Bill, In finding desense first one must locate the problem. A given.
I would start with putting GOOD dummy load on the duplexer output and do a desense test. This can be done with a local signal gen where you can vary the gen output and keying and unkey the transmitter. You should see no difference in the received signal with tx keyed or unkeyed. If you do then the desense is in the repeater. Next do the same test with the dummy load on the transmtter only. This will test for the desense being inside or outside the radio part of the repeater. I would look at your LMR400 and antenna. I think the LMR400 is a double shielded cable with different metals for the 2 shields. This is a no no in duplexed system. It generates noise. It has been discussed here on this board many many times and for good reason. If this is a problem replace with a good heliax. It is worth the cost. This is a start. 73, ron, n9ee/r >From: Bill Wilson <[EMAIL PROTECTED]> >Date: 2008/07/06 Sun PM 10:24:30 EDT >To: Repeater-Builder@yahoogroups.com >Subject: [Repeater-Builder] New Repeater Desense Problems > >Greetings Group, > >I had hoped to be able to purchase off-the-shelf components for a 2 >meter repeater for our local club to use. > >I purchased a Maggoire HiPro R1VHF35, 35 watt version, and with their >recommendation a set of FiPlex DVN-1533L Duplexers (6 cans). These >were installed with LMR-400 cabling and a 50 foot run to a Diamond X- >50 Dual Band vertical installed about 40 feet above ground. We have >a good ground, and the equipment is mounted in an open rack. > >The controller is a CAT-250, currently with COR receive only, >although the CTCSS module is installed but not turned on at this >time. I have been told however that the PL is being transmitted on >the transmit signal for decode. I also had the Narrow Band IF Filter >installed. > >Ever since we first hooked everything up, we have had receive desense >problems. The cans have been retuned 3 times by two different radio >shops in the area, and the problem still exists. With the exception >of the duplexer and repeater itself, every other component in the >system has been swapped out at least once. > >One observation by the first radio tech (ham) that came out was >the "very sensitive" receiver on the order of 0.10 uv, versus the >0.20 uv as advertised. On the third tune-up of the duplexers, >they "discovered" that the loops were 180 degrees out of phase, and >when they turned them, the duplexers "came right in". However, the >problem persists. Great for in town use, but that's about it. > >We have used varied lengths of cables between the repeater and the >duplexers, without any significant change in results. Next weekend, >I am planning on looking at the tuning myself with borrowed test >equipment. I know what I'm looking for pretty much, and it's got to >be close to correct to work as it does. > >Any thoughts ideas, etc., would be appreciated. > >Bill KJ4EX > > > Ron Wright, N9EE 727-376-6575 MICRO COMPUTER CONCEPTS Owner 146.64 repeater Tampa Bay, FL No tone, all are welcome.
