Mike, that RG-8X is the first thing I would get rid of. It is typically only %95 covered in shield braid, and that is a path for a lot of leakage.
The rule of thumb in the whole repeater cabinet is double shielded RG-214 or RG-142. That takes away a lot of paths for leakage to bypass your duplexer. 73 - Jim W5ZIT --- On Fri, 7/18/08, Mike Besemer (WM4B) <[EMAIL PROTECTED]> wrote: From: Mike Besemer (WM4B) <[EMAIL PROTECTED]> Subject: [Repeater-Builder] Duplexer Tuning / Desense To: [email protected] Date: Friday, July 18, 2008, 9:33 PM My club has been offered a sweet deal on a repeater site (free rent, free electricity, free antenna and feedline) and I’m trying to hurry to get a repeater on the air. We have a Kendecom Mark IV which was hit by lightning (through the phone line, not the antenna) which I’ve resurrected and interfaced with a CAT-1000 controller. The old cans (Decibel DB4060 – 4 cans total) had been in storage and were apparently used by another ham for experimentation after they were taken off line. I’d replaced the notch caps a couple of years ago because whoever was experimenting with them tightened them until they cracked. I’ve also disassembled them and made sure they were clean and had no signs of arching or other damage. They were originally tuned on 146.25/85 (but as I said, they’d been ‘played’ with) and I’m moving them to 145.11 (minus 600 KHz). I first tuned them with a signal generator per the DB Products instructions and they tuned in very well. The peaks were good on both sides and the notches were about -85 db, using my crude measuring system. Losses on the pass-band were less then 2 db (probably more like 1 db). This pre-tuning was done with an HP-8640 signal generator, a scope, and other miscellaneous goodies. Tonight I got a service monitor (HP-8920) from a buddy who works for the FAA. I set it to duplex and checked my peaks. They were very close to perfect already, but I managed to get the last little bit of RF through the pass-cavities. Next I set out to tune the notches. I was able to get decent notches, but apparently they are not deep enough (despite having measured -85 with the signal generator) because I still have about 15 db of desense. I’ve been through them 4 or 5 times tonight (until I’m sick of them) but that’s the best I can do. (I did discover that if I tune the receiver notch enough, I can pull the transmitter down to nothing!) I just reviewed (again) Kevin’s article on Repeater-Builder entitled ‘Getting the most from your Repeater System’, and the one thing I need to do yet is check for internal desense. I had to replace the Teflon-type cable which leads from the Mark IV transmitter to the back panel, and when I replaced it, all I had was some RG-8X. I’m (sort of) hoping that’s the problem, but I won’t know until I check. I can’t see where I’ve missed anything obvious, although tuning these types of duplexers is new to me. I spent 17 years in the Air Force working navigation and comm gear, so I’m pretty well versed in what I’m doing, but since this is a new venture for me, I’m certainly open to suggestions. I’ll check for internal desense tomorrow, but in the meantime if anybody has anything to add, please chime in. TIA, Mike WM4B Kathleen, GA ._,___
