Moscow

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