Hi there, [[ Please excuse if this is "double posted.]]
I've had some success rebuilding & tuning 1/4 wave cavities for 146 MHz by playing around with loop sizes, orientation etc and being fortunate to have had access to an HP-Agilent VNA to "help along". (W6NBC's excellent article on cavity loop optimisation was found <after> I had pulled out most of my hair ...but I digress.) Our local radio club in VK2 is attempting to get a 1296 MHz D-STAR repeater operational and we need a suitable diplex filter. (We're currently running a 6 cavity filter for 2 metres using a hybrid combiner for the D-STAR & analogue repeaters.) First thoughts were to use 450 MHz cavities operated in 3/4 wave mode. Bench testing showed the cavities performed as expected in 1/4 wave mode, but no amount of loop tweaking or black magic cursing could get the insertion loss to acceptable levels when operated in 3/4 wave mode (1296 MHz). Typical IL was 2 to 3dB per can at these frequencies. All test cable losses were "accounted for". Are we doing anything wrong or missing some critical bit of magic "fairy dust" that needs to be done when operating cavities in this mode ? There seems to be an total lack of information on the web on 3/4 wave cavity building and operation. If this is approach is a "lost cause", what do others use for 1296 MHz diplex filter hardware ? All suggestions gratefully received either on or off-line. ([email protected]) Cheers, Kit VK2LL Sydney
