Steering machine in the loop antenna. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Antennas. Theory

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In the manufacture of a transmitting loop antenna, which has received the name "magnetic loop" (magnetic loop) in the literature, the radio amateur will have to solve several problems due to its high quality factor. Such an antenna requires tuning within the amateur range, i.e., a remotely tunable variable capacitor with appropriate control circuits and a node that displays the position of its rotor (the operating frequency of the antenna) in the radio station room is required. Note that the KPI for such an antenna is a deficit, since it must have a high operating voltage, and the rotor current collector must be lossless or absent at all (for example, butterfly-type capacitors).

A simple solution to all these problems was proposed by the Belgian radio amateur ON7NU (Daniel Elias, "Servo-CV pour antenne loop magnetique". - CQ-QSO, 2005, No. 9/10, p. 22-25). In his antenna for a range of 40 meters, to tune the antenna and indicate its operating frequency, he used a ready-made servo drive - an analog steering machine that is used in radio-controlled models.

Such steering machines contain a DC motor, a gearbox and a control unit. The control channel uses pulse-width modulation. They are connected to the control channel (output of the receiver decoder, etc.) with three wires. Two wires supply voltage, and the third - pulses, which ultimately determine the position of the output shaft of the steering machine. The repetition period of the control pulses should be approximately 20 ms, and their duration should be within 0,5 ... 2 ms. It is determined by the model control, which is mechanically connected to a variable resistor that sets this parameter of the control oscillator pulses.

The servo control unit also has a generator, the output pulse duration of which is determined by a variable resistor, which is mechanically connected to the servo output shaft. These pulses are compared with those coming through the control circuit. As a result, a motor control signal is generated - it starts to rotate and turns the output shaft of the steering machine and the variable resistor associated with it until the duration of the pulses of the built-in generator is equal to the duration of the pulses coming through the control circuit.

The maximum angle of rotation of the steering machine shaft in direct form is usually not standardized in the technical specifications - only the time of rotation through an angle of 45 or 60 degrees is indicated. Despite their small dimensions, steering machines develop a noticeable torque (several kGcm). On fig. Figure 1 shows the appearance of a typical steering servo of the "standard" class - model HS311 from Hitec. It measures 41x20x37 mm and weighs 44,5 g. The supply voltage is 4,8 or 6 V. The turn time for an angle of 60 degrees is 0,19 s. Torque -3,7 kgcm.

Fig.1. The appearance of the steering machine class standard model HS311 from Hitec

The actual angle of rotation of the servo output shaft is greater than the above values ​​(depending on the model), but they are still quite sufficient for tuning a single-band loop antenna, since a relatively small change in capacitance is required from the capacitor. So, to cover the frequency band 7000 ... 7100 kHz, it is only about three percent.

In the ON7NU antenna, the issue of a high-voltage tuning capacitor is also solved in an unusual way. The steering machine rotates the rotor plate of a self-made capacitor of a relatively small capacity (Fig. 2), and the necessary full capacitance of the capacitor, which provides the resonance of the frame in the range of 40 meters, is achieved by connecting a “capacitor” parallel to it in the form of a piece of coaxial cable. If the ends of such a cable of the required length are carefully cut (to prevent breakdown), then the "capacitor" will work normally at a voltage of several kilovolts. It is this voltage that will act on it in the loop antenna with a transmitter power of 100 watts. In this case, to reduce the linear dimensions of the "capacitor", the cable can be wound on a dielectric frame.

Rice. 2. Rotation of the rotor plate with a homemade capacitor

The plates of the self-made capacitor in the ON7NU design are made of foil fiberglass. The movable plate is made of a material having a foil on both sides, which are electrically connected by soldering along the contour of the plate. Fixed plates can only have foil on one side, but then they must be installed with the foil side so that the foil "looks" at the movable plate. Electrical contact with the movable plate is provided by a flexible wire soldered to it.

The movable plate is fixed on the washer of the output shaft of the machine, and the fixed ones are fixed with the help of two long screws that are installed in the "lugs" of its body. Variable capacitor plates can also be made of copper sheet or brass.

The scheme of the control pulse generator, made on the timer 555 (KR1006VI1), is shown in fig. 3. The limits for changing their duration are set by selecting resistors R1 and R3. The variable resistor R2, which changes the duration of the output pulses within the required limits, must have a scale ("Antenna frequency"). It marks the operating frequencies, which are determined when tuning the antenna to a minimum SWR. The generator, batteries and controls are placed in a small case (Fig. 4).

Rice. 3. Diagram of the control pulse generator, made on the timer 555 (KR1006VI1)

Rice. 4. The generator, batteries and controls are placed in a small case

The assembled ON7NU antenna is shown in fig. 5. The cross, on which the frame is attached, is made of sanitary plastic pipes with a diameter of 40 mm and the corresponding T-shaped adapters. Steering servo with KPI and constant capacitor

Rice. 5. Antenna ON7NU assembled

Publication: radioradar.net

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