Thanks. This is a great write up and one for the archives. As soon as we get over to pickup the service monitor (and some free tower sections) I will let you know the specs on it and we will start trying some of these tests.
Vern KI4ONW On Tue, 16 Oct 2007 13:25:44 -0600 Nate Duehr <[EMAIL PROTECTED]> wrote: > [EMAIL PROTECTED] wrote: >> I think it's an HP of some sort but someone is using it >> right now and we have to get it back. The person who >>owns >> it actually has 2 of them but one needs some work and he >> is probably going to sell it rather than fix it. > > If it's an HP 8920 series, that's virtually top of the >line for analog > repeater work, and it would be very wise of you to both >get it back (!) > so it doesn't wander off, and also to learn how to use >it. They're > worth a bunch of $. > > There's virtually nothing you can't fix and/or >measure/see with one of > those... especially if they have the Spectrum >Analyzer/Tracking > Generator option installed, which you said it doesn't >have. > > But even without it, if it's an 8920 series, that's an >excellent piece > of test gear. > > If it is one of the later model HP's, you shouldn't >waste any time in > learning its features and how to use it! > > Be cautious, some models (ones with the option for high >accuracy) have > power input limitations, and you can damage them if you >put too much > power into them. They're usually labeled clearly with >the power > limitation near the input N-connector. > > Get it in-hand, get the model number off of it, and ask >here on the list > for assistance... a number of folks here can help with >how to set up > basic tests on your repeater with that test gear, if >it's what I think > it is. > > I'm not going to guess, though. Guessing leads to >assumptions and that > leads to ... well we all know what the word assume >stands for -- but it > also leads to poor repeater performance. (GRIN) > > The number one advice I got from veteran >repeater-builders when I > started working on these things... don't guess. Be >scientific. Measure > everything, measure again, then compare the measurements >against the > manufacturer's specifications for the radio. If they >don't match, come > up with a theory and then make incremental changes to >alleviate the > problem(s) one at a time, re-measuring along the way. A >scientific > versus an "amateur" approach, if you will. > > It'll pay big dividends in your repeater's performance, >and also in > maintenance time spent hunting problems down. It's a >lot easier to KNOW > what's not performing to spec versus guessing or >"hunches". > > The basics are: > > - Everything on-frequency (receiver and >transmitter/exciter), and > deviation levels set accurately? (Your service monitor >can help here, > as long as it's accurate... good to make sure when >starting with an > unknown service monitor, and to always check its >receiver against a > known source before starting on-site checks.) > > - Whether or not the system exhibits any desense >(easiest test of all... > already documented in previous posting) > > - Receiver sensitivity without antenna system connected. > (Known power > level signal generator/RF signal source needed here and >preferrably a > way to measure the 12dB SINAD point.) > > - *Usable* receiver sensitivity with antenna system >connected (Iso-T or > directional coupler additionally needed, and a way to >measure the 12 dB > SINAD point of the receiver.) > > ^^^ All of the above affected by whether or not you have >a pre-amp > installed, the quality of your interconnect cables, and >transmitter > power versus isolation offered by your duplexer. > > Other things which can factor in: > > - Transmitter "cleanliness" -- is it clean or is it >throwing crud up and > down the band. (A great way to become an unwelcome >neighbor to all, > very quickly.) > > - Antenna performance (does it duplex well?) > > - Feedline quality (forward loss, return loss -- how >much RF can get > through that cable?) > > - Site noise or other high-power transmitters (even >transmitters > off-frequency outside the ham bands can overload the >front end of > low-quality receivers if they're really powerful on a >nearby antenna!) > > Etc etc etc... it's really good to measure everything >before starting > and then, as I mentioned, make incremental changes and >see what helps > and knowing exactly how much. > > Your repeater users will love it if you can squeak the >maximum > performance out of your systems. Although, there's >always going to be > "that guy" who wants to hit the repeater using 150 mW >from his basement > 20 or more miles away... some people just don't realize >when they're > asking you to push the laws of physics and the typical >performance of > radio gear right to the limits! (GRIN) > > Nate WY0X
