Tony VE6MVP wrote: > This will be the only repeater covering a fair bit of the terrain with > some overlap at the edges. I'm not quite sure just how much difference > 5 watts output vs 100 watts will make.
Why not do the math? :-) C'mon. It's not a secret. Nothing hard about it. Start at the transmitter... power level. You got that part. Now work your way to the other end of the "RF system", the mobile user... Find out these items: Feedline loss at the site from the transmitter to the isolator. Loss through the isolator, duplexer and any necessary filters. Loss through the feedline from the "filter system" to the antenna, including any accessories like lightning protection and/or connectors. Antenna gain in dBd or dBi. Antenna height above average terrain, and a bit of knowledge about what terrain features "block" your signal to various places you want to cover, or otherwise know you can't cover. Free-space path loss at your chosen frequency of operation. "Typical" user's antenna gain and feedline losses. Once you have all of that... you can tell a lot of things. One of the FIRST things you learn by documenting all of that stuff is what power levels you SHOULD be seeing at each test point, even so far as hooking a wattmeter right below your antenna and reading it there... if the numbers don't match, check your assumptions, or something is broken. (In other words, how can you KNOW your system is working if you don't have the above numbers committed to an RF "engineering" book for each repeater? Do it right. Write it all down. Keep a copy of the engineering book on-site, too.) > I'll let the other guy who > understands the modelling a lot better than I do generate the coverage > maps. Models are only as smart as the person who wrote them and the guy driving the modeling software. :-) > And about half the activity is due to mobile users such as > truckers as this repeater covers a chunk of a major highway. Not sure what this has to do with the RF performance, other than you're saying that you at least know what kind of rigs and antennas they typically use, and their usual routes within the repeater's coverage area (highways and truck stops). So you know a little about what your users will be like and what they want. That's good. :-) But not related to getting the maximum possible performance out of your system within any design limitations (like power utilization). Now... if you DO all of that homework, you'll often find for mountaintop sites that "altitude trumps power", and antenna gain trumps all... it's just basic RF physics. We have a site at 11,440' MSL that's about 5500' HAAT, and if we lose a PA, it generally loses some coverage, but the repeater is still copyable by mobile users 80+ miles out... it's just not as strong as it usually is. In closer, there will be users who are using outside antennas (not rubber ducks) who didn't notice the repeater dropped from 12W out the antenna to less than a watt. Mobile users will notice because when they're in areas where the repeater was weak before, it may drop out completely... which will just make the repeater sound "fluttery" as they move. And if the mobile users are out where they're away from obstructions and things that cause the weak spots, the repeater that was full-scale on their receivers, will now be a couple of S-Units less. They'll only notice that the S-Meter went down and maybe some slight additional white noise... since FM receivers aren't very "linear" about how they receive weak signals. This all assumes you have at least some decent level of antenna gain. Without antenna gain, you need power to make up for the inefficiency in the antenna's ability to get a signal to your users. In the end it comes to these priorities... - Best antenna possible. - Altitude, altitude, altitude. - Best receiver possible at the repeater (sensitivity and selectivity) - Low loss feedline, best filtering system possible (low loss but filters what's needed to be filtered) And then.. oh yeah, throw some power at it with a PA after all that above works, and "balance" the system for whatever users you are shooting for... mobiles, HT's with rubber ducks, whatever... Soooo many beginning repeater folks buy decent repeater gear, but... They don't buy good filters, don't buy pre-amps or anything to assist the receiver, don't buy good feedline, and don't protect their investment with lightning protection, etc... The poor repeater antenna is often a complete afterthought. Which is strange. The same guy who decided to put up a repeater might have an entire FARM of antennas for his HF part of the hobby, but it doesn't dawn on him that the antenna will make or break his repeater... And if you talk to him while he's building his first repeater, he's usually worried about power output before all else. Money spent getting the repeater higher (sounds like you guys have already done that part) with the best antenna system possible, will pay off in spades over adding 15dB more power to the transmitter. And remember that antenna gain is multiplicative BOTH directions... so a better antenna helps the system far more than a bigger PA. Nate WY0X
