A High-Power UHF Power Meter
I was very fortunate to be loaned, on a long term basis, a Tokyo HL-130U solid-state PA for 70cm. Although I have been QRV on 70cm since 1977, I have never had the capability to run more than 30W at any time. This is about to change! My initial concern was that since the PA belonged to someone else, I should ensure that my antenna VSWR was acceptable. My old Pye reflectometer for that band did not cater for power levels above 75W and I was increasingly suspicious of it’s reliability in respect to VSWR indication.
I had read about using parallel lengths of semi-rigid coax to make a directional coupler. I didn’t have any 0.25” semi-rigid but did have some FSJ-2/50 coax. The photograph on the right, I think, explains the construction. The copper outer is filed away on each length of coax over a length of about 45mm and maybe 3mm wide. The corrugation of the outer can be a problem, but with care, it can be done. Bind the lengths together with copper wire such that the openings in the outer are face to face, and carefully bend up the ends of the coupled line. Finally solder the two lines together. I sealed the assembly with conductive tape for good measure. I could have kept it simple and just gone for a single directional coupler, but that would have required turning it around to compare reflected and forward power levels. I didn’t want to have to do that, so this had to be a bi-directional device, and given the relatively high
isolation between lines (~50dB, I’m told), I figured that a little PCB (left) would suffice for ‘swapping ends’. Two miniature relays are used to alternatively connect the 47R resistors or BAT85 diodes to the appropriate ends of the coupled line. One of the relays also selects the appropriate diode output. It is true to say that I could have used just one diode and switched it from one end of the line to the other but since I was planning on separate calibration pots for forward and reverse indication anyway, I included the second diode. The photograph below shows the detector PCB in place.
Testing . . .
After a slight hiccup which revealed a fault actually inside the 100uA meter, . . .
. . . I’d would have liked to have said that I was satisfied with the results, but . . .