VK4ZXI

Three cavity notch RX filter for 2 m repeater with 1.6 MHz RX/TX spacing Introduction A three cavity notch filter was constructed and tested. The RX filter has over 80 dB of TX rejection. Construction The club has a 2 m repeater that is to operate on the new 1.6 MHz RX/TX spacing, as opposed to the usual 600 kHz spacing. The existing high/low pass reject cavities could not accommodate the wider spacing, limited to about a 1.1 MHz spacing. The club has five 1968 band pass filters, old but well made. A pair of them had been used with a phasing harness to act as notch filters, but the rejection was not high enough for TX reject on RX. Two are probably adequate for the TX filter to reject spurious noise at the RX frequency. I decided to try and make a three cavity notch RX filter, necessitating a new phasing harness. Each of the cables are a quarter wavelength, with those going to the cavities a little shorter to allow for the probe inside the cavity. I used LMR-400 ultraflex fo...

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...