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We ran the "live" test today - a simulated emergency based on an actual one 5 years ago with all the public safety services involved.

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?This time, we had a 3 element Yagi instead of the J-Pole used previously at north, with the same 3 element Yagi at south. We found that voice communications through that system went from a bit scratchy to full quieting with the new antenna. This was all at about 9:30 AM.

This just conforms what I said in a previous post -- that FM voice comms must be fully hard-quieted before data is going to work decently.? Signal levels with hiss or "popcorn" on them, that remain intelligible on voice,? just won't work for data.? At least for high-speed data.??? 300-baud HF-style packet or VARA-HF run over VHF/FM radios can tolerate a bit of noise but will be VERY slow.

I have actually experienced this.? Several years ago, I was experimenting with APRS tracking for an event in Evergreen Colorado.? Evergreen is located in an L-shaped canyon in the Front Range of the Colorado Rockies, with mountains thousands of feet higher all around it. VHF radio is a nightmare of non-line-of-sight paths with massive amount of multipath reflections, even on strong signals.? 1200-baud packet APRS failed miserably, even on paths of only a few miles.??? On a hunch, I tried switching my UZ7HO Soundmodem soft TNCs to 300-baud HF-style packet.??? BINGO!?? Things worked perfectly even on noisy non-quieted multipath-riddled paths.? Over TEN TIMES as many posits succeeded compared to the 1200 baud built-in TNC of the Kenwood D700.????

For passing substantial amounts of traffic, the speed hit might be a problem, but in my case I only needed to pass compressed position reports.? Even at 300 baud, they were only about a second each.

On the issue of paths working in the morning but failing later in the day, this is quite normal for marginal horizon-grazing non-line-of-sight. paths on VHF & UHF.???

Beyond the optical horizon, VHF propagation depends on refraction (bending of the wave front).? In turn, this refraction depends on air masses of non-constant density. ? In the morning, when the ground is rapidly heating from the sun, temperatures near the surface rise faster than a few thousand feet up, creating a layer of less-dense air near the ground.? This bends VHF signals beyond the horizon.? By noon, when the air has heated more uniformly up to several thousand feet, the refracting effect is far far less - signal levels can drop by 10-20-even 30 dB on non-line-of-sight paths.??

At sundown, when the solar heating of ground & air stops, the same thing happens.? Signal levels in the early evening shoot up when air near the ground doesn't cool off as rapidly as air higher up.? By several hours after sunset, when air has cooled off more uniformly at all levels, signal levels drop off again, until the next morning's sunrise.?

The key to reliable paths is to have enough excess signal level (power plus antenna gain) that even with 20 dB or so loss, you remain hard-quieted at the receiver.?

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Stephen H. Smith??? wa8lmf (at) aol.com
Skype:??????? WA8LMF
EchoLink:? Node #? 14400? [Think bottom of the 2-meter band]
Home Page:?????????

APRS-over-VARA igates now operating on 30 & 60 meters
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"Studio B" Ham Shack on Wheels
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-- APRS over VARA? --
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