Celwave PD-497 duplexer/ general duplexer jumper cable lengths
Posted: Sun Dec 04, 2022 7:24 pm
Good evening:
It's been a really long time since I posted here and was active on the forum. I've been around though. My work retired some Celwave PD-497 6-cavity VHF duplexers some years ago and I snagged one. It was tuned for the railroad but could be tuned for ham. Recently I've had the desire to retune it and using an inexpensive NanoVNA, tuned the cavities and notch frequencies on each individual can the best I could for a 600khz VHF, 2-meter split. The repeater seems to be somewhat deaf... but I if it's not a radio receiver issue, my question for the membership here is a about a more abstract topic regarding coaxial cable jumper lengths and their absolute importance to a repeater system.
My duplexer cans had factory jumpers between them, but no 'T' antenna connector cables. In my test setup, I had the cables to the repeater mismatched by both type of cable and lengths... I think one was 8ft, the other maybe 6. I put a 'T' connector on the transmitter side of the duplexer to the antenna and and a 1ft jumper over to the rx side.
So, in my "test" setup, which according to my SWR meter on TX is really good, and field tests show that to be true, the RX side is really lacking. How much of a factor does equal length cables between the transmitter and duplexer, receiver and duplexer, and both legs to the T-connector for the antenna truly play when it comes to performance? I expected it all to be fair. My estimation is poor to marginal. Either the receiver is bad, or the repeater is being desensitized but I can't say which so far.
I plan on testing more tomorrow and have matched the cable lengths to the 'T' connector as well as the transmitter an receiver to the duplexer, but I am not hopeful for a better result.
-Josh
It's been a really long time since I posted here and was active on the forum. I've been around though. My work retired some Celwave PD-497 6-cavity VHF duplexers some years ago and I snagged one. It was tuned for the railroad but could be tuned for ham. Recently I've had the desire to retune it and using an inexpensive NanoVNA, tuned the cavities and notch frequencies on each individual can the best I could for a 600khz VHF, 2-meter split. The repeater seems to be somewhat deaf... but I if it's not a radio receiver issue, my question for the membership here is a about a more abstract topic regarding coaxial cable jumper lengths and their absolute importance to a repeater system.
My duplexer cans had factory jumpers between them, but no 'T' antenna connector cables. In my test setup, I had the cables to the repeater mismatched by both type of cable and lengths... I think one was 8ft, the other maybe 6. I put a 'T' connector on the transmitter side of the duplexer to the antenna and and a 1ft jumper over to the rx side.
So, in my "test" setup, which according to my SWR meter on TX is really good, and field tests show that to be true, the RX side is really lacking. How much of a factor does equal length cables between the transmitter and duplexer, receiver and duplexer, and both legs to the T-connector for the antenna truly play when it comes to performance? I expected it all to be fair. My estimation is poor to marginal. Either the receiver is bad, or the repeater is being desensitized but I can't say which so far.
I plan on testing more tomorrow and have matched the cable lengths to the 'T' connector as well as the transmitter an receiver to the duplexer, but I am not hopeful for a better result.
-Josh