On May 26, 2008, at 5:20 PM, Randy wrote: > Hello! I have a host of questions I hope you can answer for me or at > least comment on. > > 1. Our D-Star full stack is on a 9200 ft mountain. It is a > relatively high RF area. > All I can do is offer up what we've done at our test site, so far...
Commercial grade single-band antennas. UHF is a 4-bay folded-dipole Sinclair, VHF is currently a borrowed antenna. High quality hardline, no lossy stuff. 7/8" or better, depending on your cable run length. Good filtering to deal with the non-selective front-ends. UHF is a four cavity duplexer with an additional good quality pass cavity on the RX side, and room to integrate a second pre-selector can. VHF is a very high quality four cavity duplexer with really high-Q cans (big). No pre-amps. The receivers are very sensitive without them. No one (that I've seen) has done any measurements or studies on their noise- figure, so why drive the receiver down into the noise, without knowing? Also the pre-amp at a high-RF site is just as likely to help as to hurt, by dragging in off-channel junk that will just overload the receiver. No amplifiers. Our final site is almost 5000' HAAT. Our "test site" is about 2000' lower and blocked from most of the Denver Metro area, and places where big rocks are in the way... the thing still works. Impressively so, really. Putting amps on it just turns it into an "alligator". Balanced operation for HT users was our goal... if they can hear it, they can work it. Mobiles obviously perform better. If you're going for a balanced system for mobiles and unbalanced for HT's... add the amp. "Maximum coverage" is always a ham goal, but at the cost of HT users... they can hear the system but not reach it with 5W and a rubber-duck. Why? I think you might be surprised at how well the "bare" repeaters work with good filtering up front, especially at a high site. I think the techniques many have used to add amplifiers and pre-amps are really only needed to either a) get the system to work better at low-level sites compared to our mountain sites out here in the West, and of course... b) to squeak every last bit of performance out of it. Nothing wrong with either goal, but for your site, I would say: Take it in stages. Try it without all the add-ons and see how it does... then make careful, one-at-a-time, changes and carefully measure the performance difference. (Really this is no different than any other repeater.) You mentioned that other machines are 50W machines at the same sites... how many dB is 20W to 50W...? Not much. Mark N5RFX's work he did in his small but informative article for TAPR recently, is some of the best work on describing the receiver sensitivity that I've seen yet, and explains well why the digital systems seem to outperform the analog (even when the receivers he tested actually drop out 3dB sooner than analog would be copyable) but there's about a 17dB "slide" where analog sounds worse and worse, and digital is just fine. -- Nate Duehr, WY0X [EMAIL PROTECTED]
