John, While incorporating feedthrough capacitor filters is fairly easy in a new design or a homebrew project, it is not so easy to convert an existing commercial product. In my own Yaesu/Vertex repeater, both the receiver and the transmitter modules have their control and power connections brought into the module with plastic "mini-Molex" inline connectors. It would be next to impossible to "convert" such connections to feedthrough filters.
I suggest you go back to your original plan of replacing the single-braid coaxial cable jumpers that exist inside the repeater cabinet, before getting sidetracked into other potential cures. In any case, as I noted in a previous posting, replacing the gray single-braid jumper cables with RG-400/U cable completely cured my own desense problem. I did not need to modify the modules with feedthrough filters or perform exhaustive probing of the radio to locate or cure the desense. From the outset, it seemed likely (to me, anyway) that leakage from the transmit side was causing the desense. As I recall, the replacement of the bulkhead-to-RX-module jumper with RG-400/U cable resulted in about a 10% improvement. Replacement of the exciter-to-PA jumper resulted in about a 10% improvement. An 80% improvement resulted when the PA-to-TX-bulkhead jumper was replaced. Even though the latter jumper was only about six inches long, it was "bathing" the inside of the repeater cabinet with RF. Based upon that experience, I suggest replacing that jumper in your repeater first, and then testing for any improvement. If it results in a cure, you can button it up and get on with your life! Here's another thought: In another thread, we had some discussion of 3M (Scotch) #1170 conductive-adhesive-backed aluminum shielding tape. If you can get your hands on some of that stuff, you can try over-wrapping the jumper cables in your repeater with shielding tape, to see if an improvement results. Who knows- you might have a cure without doing anything more intrusive... 73, Eric Lemmon WB6FLY -----Original Message----- From: [email protected] [mailto:[EMAIL PROTECTED] On Behalf Of John Transue Sent: Sunday, August 31, 2008 6:42 PM To: [email protected] Subject: RE: [Repeater-Builder] Wits End -- Desense JohnR, Thanks for the explanation. I'll look at the web site. I suppose the capacitance should be chosen just to conduct the UHF RF to ground. JohnT -----Original Message----- From: [email protected] [mailto:[EMAIL PROTECTED] On Behalf Of John J. Riddell Sent: Sunday, August 31, 2008 8:16 PM To: [email protected] Subject: Re: [Repeater-Builder] Wits End -- Desense John, Feed thru capacitors look like a peice of wire with a "blob" in the middle, and have a 1/4" locknut on one side. You drill the chassis and insert the feed thru cap then mount it tightly with a nut. Then attach the incoming wires to the outside of the cap. Check Maggiore's web site...you might see a picture of them there as they use them on their Tx and Rx assemblies. John VE3AMZ ----- Original Message ----- From: John Transue <mailto:[EMAIL PROTECTED]> To: [email protected] <mailto:[email protected]> Sent: Sunday, August 31, 2008 4:55 PM Subject: RE: [Repeater-Builder] Wits End -- Desense Chuck, John Bob, Derek, Eric, Mike, JohnB, and Tom, Thank you all for the helpful suggestions. The connectors appear to be perfect, and they check out with DC. The cable on the RX side is double shielded RG142B/U. The cable on the TX side is not marked with a complete designation. It says AWM Style 1354, but there are many cables that are said to be UL AWM Style 1354. Some of these are 75 ohm, some are 50 ohm. This cable is only six inches long. I plan to replace this as soon as I can get RG400 or other suitable cable. BNC connectors are used on both the RX unit and the TX unit. I don't know what it means to have a feed through capacitor in series with a control or audio or power cable. However, a capacitor to ground might be good to get rid of stray RF. Pray if you are so inclined. Otherwise, hope we can make the physics work. Thanks again, JohnT -----Original Message----- From: [email protected] [mailto:[EMAIL PROTECTED] On Behalf Of Chuck Kelsey Sent: Sunday, August 31, 2008 3:06 PM To: [email protected] Subject: Re: [Repeater-Builder] Wits End -- Desense I was wondering the same thing. Has someone added some wiring and subsequently breached the integrity of the shielding effectiveness somewhere? First thing to come to mind is controller interfacing and CTCSS wiring. Chuck WB2EDV ----- Original Message ----- From: John J. Riddell <mailto:[EMAIL PROTECTED]> To: [email protected] <mailto:[email protected]> Sent: Sunday, August 31, 2008 2:43 PM Subject: Re: [Repeater-Builder] Wits End -- Desense John, Make sure that every wire going in to the RX and TX box has a feed thru cap in series with it. Also you might put a few small ferrite beads on each wire as well. John VE3AMZ __________ NOD32 3192 (20080616) Information __________ This message was checked by NOD32 antivirus system. http://www.eset.com <http://www.eset.com>
