A High-Power UHF Power Meter . . . / cont
. . . Something wasn’t right. There was little distinction between forward and reverse power when remotely ‘swapping ends’. So much so that I had to concede that the directivity between the two ports on the coupled line was very poor. The book which had inspired this project made reference to an article in a issue of the RSGB Microwave Newsletter from 04/82. I posted a request for further information on the UKUG reflector and very quickly received the information that I was looking for. It became very clear that my decision to use FSJ-2/50 coax was nothing less than folly! With no 0.25” semi-rigid coax to hand, I decided to make another coupler out of UT141 semi-rigid coax.
The new coupler (left) unlike the previous, was very easy to make. Clamping the coax in a vice such that only a small area is above the level of the jaws makes it easy to ‘machine’ with a small file, a 1mm wide slot in the coax outer jacket. The coupled line needs to be bent in such a way that the mating faces are in good mechanical contact. Initially I used steel wire, tightly twisted around the two
lines whilst I soldered them together. The end result (right) is a much neater and confidence inspiring assembly. I can also confirm that the new coupler exhibits a far greater level of directivity between the coupled ports to the extent that there is now a very clear distinction between forward and reverse measurements. So much so that although it is possible to ‘cheat’ by adjusting the level at the controller end, very little adjustment is in fact required.
Below are a couple of photographs of the controller . . .
. . . A fancy name for a simple box.
In keeping with the present colour-scheme in the shack, where all my home-brew kit is currently matt-black, the controller is no different! As can be seen, there is little within the box other than the meter, a switch and two trimmer resistors which simply serve to prevent the meter movement being damaged and to null out the difference between the two (unmatched) diodes.
As can be seen, the meter has evidently been salvaged from some other piece of RF test kit. At some point in the future I may actually undertake to create a new scale for the meter, but until then I am satisfied with a simple relative indication.