I'm tuning up a cavity-type duplexer for a 70cm repeater, and in the process, I showed my results to a fellow ham. He asked a couple questions that cause me to think; so I decided to ask the experts...
FIRST QUESTION TOPIC: For reference, my primary cavities were made by Tx-Rx Systems, and have both bandpass and band-reject tuning controls on each cavity. When tuning them (either singularly or as pairs in-series), I see and can adjust the band-pass and notch for the desired Tx and Rx frequencies. However, in the broad-band sweep, I can also see a bunch of other signals passing through the cavities; all of which are well away from my Tx/Rx frequencies. I suspect that these are normal, and are a side effect of how the can-type resonant cavities work. Am I correct in this assumption? As a sanity check, I combined this set of cavities with another (Phelps-Dodge) set I have, and tuned these supplemental cavities for band-pass only (one cavity for my Tx frequency, and one cavity for my Rx frequency). Sure enough when I put these in series with my primary set, I can eliminate nearly all of the "other signals" from the broad-band sweep. This observation seems to reinforce my initial assumption about the Tx-Rx cavities. Do you agree? SECOND QUESTION TOPIC: The other ham thought that I should end up with a band-pass that is narrow enough to eliminate adjacent repeaters (at 16kHz spacing, as per the current SCRRBA band-plan separation). I tried but I cannot get either set of cavities to have that narrow of a band-pass; at least not without sacraficing most of the signal in the process. I suspect that the receiver and transmitter need to actually inforce these much narrower bandwidth requirements within the broader protection provided by the cavity-duplexer. I suspect that I should focus my duplexer tuning on passing the desired frequency, notching the alternate repeater frequency, and trying to do so with the least amount of signal attenuation. Am I correct in these understandings? For both of these question topics, feel free to point out anything that I might be missing or misunderstanding. I'm by no means sensative about this stuff, and still consider myself on the learning curve about duplexers and repeaters in general. Thank-you (in advance) for your time, thought, and opinions. <<< vern >>>
