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#17
November 26th 03, 06:16 PM
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O.K. CAN YOU HEAR ME NOW ?
"Tarmo Tammaru" wrote in message ... There is almost always some kind of enhanced propagation. With 50W SSB to a 10 element 2 meter beam at 40 feet, 90% of the time my range is 75 - 100 miles when talking to another base station. I don't think they all have 1000 foot towers. Tam/WB2TT "Richard Harrison" wrote in message ... K1YW wrote: "So Artsci should help him (KC5CQA)." 50-watt radios will talk with plenty of exess fade margin over any line-of-sight path from a 100-foot tower to any land mobile in the two-meter band. Beyond line-of-sight, VHF propagation is kaput because the signal does not follow earth curvature as Reg`s low and medium frequencies do. Over smooth earth or sea, the horizon is sq rt of 200 from a 100-foot high antenna. That is about 14 miles. Add 4 or 5 miles of range due to the mobile antenna height and that is near the maximum range. 5 watts, 100 watts, or 500 watts make very little difference in range. Range is extended with increased power very slightly. There is some noise reduction. In my experience, I found it advantageous to use 50-watt mobiles and 500-watt base stations. This is because the mobile is often in a noisier environment than the base station. Obviously the path length is the same in both directions in this VHF application. The dominant requirement in VHF communication is a line-of-sight path. Once that is obtained, all else is secondary. Most of the microwave stations I`ve put in have 100-milliwatt transmitters and these produced 30 dB fade margins on paths of more than 20 miles. For land mobile VHF service, you usually have no significant antenna gains, but you also have significantly lower path and transmission line losses, higher receiver sensitivities, and higher transmitter powers. Received carrier power to produce full quieting in a good receiver is very small indeed. This makes line-of-sight the only significant requirement for single-channel, single-hop VHF communications. Noise is cumulative, so for a multi-hop system, big signal surplus is required on each hop to supress noise to make the cumulative total acceptable. Best regards, Richard Harrison, KB5WZI |
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