Antenna tuner. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / HF antennas

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With all the variety of circuit solutions, matching devices perform the same task - they transform the complex input impedance of the antenna into low-resistance (50 ... 75 Ohm) and compensate for the reactive component in a wide frequency range. In English literature, such devices are called ATU (short for antenna tuner unit). The same abbreviation is sometimes used for CW QSOs. Coordination is controlled by an SWR meter installed between the transmitter and the antenna tuner according to the minimum standing wave.

The SWR meter can be made according to any scheme, for example, see [1]. It also says how to properly calibrate the SWR meter. Calibration must be performed using an equivalent value equal to the output impedance of the TX output stage.

The most common antenna tuner circuit is a T-shaped circuit (Fig. 1), which has good broadband and excludes adjustment when the frequency changes within the same range. Such a scheme is distinguished by broadband and harmonic suppression of 10 ... 15 dB, depending on the type of antenna and operating frequency.

Figure 3 shows the most narrowband circuit, but harmonic suppression reaches 28 dB. Such a tuner is able to work with antennas of various resistances - from a dozen ohms to tens of kOhms. Leading device manufacturers, such as MFJ and others, stick to T-patterns.

There is another circuit solution (Fig. 2). This is a relatively inconvenient scheme that requires frequent tuning when moving around the band, however, a narrow bandwidth can be useful both in terms of suppressing TX harmonics, and in reception for tuning out from QRM.

In all cases, a variable inductance from the old RSB-5 transmitter was used, however, a similar one with an inductance of 26 ... 28 μH can be used to operate in the range of 1,8 ... .30 MHz. Capacitors of variable capacitance should be with a gap of 2 ... 3,5 mm, depending on the power of the transmitter. When it comes to the capacitors used, it is the maximum, not the minimum, capacitance value that matters. In a T-shaped circuit, the capacitance of each capacitor can be at least 180 pF, but it is better to take capacitors of 240 ... 250 pF. In the case of using long-wire antennas, just such capacitance values ​​will be required. If there is no necessary variable capacitance, you can add a constant capacitor in parallel with the variable using an RF relay.

Any of the matching device circuits transforms the complex resistance of any piece of wire to 50 ... 75 Ohms, however, the efficiency of such a piece of wire as an antenna will be low with a high transmitter efficiency, since it is loaded with the necessary load. Therefore, if we already use a wire of arbitrary length as an antenna, it is desirable that it has the maximum possible length.

For any matching device, good grounding is necessary, the efficiency of the tuner depends on it. To work with symmetrical transmission lines, a balun transformer 1: 4, made on a ferrite ring, is used at the output of the matching device, or such matching device circuits are used that exclude the use of ferromagnetic materials. The latter are more preferable and stable in terms of TVI. A low-pass filter, which can be used to suppress transmitter harmonics, must be installed between the SWR meter and the input of the matching device.

Literature

1. Shortwave Handbook, 1984, "SWR-meter", S. Bunin, L. Yaylenko.
2. "RL" N 1,1991 "Matching device", UC2AGL.
3. "RL" N 3,1991 "Wattmeter instead of thermocouple", UC2AGL.
4. "RL" N 12, 1991 "How to improve the efficiency of the radio station", UC2AGL.

Author: I. Podgorny (UC2AGL); Publication: N. Bolshakov, rf.atnn.ru

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