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Automatic SWR meters have gained well-deserved popularity due to the fact that they do not require constant calibration. This greatly simplifies the measurement process itself and makes it possible to quickly control the quality of the antenna-feeder path matching when working on the air. A large number of successful circuit solutions proposed by radio amateurs can be divided into two groups.

The first one includes solutions based on SHI controllers [1-4]. These are relatively complex circuit devices, consisting, as a rule, of two blocks - the actual auto-calibration unit for three or four op-amps and an indication unit (analog on a pointer device or LED digital with its rather complex converter). The second group includes devices based on resistive dividers [5-7], which are distinguished by their simplicity of execution. The principles of their construction and the method for calculating the SWR meter based on resistive dividers are quite simply and accessible in the article by I. Goncharenko [5].

Very attractive in terms of ergonomics, design and ease of visual control, SWR meters with LED indicators. It is worth noting two important features of these devices. Firstly, the operation of calibration or auto-calibration, as such, is absent as unnecessary. The measurement accuracy is determined only by the accuracy of the selection of resistor values ​​and the sensitivity of the comparators. Secondly, good speed allows us to recommend their use for operational monitoring of the operating and emergency conditions of the antenna-feeder path. In this case, it is sufficient to count two or three threshold levels, for example, as in [7]. But for comfortable use as the main SWR meter, it is desirable to increase the number of displayed levels to at least 5-7.

The version of the automatic LED SWR meter with unipolar power supply that we offer to your attention has ten levels of reference and is extremely simple due to the use of an affordable and inexpensive LM3914 microcircuit [8]. This microcircuit - a specialized controller for controlling linear LED scales - has everything we need, namely: a precision ten-stage voltage divider with a linear division step of 0,1, ten comparators and an LED control unit.

The scheme of the device is shown in fig. 1. Voltage direct U_pr and reflected U_neg waves from the SWR sensor are fed to the inputs of the DA1 chip. The allowable voltage of the direct wave is + 1 ... + 11 V. It is set during the adjustment of the sensors when the rated power of the transmitter is applied to the matched load. It is desirable to limit the lower value of this voltage at a level of approximately 2 V in order to minimize the effect of the nonlinearity of the germanium diodes of the SWR sensor on the measurement accuracy. Direct and reflected wave sensors are any known devices on directional couplers, on current transformers or bridges, which are repeatedly described in the literature. I would like to recommend a good design by E. Gutkin for manufacturing, which is accessible and described in detail in [9].

Rice. 1. Scheme of an automatic LED SWR meter

The direct wave voltage through the resistor R2 is fed to pin 6 DA1 - the upper arm of the internal resistive divider, which is ten identical resistors connected in series with a resistance of about 1 kOhm. The use of an additional external resistor R2 made it possible to obtain a certain flexibility in setting the comparator thresholds and, accordingly, in choosing the SWR values ​​indicated by the LEDs. In the author's version of the indicator, with the values ​​\u1b\u1,2bof these resistors indicated on the diagram, the glow of the HL2 LED corresponds to SWR 1,4, the HL3 LED - 7, the HL4 LED - I.2, the HL5 LED - 2,5, the HL6 LED - 3, the HL7 LED - 4 , LED HL8 - 5, LED HL9 - 7, LED HL10 - 11, LED HLXNUMX - XNUMX.

These values ​​are valid if the total resistance of the internal divider is 10 kOhm, but in reality, due to the technological spread, it can be from 8 to 17 kOhm. Therefore, to ensure high accuracy of the SWR meter, it is first necessary to measure the total resistance of the internal divider by connecting an ohmmeter to pins 4 and 6 of DA1.

To do this, it is best to use a "Chinese" digital multimeter - in its ohmmeter mode, a low voltage is applied to the output (no more than 0,2 V), which is lower than the opening voltage of silicon pn junctions. This ensures high measurement accuracy. In the author's version R_int = 9,92 kOhm. Measured value R_int will allow you to select the specific resistance of the resistor R2 for the desired indication characteristic.

The formula for calculating the steps of the SWR indication for a specific instance of the microcircuit and the selected value of resistance R2 is simple: SWR \uXNUMXd (R_int + R2 + R_tech)/(R_internal + R2 - R_tech). Here the resistance R_int and R2 - in kiloohms; R_tech - resistance of the steps of the resistive divider in kiloohms (i.e., in this case it is 1, 2, 3 ... 10).

About the appointment of other elements. Resistor R1 equalizes the load resistance of the SWR sensor rectifiers, so its resistance must be equal to the sum of the resistances R2 + Rext. Resistor R4 determines the current through each LED, in this case it is chosen to be approximately 10 mA. Capacitors C3 and C4 protect the inputs from RF interference. The circuit variant shown in fig. 1 corresponds to the operating mode of the scale in the form of a luminous column. If pin 9 of the DA1 chip is left free, only one significant LED will light up.

It turned out that there are often instances of the LM3914, in which the bias voltage at input 5 is quite large. This results in an indication without input signals. To eliminate this, it is necessary to apply a small positive voltage to pin 4, for which a tuning resistor R4 with a resistance of 3 ... 220 Ohm is connected between pin 330 and the common wire. Turning on the power, by adjusting this resistor, we remove the background (without signals) glow of the indicators.

LEDs can be used any available. Structurally convenient imported monoblocks of ten independent diodes in one package. In the author's version, a KingBright DC-763BWA block was used, in which seven diodes are green, and three diodes (in our case they correspond to SWR> 4 levels) are red.

If desired, this SWR meter can be supplemented with a device for sound indication of exceeding a certain SWR threshold and automatic relay protection against high SWR. A diagram of such a device is shown in fig. 2.

Rice. 2. Diagram of the device

In this case, the following operation algorithm is implemented: when the SWR reaches level 3, the HL6 LED lights up (according to the diagram in Fig. 1), the voltage drop across it opens the VT1 transistor, which turns on an acoustic emitter with a built-in generator. It can be of any type - as long as it works loudly enough when +5 V supply voltage is applied to it. A warning sound signal sounds. If the SWR continues to increase and reaches 7, transistors VT2 and VT3 open and a relay is activated, the contacts of which (they are not shown in the diagram) can put the device into receive mode or, for example, significantly reduce the output power.

Positive feedback through the VD1R5 circuit "latches" the keys VT2, VT3 in the open state. They can only be removed by closing the contacts of the SA1 reset button or by completely de-energizing the protection unit. Capacitor C2 provides a slight delay (about one second) to the tripping of the protection relay, and its capacitance can be changed based on your own preferences.

Transistors can use any silicon of the corresponding structure: VT1, VT2 - series KT209, KT361, KT3107, 2N3906, etc., VT3 - series KT315, KT3102, 2N3904, BC547, etc. Diodes - any silicon low-power series KD522, KD102 , Sh4148, etc. Relay - with an operating voltage of 5 ... 6 V.

Literature

1. Pogosov A. Automatic KBV-meter. - Radio, 1985, No. 10, p. 20, 21.
2. Automatic readings when measuring SWR. - URL: cqham.ru/swr_12.htm.
3. Dobrokhotov I. Automatic SWR meter. - URL: cqham.ru/un7gm_swr.htm.
4. Nechaev I. SWR-meter with automatic calibration. - Radio, 2005, No. 3, p. 64, 65.
5. Goncharenko I. SWR meter indicator. - URL: dl2kq.de/ant/3-21.htm.
6. Kabaev A. Automatic SWR indicator. - URL: cqham.ru/swr14. htm.
7. Nechaev I. Automotive automatic SWR meter. - Radio, 2005, No. 6, p. 68, 69.
8. LM3914 Dot/Bar Display Driver. - URL: datasheetcatalog.com/datasheets_pdf/L/M/3/9/LM3914.shtml.
9. Gutkin E. We measure SWR: theory and practice. - Radio, 2003, No. 5, p. 66-68; No. 6, p. 61-63.

Author: Sergey Belenetsky (US5MSQ)

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