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SWR Meter information & software

# Part 1 - Design, Calibration & Performance of Standing-Wave-Ratio Meters - Introduction & Principle of Operation

Author: R.J.Edwards G4FGQ © 28th October 2000

An SWR meter is a fixed-ratio resistance bridge. The external "unknown arm" is the input impedance, R+jX, of the antenna system. The bridge unbalance voltage is displayed on a meter calibrated to indicate a choice of system parameters.

This program assists with design and models the behaviour of the most common form of HF SWR meter. It is located in a coaxial line between the transmitter and the antenna where, subject to certain conditions, it indicates SWR on the section of line between transmitter and meter. It is often placed immediately at the transmitter output where an indication of the reflection coefficient of the system input impedance is a more appropriate parameter to display.

Some or all of the following meter scales may be used:

1. SWR. Very non-linear scale from 1 to Infinity. SWR = 3 at 1/2-scale
2. Magnitude of Refln.Coeff. Scale graduated linearly 0 to 1 at full scale
3. Reflected Power, watts. Square-law scale, cramped at lower end
4. Forward Power, watts. Same square-law scale. Full scale > Tx rated power

Actual Tx Power = Indicated Forward Power - Indicated Reflected Power.

Principle of Operation
A generator with internal impedance Zs feeds a coaxial line of impedance Zo. The end of the line passes through the meter. The rest of the system beyond the meter, for the purposes of these notes, is considered to be the Tx load ZL. Inside the meter two relatively small voltages are obtained. One is proportional to and in phase with the load voltage. The other is proportional to and in phase with the load current.

From this pair two other voltages are produced: their difference Vd and their sum Vs. The bridge is standardised by terminating the bridge with a dummy load equal to Zo, then adjusting the fraction of load voltage tapped off such that the rectified Vd is precisely zero as indicated on a high-Z DC voltmeter.

Any load other than Zo will now unbalance the bridge, the unbalance voltage Vd being proportional to the reflection coefficient RC = (ZL-Zo)/(ZL+Zo). The RC scale is graduated linearly from 0 to 1. SWR = (1+RC)/(1-RC) may be indicated on the same meter face, the scale being graduated non-linearly, 1 to Infinity.

The square of the meter deflection Vd is proportional to power reflected back from a mismatch at ZL. The scale, calibrated in watts, can be shared with the forward power indication which is proportional to Vs squared. But forward and reflected watts cannot be displayed simultaneously on a single meter movement.

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