Using the VFD power meter for VHF/UHF
I purchased a HF digital power meter from Array Solutions. The wattmeter samples very fast and holds the peak value, and can therefore give a true SSB peak power reading. This makes it a lot easier not to overdrive a PA. The bar graph at the top gives the instantaneous power value, and can be used for tuning the PA. The meter ranges from about 20W to 3kW with a good linearity.
The readout unit is simply sampling a DC value from the coupler, squaring it and giving the corresponding power reading. The HF coupler is about -29.5dB, and uses two diodes in series to detect the signal. The diodes are operating in the rectifying region (also known as linear detection). For a further description of how the power meter works click here.
I got hold of some adjustable directional couplers made by Ericcson. They have the coupling lines contained on small PCBs, that can be moved back and fourth to provide the desired coupling. After making some detectors in a voltage doubling configuration with HP5082-2800 diodes, and integrating them with the coupler, I calibrated the couplers (with this kind of detectors approximately -35.5dB of coupling is needed) for 70 MHz, 144MHz , 432MHz and 1296MHz , afterwards I connected all the couplers in parallel to the readout unit, so that I now have one central readout of power on all bands from 160m to 23cm.
I learned the hard way to be careful, when measuring a signal with high harmonic content. The higher frequencies couples much better trough the directional coupler (eg. +9dB for the 3'rd harmonic), and contributes to a too high power reading.
