SWR meter

An SWR meter (English SWR meter ) is a device for measuring the standing wave ratio. With it, for example, the current in a coaxial high-frequency waves can be recorded separately for each direction in their amount. It provides an indication of the degree of mismatch an antenna or dummy load at the end of the cable.

The standing wave ratio is in the ideal case (ie no mismatch ) the value 1

Measurement with directional couplers

A standing wave knife for very high frequencies consists of one or two directional couplers, rectifiers and a voltmeter, which is selectively connected to one of two outputs, or two measuring stations, whose hands cross over each other (cross pointer instrument ).

The a value is a measure of the voltage of the departing, the other being a measure of the voltage of the returning wave. The length A of the coupling wires can not be chosen arbitrarily:

  • A must be less than the wavelength of the measurement signal. Absolute upper limit is λ / 4
  • If A is too short and the coupled voltage is only about as large as the threshold voltage of the diode, the measurement error increases enormously. Therefore, this type directional coupler at wavelengths greater than 50 m are hardly usable.

Breitbandrichtkoppler

At frequencies below 5 MHz replacing the pieces of wire through current transformer ( through transformer). Thus, the measured voltage is nearly independent of wavelength.

One possibility is the " Bruene directional coupler ", which combines a current transformer with two variable capacitors.

The directional coupler according to Sontheimer Frederick two identical power converters are used to

  • Herabzutransformieren and 1: T1 with the power of the inner conductor n in relation
  • 1 herabzutransformieren: T2 is the tension between inner and outer conductors in the ratio n.

Thus, the impedance V / I is preserved. The coupling constant is calculated to be C3, 1 = 20 * log ( n ). The two resistors R1 and R2 of the transformer T2 must have the same value as the characteristic impedance of the coaxial cable between P1 and P2.

Direct voltage measurement

At sufficiently short wavelengths and very high demands on the accuracy of the voltage profile at the inner conductor of a coaxial cable is curled (English: slotted line ) determined. This is not to measure the ( often high ) voltage of the inner conductor directly, but couples a small fraction of capacitive and so stimulating the orange subscribed λ/4-Schwingkreis, at the tap of the RF voltage is rectified. With a sliding short circuit at the left end of the tank circuit resonance is set. The minimum length of the slotted line is λ / 2, so the applicability is limited to the FM band.

Measuring method: Man looking for a job on the waveguide, where a particularly large rms voltage U Max can be measured. At a distance λ / 4 of which the voltage UMin must be particularly small. The sought standing wave ratio is calculated as

Calibration

The SWR meter to be calibrated is connected between the transmitter and the dummy load, the connecting coaxial cable should be considerably shorter than a quarter of the wavelength at which is to be measured in order to avoid distortions due to cable resonances. The standing wave ratio for different values ​​of the equivalent load can be calculated and used to control the displayed values.

Where should the SWR be measured?

For long cables, cable resonances are the reason that some outlying standing wave ratios are displayed at different measuring points. The optimal point is at the junction of the cable and antenna, because the actual base impedance of the antenna then is compared with the calibration value of the standing wave meter. However, this point is often inaccessible or inconvenient to reach. Therefore, the meter is usually connected directly after the service. At this point, we measure only the correct value when the ( mechanical ) cable length is an integer multiple of shortening factor × λ / 2, because then there is a 1:1 transformation.

Enormous deviations from the true value occur when the cable length is an odd multiple of shortening factor × λ / 4 and displays the SWR values ​​above 2.

Each waveguide attenuates the passing waves by a certain percentage, which is why the measurement result is different at the beginning of the cable at the cable end. The often serious consequences can show a simple example: A transmitter of 100 W is connected via a coaxial cable to an antenna, the cable attenuates by 3dB at the antenna are only 50 W at.

  • If the antenna is shorted or the connection is torn down, this service is completely reflected, why do you measure and calculated directly at the antenna SWR = ∞.
  • The reflected power is also attenuated by 3 dB, there are only 25 W at the transmitter, and here the meter shows SWR = 3 at. That may be enough for modest claims, but it is completely wrong.
  • Increased moisture due to cable attenuation 4 dB, the decreases measured at the SWR on the acceptable value of 2.3 and indicates a functioning system, although the antenna radiates nothing.

Nevertheless, the SWR meter is almost always directly operated on the channel, because there is much more accessible than in the antenna.

If signal attenuation is sufficiently large along the cable, in spite of complete mismatch acceptable SWR can be displayed. For example, a 30 m long piece of RG58U coaxial cable at 432 MHz as a dummy load to about 200 W was used, because the reflected wave is damped by a total of 20 dB. Therefore comes - regardless of the terminator - a maximum of 1 % of the power fed back to the transmitter, resulting in the very low SWR = 1.22 is calculated. At lower frequencies corresponding to a longer wire must be used to achieve the necessary total attenuation.

Application

The wave impedance of the high frequency cable, and the antenna or load must be matched to one another as possible in order to avoid unnecessary losses due to reflected power in it. The SWR meter is not an indicator of how well the waves are radiated from the antenna. The impedance of the transmitter deviates at high power from highly characteristic impedance of the cable to allow an efficiency over 50% ( See line matching ).

In the construction and operation of high- frequency systems are used standing wave gauges

  • For impedance matching of transmission antennas
  • Here often provided for monitoring of transmit antennas during operation with additional devices which restrict the transmission power in case of large mismatch or turn off the transmitter
  • For monitoring of devices for high-frequency heating, plasma generation and excitation of gas lasers (eg RF excited CO2 laser).
Power dividers and directional couplers Electrical impedance Gaslaser
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