Measuring SWR with an extreme QRP transmitter

As I've used the prototype RF breakout boards for the Raspberry Pi, I've found one or two unexpected challenges posed by such a low power transmitter. This post deals with one of them, the problem of measuring SWR with not enough power to operate my SWR bridge.

When your transmitter only supplies 10mW of RF power it is extremely important that all components handling that RF do so with maximum efficiency. When you have plenty of power to spare it does not matter if you lose a bit along the way, but when you have so little in the first place your every loss will be felt. You thus need to ensure that your connectors and feeder are as low-loss as possible, and that your antenna has as good an SWR as you can make it. If you have a higher power transmitter on the same band in your shack it will be easy to use that to set up your antenna, but if all you have is a Raspberry Pi you may have to take a different tack from that which you are used to.

This might seem like a blindingly obvious revelation, but a 10mW transmitter does not have enough power to operate a lot of commercial SWR meters. Devices sold for use with watts or more of RF power may not have a range with the sensitivity required to give a reading when supplied with small numbers of milliwatts. My trusty Howes resistive SWR bridge for example is nominally a QRP device, but QRP in that case seems to mean watts in the single digits and it starts to have problems as the power dips under 500mW. When presented with 10mW it gives no meter deflection at all, to all intents and purposes it has become a useless instrument.

My solution when I needed to measure the SWR of my 70MHz WSPR dipole was to go back to SWR basics and use a directional coupler on its own with my multimeter measuring the voltage on its reverse port. My directional coupler dates from my days experimenting with 435MHz ATV, and is a few inches of coupled stripline on a PCB in a diecast box with Schottky diodes feeding its ports. 70MHz is probably not its ideal frequency, but just as an example with the Raspberry Pi as an RF source I measured between about 10 and 100mV on the reverse port increasing with the SWR of the termination. For the record the 70MHz dipole measured 81mV compared with a 50 ohm terminator's 10mV, indicating a poor SWR that turned out to be from a detached co-ax braid connection.

Some more information on directional couplers:
W2AEW video on directional couplers
https://www.youtube.com/watch?v=byF1FLdbUiA
GM8OTI stripline directional coupler for UHF
http://www.marwynandjohn.org.uk/GM8OTI/projDirCoupler/projDirCoupler.html
VK1HW ferrite directional coupler for HF and VHF
https://sites.google.com/site/vkonehw/home/homebrew/Bidirectional-Coupler/building-a-bidirectional-coupler