At 07:24 PM 07/06/08, you wrote: >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
Who told you to use the LMR coax for duplex use? Doesn't anybody read the archives of this group? There has been at least a dozen instances over the last few years where people have popped up and been told that LMR CABLE IS JUNK WHEN USED IN DUPLEX SERVICE. Go to this page: <http://www.repeater-builder.com/antenna/ant-sys-index.html> Read the 4 paragraphs under "System Engineering" and also the second and third articles there. This is also of interest: <http://www.repeater-builder.com/antenna/andrew-shielding-effectiveness.pdf> As to what cable you should use... it all depends on how well you want to hear. Increasing the talk range is easy - how much amplifier and duplexer can you afford? The average amateur radio repeater is limited by the fact that every user wants to use a 2w or 4w handheld instead of a 30w of 50w mobile. If you can't hear them you can't repeat them, so a repeater that has a 1.5db of feedline loss (100 feet of LMR400 at 2m) from the antenna to the receiver is not going to hear as well as one that has 0.5db of loss (100 feet of 7/8 inch heliax at 2m). People never seem to think about feedline loss on received signals... Look at the bottom third of this page: <http://www.repeater-builder.com/antenna/wa2ise-coaxial-cable.html> and look at the loss per 100 feet. Always favor your receiver. Once you have lost received signal (i.e. it's down into the noise level) all the preamps do is bring up the noise. This is also of interest: <http://www.coasteltools.com/tech_cable_db_loss.htm> And some evening spend some time reading this: <http://www.repeater-builder.com/db/db-about-rf-communications.pdf> Yes, some of the info is 40 years old, but the physics of RF hasn't changed. Mike WA6ILQ
