On Aug 30, 2007, at 12:35 AM, ldgelectronics wrote: > As a quick and dirty method, the radio horizon is: > > Distance (in miles) = Square Root of (2 * height in feet).
It's a rule of thumb, but isn't nearly as accurate as doing the real engineering on a system. > Power and frequency do not really play that much into it. This has > been mentioned in many stories of a repeater running just on the > exciter and not many noticed. Once you get past the radio horizon, > you cannot practically increase the power to get more distance. Humbug. Please tell all the VUCC holders at VHF and up that, and see if they laugh pretty hard. You can't just increase transmitter power, though -- you have to increase the overall gain of the system on both transmit and receive. The biggest bang for the buck in dB? The antenna system. Ask any Moonbounce specialist, or satellite chaser. They'll tell you the same thing. Ever seen the size/effort involved in a 2m EME array for CW? > So a radio transmitting with an antenna on a 200 foot tower will give > about 20 miles of coverage. With an average gain antenna, average feedline, and average power levels on both ends. > VHF goes a little farther than UHF, but it's not by a lot. Only part of the story. Try 220 MHz. It'll out-perform VHF by a large margin, in most cases. Why? Noise floor is lower in densely populated areas. (This won't hold as true out in the sticks.) > RadioMobile does a great job of factoring in many other things like > TX power, RX sensitivity, frequency, coax and duplexer losses and > some antenna modeling. After the learning curve, you can closely > approximate typical systems with ease. Yes! That's real engineering. The problem is... most people think that the rule of thumb is some kind of law, and don't bother doing the real calculations. Will this low-loss coax help my system? Will a bigger antenna or one with more directional characteristics help? Additionally, between 2m and UHF, most people run MUCH higher gain antennas, because they fit in the same physical space. One's UHF station can out-perform one's VHF station by a mile (no pun intended) if it has similarly sized antennas. I think rules-of-thumb in most things are great AFTER you "do the math" and know what their limitations are. If you haven't done that, they're a crutch and/or worse, a perceived limitation that really isn't there. A mediocre repeater, not tuned/optimized on a relatively low gain antenna with average system losses (cheap feedline, smaller antenna) out here on a mountain top (5000' HAAT) will perform approximately to your rule of thumb. A repeater with excellent receiver sensitivity by adding a good low-noise pre-amp (GaAsFET or PHEMT), very low loss feedline (7/8" hardline or better), and a relatively high gain antenna (perhaps with slight down-tilt) will perform FLAWLESSLY at your rule-of-thumb radio horizon and be useable for another 25-50 miles beyond that. Especially if the mobile user is also blessed with a better than average gain antenna with a nice pattern as close to the horizon as possible! Give that mobile an optimized yagi, and they'll be working the repeater far into the next state out here, and our states aren't very small! http://www.repeater-builder.com/antenna/3db.html <- A good article on the topic and the trade-offs for a repeater. Many people simply won't take the time, expense, and effort to do that level of engineering work on a repeater -- or a home station. But those that will, are rewarded with communications capability far beyond where the rule-of-thumb says it shouldn't work anymore. Like I mentioned above, the VUCC roles tell the tail. Never limit the new guy's imagination with a rule-of-thumb. Show them the TOP operator's achievements and tell 'em it's attainable by anyone with enough time, resources, and patient application of ALL the radio theory they can learn. Apply what those guys know to your repeaters and it'll make a significant improvement in performance, at the edges. ALWAYS recommend that anyone upgrading a station or building a repeater put their maximum effort into the antenna and antenna system. As my first Elmer used to say, "The antenna makes the radio." -- Nate Duehr, WY0X [EMAIL PROTECTED]
