VK4ZXI

Tuning a commercial UHF duplexer with RigExpert antenna analyser The club is assembling a 70 cm repeater with some equipment kindly loaned to us to use; a Kenwood TKR-820 repeater and a good quality commercial six cavity band-pass duplexer. The tuning was done with a RigExpert antenna analyser that gives a plot of SWR swept across a frequency range. The analyser was able to tune each cavity, but the overall response, while good for separation was compromised, possibly by incorrect inter-connecting cable lengths. Tuning the duplexer The duplexer is a band-pass type, with six cavities, three each for RX and TX with a "T" combiner. The cavities use N connectors and thick LMR400 type cable, and are about twenty years old. The probes are set for maximum coupling, which is desirable for selectivity but for some transmission loss. As a band-pass cavity filter, each cavity has a N connector in and out, making tuning each cavity quite easy; put a 50 Ohm termination on ...

Homebrew cavity resonator/duplexer for 2m repeater There has been some interest in the club establishing another 2m repeater, but lacked a duplexer. A couple of old cavity resonators were discovered in the back shed. We were able to tune these very quickly and easily using the new Chinese KC901H network analyser (more about that in another post). Why and how they work: an antenna in a box! Up to that point I had heard of cavity resonators but had little idea of how they worked or how they were made. Similarly, so was my knowledge of repeater. However, as I started to learn about them, I became quite intrigued with the technical finesse of being able to transmit and receive with the same antenna simultaneously, albeit on different frequencies. Further, as desirable repeater sites are restricted in number, many repeaters share the same site, again on different frequencies and bands. The solution to the repeater problems is in using very selective band-pass and/o...