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Summer 2003 Issue |
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Watch the Weather |
 Paul Lieb, KH6HME, and the first California to Hawaii multi-band transmitter. |
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Summer
and fall bring tropo DX to its highest levels. Just imagine an FM and an
ATV path completed over 2500 miles. |
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Serious VHF and UHF weak-signal operators spend most of the winter gearing up for summertime tropospheric-ducting DX. New FM operators might be surprised when they answer a general call on their repeater from a base station located 1200 miles away. Imagine the surprise of the Long Island Amateur Radio Simplex operators on 147.510 MHz yakking FM when Frank, AA2DR, a well-seasoned VHF/UHF DXer, begins to hear faint signals from a mobile driving by the Kennedy Space Center. Without fail, summer and fall bring tropospheric-ducting excitement. If you watch the weather patterns, you might be able to predict that the band will literally go “wide open” for tropo DX the day before it actually happens.. The term tropospheric ducting refers to the stratification of the air within our weather atmosphere, abruptly changing the normal refractive index. An abrupt change of refractive index from one medium to another is easily visualized by dropping a pencil into a glass of water that is half full. The pencil looks bent, doesn’t it? Now you might imagine VHF and UHF radio signals that normally travel in a straight line becoming “bent” within the sharp edge of an approaching cold front or a long ridge of high pressure pushing down from a cell above. “The formation of a duct is the result of the physics of the fluid. Any combination of fluid parameters which contribute to a sufficiently thick layer of super-refractivity may cause ducting,” comments Bruce Eggers, WA9NEW. He points out that a “sufficiently thick” super-refractive layer may minimize signal losses through the top area of the duct and be sufficiently elevated so as to minimize losses on the bottom side of the duct. He indicates that this could cause field-strength attenuation nearly proportional to 1/R rather than 1/R squared. “Under ideal ducting conditions, a signal from a transmitter 5000 miles away may have the same field strength as an identical transmitter only 70.7 miles away from common frontal layers and early morning radiational cooling of the lower layer of the atmosphere,” adds Eggers. “I can spot a tropo duct as soon as I take off,” comments Bill Alber, WA6CAX, an avid airborne ham operator, who delights in working within the duct. “I look for the suspended brown air, fly into the duct, and watch my outside temperature gauge jump by 8°F, then start working 2 meter and 432 MHz SSB 500 to 700 miles away along the duct,” adds Alber, boasting his best airborne record of 2500 miles to KH6FOO in Hawaii. Paul Lieb, KH6HME, who operates at the 8000-foot level on the side of the Mauna Loa volcano, agrees with tropo experts about watching weather patterns, looking for the cloud boundary layers and observing that 8° to 10°F rise in temperature.
“I worked over 350 contacts during three days
one summer when the cloud layer was just below the transmitter site, and
my outside thermometer showed a 10°F temperature increase from what the
Hilo Airport was reporting down on the deck,” comments Lieb, world-record
holder of tropospheric ducting on multiple VHF/UHF bands. It takes him
over two hours to drive up to the operating site on the side of an active
volcano, but when he hooks into his Yagi antennas beamed toward the west
coast of the United States 2500 miles away, he talks from Mexico to
Washington state with ease. |
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