Encyclopedia of radio electronics and electrical engineering / Civil radio communications

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A standing wave ratio (SWR) meter is one of the most essential instruments in an amateur radio station. However, the most common SWR meters have a limited bandwidth. The article describes a broadband device with an upper operating frequency of 2,5 GHz. The device is small in size and easy to use.

SWR meters make it much easier to set up, operate and monitor the condition of the antenna-feeder path. They are made either on the basis of directional couplers or AC bridges. Directional couplers have a noticeable frequency dependence and do not allow you to create a broadband SWR meter.

The attention of radio amateurs is offered a description of the design of the SWR meter, made on the basis of an unbalanced bridge. The device also allows you to measure the output power of the transmitter or transceiver. The meter operates in the frequency range from 1,5 to 1300 MHz, and with reduced accuracy even up to 2500 MHz.

The device consists of two nodes: high-frequency and indicator. Each of them is made in the form of a separate block. They are interconnected with a shielded wire. Thanks to this solution, the high-frequency unit can be placed directly on the measurement object, for example, on an antenna, and the indicator unit can be installed in a place convenient for observation. The signal source is a standard signal generator, transmitter or transceiver.

The scheme of the high-frequency block is shown in fig. 1. It consists of a resistive attenuator with attenuation of about 2 dB, assembled on resistors R1 - R6, and a resistive bridge on the elements R9 - R14. One arm of the bridge is the load whose SWR is measured. The load is connected to socket XW2. To reduce the inductive component of the impedance and increase the dissipated power, two resistors are connected in parallel in the bridge arms.

Diode VD1 rectifies the RF voltage generated by the resistors R10, R12, and it is used as a reference for calibrating the device and measuring the transmitter power. Diode VD2 rectifies the voltage in the measuring diagonal of the bridge, which depends on the SWR of the load connected to the device.

The presence of an attenuator leads to the fact that increased power must be supplied to the device, but at the same time, this ensures a satisfactory matching of the input of the device with a high-frequency signal source, most often the radio station itself. So, for example, without an attenuator in (depending on the SWR of the load) the SWR of the device at the input can reach 2, which is not always acceptable for the transceiver. With an attenuator, the SWR at the input of the device in any case will not exceed 1,5 ... 1,6. The diagram of the indicator assembly of the device is shown in fig. 2. It uses a small-sized pointer device - an M4247 microammeter with a total deflection current of 100 μA. Diodes VD1 and VD2 protect the device from overload.

Work with the device as follows. The load whose SWR is to be measured is connected to the XW2 "Load" jack, and an RF signal with a power of at least 1 ... 0,08 W is fed to the XW0,1 "Input" jack. In the position of the switch SA2 "SWR" and SA1 "Calibration" resistor R3 "Calibration" set the instrument pointer to the last division of the scale. After that, switch SA1 is switched to the "Measurement" position and readings are taken from the scale of the dial indicator. To measure the output power, switch SA2 is switched to the "Power" position, and a matched load with an SWR close to 3, a dissipation power of at least 1 ... 0,5 W is connected to the XS1 "Load" socket, and the readings are read from the indicator scale.

The following parts can be used in the device: resistors - RN1-12, size 1206, they can operate at temperatures up to 125 ° C. With a resistor dissipation power of 0,25 W, power up to 3 W can be supplied to the device for a long time, and several times more for a short time. If you use 0,5 W resistors, the input signal power can be doubled. Trimmer resistors - SPZ-19, variable - SP4 or SPO, capacitors - K10-17v or similar imported ones.

Diodes VD1, VD2 of the high-frequency unit are microwave detectors, preferably with a Schottky barrier. You can also apply KD922, 2A201, 2A202, and for frequencies up to 500 MHz - KD419 with any letter index. Diodes in the indicator block - any low-power pulsed silicon. RF connectors XW1, XW2 can be of any type, but they must be designed for joint installation directly with the microstrip line.

Three-pin (stereo) connectors for headphones (1 mm in diameter) or microphones (3,5 mm in diameter) can be used as XS2,5 bass jacks, in addition, you will need two appropriate plugs and a shielded cable several meters long to connect the high-frequency and indicator blocks. Another microammeter can also be used, including a large one, with a total deflection current of 50-100 μA and a loop resistance of several kOhm. Switches - any low-frequency two positions and two directions.

Structurally, the device is also made of two blocks. Most of the parts of the high-frequency unit are placed on a printed circuit board made of double-sided foil fiberglass, a sketch of which is shown in Fig. 3.

The second side of the board is left completely metallized. Through the holes indicated on the sketch by light circles, the metallization on both sides of the board is connected with short pieces of wire. The board by soldering along the edge on both sides is installed in a tinned metal case of a suitable size, and nests are placed on its walls (Fig. 4).

All elements of the indicator part are also placed in a metal case of a suitable size (Fig. 5).

To set up the device, you need a 144 MHz or 432 MHz transceiver with an output power of up to 3 W, which can be adjusted, and load resistors with known SWR. Carry out the setting in the following sequence. By setting the SA2 switch to the "SWR" position, a signal with a power of 0,3 ... 0,5 W is applied to the input of the meter, and the output is left unloaded. In the position of the switch SA1 "Measurement" resistor R3 of the indicator block of the device set the arrow of the device to the last division of the scale. Then, in the "Calibration" position, the arrow is also set to the last division of the scale with the resistor R8 of the high-frequency unit. Decreasing the signal power, find its value, at which the readings of the device in the "Calibration" and "Measurement" positions will noticeably differ from each other. This will be the lowest power at which measurements can be made.

Then the scale of the power meter is calibrated. To do this, a load with an SWR close to 2 is connected to the output of the device (XW1). In the switch position SA2 "Power", a signal with a power of 2,5 ... set the instrument pointer to the last division of the scale. By reducing the power and controlling it with the help of some measuring device, for example, an RF voltmeter, the scale of the device is calibrated in units of power. For an example in fig. 3 shows an experimentally taken graph of the dependence of the measured power on the indicator readings.

Lastly, the scale of the SWR meter is calibrated, for this a signal is given that exceeds the minimum value by 1,5 ... 2 times. By connecting load resistors with a known SWR, they calibrate the scale of the device and check its operation over the entire frequency range, and also determine the range of input signal levels at which the device provides the necessary accuracy. On fig. Figure 7 shows the experimental dependence of the SWR on the indicator readings.

To quickly check the health of the device, the device must have two or three load resistors with a known SWR.

Author: I. Nechaev (UA3WIA), Kursk

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