Wednesday, June 5, 2019

RF power meters

A handy instrument around the shack is an RF power meter. It can be used to give a rough idea as to whether there is something wrong with your transmitter. Or to tell whether it's been misadjusted, for example the microphone gain is all the way back.

Being small (and in most cases) self-powered, RF wattmeters can easily be taken outside. This is handy to test whether RF is getting up to the antenna or if there is a break or bad connection somewhere between there and the transceiver.  It can be frustrating when you find after hours of investigation, that the problem is not in the antenna but at the transceiver or feedline end. 

VSWR meters normally have an RF power indicator built in. Especially in the cheaper instruments they are not particularly active. And what they show is often power for full duty cycle modes like CW, FM and most digital modes rather than PEP readings for SSB. Nevertheless they are still useful for most purposes. 

Apart from difficulties with indicating SSB power, RF power meters can deteriorate in accuracy above the HF frequency range. Dedicated UHF and microwave operators have specialised instruments for those bands. And if you're running QRP or doing micropower experiments, you will probably want an instrument that has better accuracy at lower power ranges. Trying to interpret power when the meter needle moves just a few millimetres off the stop is not fun! 

RF power meters are fun and simple to build. The easier types comprise a diode detector, a resistor whose value is set for how much power you wish to measure and a meter movement. For maximum versatility build them along with a 50 ohm dummy load. Some designs (often built into VSWR meters) can be used while operating while others are basically 'dead ends' only suitable for testing. Below are some RF power meter designs others have succeeded with: 

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