G3TXQ

Amateur Radio (G3TXQ) - SWR meter

I've always been interested in "direct-reading" SWR meters that automatically compute the ratio of Forward and Reverse powers (i.e. you don't have to adjust any sensitivity controls). Several years ago I built one to a design by G3WPO that was sold as a kit by Cirkit. It worked quite well, but it contained numerous trimmers that needed adjusting from time to time to maintain the accuracy.


SWR meter (front) SWR meter (inside)

Then I discovered a much more modern design by N2PK. Paul uses a pair of AD8307 RSSI (Received Signal Strength Indicator) chips to produce DC voltages that are proportional to the logarithm of the Forward and Reflected power levels. These DC voltages can then be subtracted in an Op Amp to produce a voltage proportional to SWR which is essentially independent of power level. The Forward DC voltage also drives a separate Power meter. I have modified Paul's design to use a "Stockton" directional coupler, and to provide a peak-reading capability.


Meter PCB

The meter automatically provides an accurate readout of SWR for any power level between 10mW and 1000W. I arranged for the Power meter to cover the range 100mW to 1000W, but it is easy to change the circuit to set upper and lower power limits to any values in the range 100uW to 1000W. You can see the circuit diagram here.


Coupler Coupler close-up

The design of the coupler is critical to the accuracy of the unit. I chose to build the coupler as a separate unit so that strong RF fields could be kept away from the processing circuitry. The Forward and Reflected RF samples are fed via RG174 coax cable to the "processing" unit.

I had been very impressed with the accuracy of the coupler supplied by N8LP as part of his LP100 kit, so I used the same design for my meter. A 6dB pad in the coupler, and a 11.2dB pad at the input to the processing unit provide isolation for the interconnecting cables.