Not to hijack the thread, but a dummy load is NOT always a perfect test to eliminate the equipment. I think it’s a good start though.
I just solved a situation where I had 15 db desense into a 600ft feedline / antenna and ZERO desense into a dummy load. This was using a sample slug ( same function as iso tee)in my bird 43 where the feed line attached to the cabinet. My assumption that the complete repeater, duplexers, isolator and cables were fine into a dummy load led me to look at the hardline and antenna and prepare for paying for a climb. Before we could properly test the feedline and antenna, the mastr II PA failed. (VHF) after replacing the amp, the 15 db desense problem was gone. So the failing amp liked the dummy load better than 600ft of 50ohm feedline and antenna . Or some product from the amp excited the feedline /antenna to create something on our input. The low level products on our input creating the desense were not visible when I had looked at the spectral output of the amp before it failed. Nothing like moving a mobile amp into a continuous duty heat sink, on you tailgate, on the 4th of July! 73 Don Kirchner W5DK From: [email protected] [mailto:[EMAIL PROTECTED] On Behalf Of [EMAIL PROTECTED] Sent: Tuesday, July 08, 2008 9:39 AM To: Bill Wilson; [email protected] Subject: Re: [Repeater-Builder] New Repeater Desense Problems Have you run the repeater into a dummy load and check for desense? David ===================== From: Bill Wilson <[EMAIL PROTECTED] <mailto:kj4ex%40yahoo.com> > Date: 2008/07/07 Mon AM 02:24:30 WET To: [email protected] <mailto:Repeater-Builder%40yahoogroups.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
