SWR Meter information & software

Part 2 - Design, Calibration & Performance of Standing-Wave-Ratio Meters - Basic Meter & Voltmeter Circuits

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

Program Notes (Continued)

Basic Meter Circuit (Fwd/Refl switch not shown)

                       Current Transformer
¦ Turns ratio N --------- ¦ ¦
¦ ,MMMMMMM, Sec. RRR ¦
Source RRR ¦ ¦ RRR R1 RRR Load
Internal RRR ¦ R3 ¦ RRR RRR RL
Resistance RRR o-SHUNT-o ¦ RRR
¦ ¦ ¦ RRR ¦
¦ ¦ '----K-->RRR Cal potr. ¦ ZL=RL+jXL
¦ ¦ RRR ¦
Source ¦ ¦ XXX Load
Internal Diode rectifier RRR XXX jXL
Voltage + DC voltmeter RRR R2 XXX
¦ ¦ RRR ¦
¦ ¦ ¦ ¦
Ground --->o---------------o-----------------o-------------'

R1 & R2 is a voltage divider across the load. Fraction K = R2/(R1+R2). Voltage across R3 is load current times R3/N which can be switched either to add to or subtract from the fraction K. When subtracting the meter responds to reflected waves and when adding it responds to forward waves. The calpot is adjusted for a voltmeter null for reflected waves when ZL equals reference resistance Zo.

To minimise power dissipated in the divider resistors R1 should not be less than 100 times Zo for a 100 watt transmitter. The preset potentiometer is included in the divider chain to permit precise adjustment of the ratio K.

If a capacitance divider is used connect an RF choke across C2 to provide a DC path for the meter current. Alternatively a high-value resistor can be shunted across C2 which then must be included in the voltmeter calibration resistance. Fine ratio adjustment is possible when C1 or part of C2 is a preset capacitor. The ratio K = C1/(C1+C2). The reactance of C1 should not be less than 12*Zo.

The voltmeter may be a 100 uA fsd moving coil meter with sensitivity adjusted by a series preset calibrate resistor. The program computes resistor setting.

Voltmeter Circuit

                             Ge diode      Preset cal resistor
---> from SWR bridge --->o------>¦------o-------RRRRRRR------,
¦ ¦
¦ ¦
Disk ceramic === 0 to 100
0.01 uF ¦ micro-ammeter
¦ ¦
¦ ¦
Ground --->o--------------o--------------------'

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