[EMAIL PROTECTED] wrote: > Well the local ham fest rolled into town last weekend and > I was able to get some cans. One of which is a DB > products bp can model sp6610C. A friend tuned it up for > me and I put it between the MASTRII on the receive side > and the receive side of the db4048 duplexer that I have > which wasn't providing proper isolation. It's like I have > a new repeater. I have not been able to do any extended > testing yet but I can run lower power on my mobile around > town then before and people on HTs that couldn't hit the > machine from in their house can now do so full quieting.
Now if you want to take the next step, and go from a "thinking" man to a "knowing" man... Use an Iso-T or directional coupler, and a signal generator on your input frequency and some way to measure the 12dB SINAD point of the receiver. (If you're using a service monitor, they'll usually do both the signal generation at an accurate power level, and the SINAD measurement, but you can also use a much cheaper standalone signal generator and something like a Sinadder S/N measurement device.) Measure the real-world receiver sensitivity numbers, put them in the engineering logbook, and track it over time. It allows for a lot of things, like knowing if your receiver suddenly has "gone deaf", etc. Also allows you to track it over time to see if you have a slow degradation going on, site noise going up, etc. While you're set up to do that test, desense is easy to test for, too. Inject a weak signal into the receiver and set it to the "noisy" 12 dB SINAD point, with the repeater transmitter off. Turn transmitter on. If weak signal disappears or gets noisy, you don't have enough isolation, you have leaky interconnect cables, or something else is wrong... you have desense. A properly working system should have none. None at all. You should hear no difference at all with the transmitter on or off. If you have desense, leave the transmitter on, and turn the signal generator up until you're back to 12 dB SINAD. Now you know how much the power level had to change to get back to your arbitrary reference point, and therefore, you know how many dB of desense your system is experiencing, so you have hard numbers as to how bad the desense is. Then you can share here on the list, and folks that have used the type of radio and setup you've built -- can tell you if that's what they would expect to see from your specific radio type, etc. If you get in the habit of doing these tests at least once a year on established systems, and/or during an installation of a new system, and recording all the numbers -- you then know right away that the system isn't performing as well as it should, opposed to finding out by "shotgunning" in more filtering later on. Knowing up-front is nicer, once you get in the habit of going through the extra (minor) work of doing the real tests. Nate WY0X
