QRP Resistive SWR bridge and Indicator

Many DIY and low cost ATU’s do not have a built in SWR indicator and so it’s an extra bit of kit to carry with you if out portable (or in the shack). Also it is easy to damage the output stage of your nice new QRP radio if you spend a long time setting up the tuner.

A tried and tested method to do this is to use a resistive SWR bridge. If you’re not sure what one of these is let me briefly explain. Now the purists will say this isn’t right but even they would have to agree that in practice the results are the same.

Normally our radios are designed to work into a 50 ohm load, we use 50 ohm cable too.  When we use an ATU we are trying to match the antenna impedance to that expected by the radio (50 ohm). The ATU (or AMU) acts like an impedance matching network.
There are many factors that should be considered when measuring SWR as it’s not just a resistive load, but in practice this type of circuit does the job fine and 1000’s of people use this type of bridge with good success.
The bridge is based around the Wheatstone Bridge circuit with the antenna being the 4 element in the bridge, when the antenna is matched to 50 ohms the bridge is balanced and our LED will go out, when the antenna is not matched the LED will illuminate, the brighter the LED the worse the mismatch so we tune the ATU for minimum brightness of the LED which will mean we have a good match.