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Part 7 - Design, Calibration & Performance of Standing-Wave-Ratio Meters - Direct Indications of Reflection Coefficient and SWR, and Obtaining Simultaneous Display of Forward and Reflected Power

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

Program Notes (Continued)

How to Obtain Direct Indications of Reflection Coefficient (RC) and SWR
As already described, when the source impedance as seen looking back towards the transmitter from the meter is not Zo, the indicated values of RC and SWR are incorrect but the true values can be calculated from forward and reflected watts.

Note that RC = Reflected/Forward meter deflections regardless of power level. So to obtain correct direct indication of RC and SWR proceed as follows:

  1. Switch to forward power. At any suitable Tx output power adjust meter sensitivity for full scale deflection to read 1.0 for RC and infinity for SWR.
  2. Switch to reflected power and read RC and SWR directly on the meter.
  3. (1) and (2) must be repeated whenever the load impedance changes. This is because the Fwd power reading changes whenever the load changes unless the source resistance is equal to Zo. This meter characteristic provides a simple means of checking how close the source resistance is to Zo.

Note that adjustment (1) upsets forward and reflected power measurements. The meter must now be re-calibrated at a standard power output level into a dummy load having a resistance of Zo.

How to Obtain Simultaneous Display of Forward and Reflected Power
Two separate 100 uA moving coil meters are needed. Each could could have its own current transformer but, in practice, both meters can be driven from one common transformer with a centre-tapped secondary.

Use the same core as used for one transformer and wind on twice the number of secondary turns with a centre-tap. Connect the centre-tap to the line voltage divider. Shunt each half of the winding with a resistor twice the value computed for single-meter operation. Connect the rectifier diodes to each end of the winding. Each meter circuit will require its own decoupling capacitor and sensitivity control resistor. Input resistance of the primary winding, sensitivity control resistors, and the amount of RF power extracted from the line to drive the meter circuits will be the same as for one-meter operation.

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