Resistive SWR Meter.
I built a resistive SWR meter during the Covid-19 lockdown. I wanted to use this to overcome problems I had experienced when using my Alan KW520 on the 5 watt range when experimenting with QRP antennas. Just a small amount of reflected power and a slight reduction in forward power below 5 watts made it difficult to set the SWR meter. This problem was made worse if my Yaesu FT-817nd output stage went in to power fold-back.
I had already experience of using a resistive SWR meter which is incorporated in the G-QRP Club “Limerick Sudden” ATU Kit. Whilst wanting to use a resistive SWR meter, I wanted to avoid the losses associated with a matching unit. Therefore, I chose to build a stand-alone meter and chose to use the design from the G-QRP Club web-site: http://www.gqrp.com/resistive_swr_bridge.pdf
In common with most if not all resistive SWR meters, this design is a Wheatstone Bridge. There is a full explanation of the Wheatstone resistive bridge at: http://www.gqrp.com/resistive_swr_bridge.pdf
Advantages of the resistive SWR bridge include, that it will work with a transmitter output power of 1 watt (and probably less), that the antenna only receives a quarter of the transmitter power during SWR measurement, limiting interference on the band, and that the transmitter should not experience an SWR of more than 3.
I built the circuit ‘bug style’ using solder tag terminal strip bought from TK Electronics at an earlier rally. Without access to the workshop at Create Oldham, the creative and technology hub in Oldham, to do some metal bashing, I used modeller’s plywood sheet for the front panel and mounted the solder tag terminal strips off the screws holding the SO239 sockets. This type of modeller’s plywood is used in model airplanes and to build tunnels on model railways. It does cut easily with a modelling knife although remains quite stiff as a panel. Although I could use the plywood panel as a template for a metal front panel in the future, the circuit works very well as it is.
Here are a couple of pictures showing the circuit construction: