Winter 2010 Issue

Rare Visual
Tropo Duct
Surprises the Microwavers

What started out this past November as a Thanksgiving Day turkey dinner on the beach for a group of amateur radio operators turned into six hours of looking inside a tropospheric duct! Here is how it happened.

By Gordon West,* WB6NOA

Photo 1. Los Angele Times newspaper photographer Wally Skalij takes a candid picture of Janet Margelli, KL7MF, carving the Thanksgiving Day turkey while Suzy
West, N6GLF, and Chip Margelli, K7JA, look on. (Photo courtesy of K7JA)


Every Thanksgiving a group of us southern California ham radio operators head to the beach to drop a turkey into boiling oil. Many times we bring along a radio and set up a small 3-element beam. However, this year, rather than operating, we were just going to soak up the sun and spot some faint indications of the common fall inversion layer.

A quick Doppler radar weather check before we headed to the sand clued us in on the last day of a persistent high-pressure system over the southland. There was not a cloud in the sky and the indicated sea surface winds were at nearly zero.

“What caught my eye was the Doppler radar return echoes of a phantom curtain of reflection about 10 miles off shore,” commented Suzy West, N6GLF. “On the short drive down to the beach, I couldn’t imagine how the weather service was getting such strong echoes,” added Suz. Soon we were to better understand what the radar was showing. However, before we get started with our understanding, let’s do a quick review of normal atmospheric dynamics.

Microwaves and light waves normally travel 1.1 times farther than the optical horizon. The formula used to calculate the distance of the radio horizon, in kilometers is D = A17h, where h is the height of the antenna above water. As ham operators, we know this as 4/3 radio horizon, traveling slightly farther than the visual horizon on a normal day. This day, however, would not be “normal”!

Our atmospheric dynamics exhibit a decrease in air pressure with altitude in an approximately logarithmic manner. The higher we go, the less air pressure there is. Air temperature also decreases with altitude, approximately 20° Fahrenheit for every mile of increasing altitude up to 40,000 feet. The number of water molecules also decreases with altitude, resulting in atmospheric density decreasing with height above the surface of the Earth.

The bending of both visual and radio waves is called refraction. The refraction of “normal” air is slightly higher than unity, around 1.000345. We know the refractive index of air increases in the presence of a stationary high-pressure system; as the heavier air within the high begins to sink, it is called subsidence). It bottoms out just above land, lakes, or seawater and becomes compressed with the continuing influx of descending air. Squish air and it gets warmer. This thin, and sometimes as occurred on this day, thick stratified band of warmer air can create a mirage, our English word that comes from the French word mirer, which means “to look at.” When considering mirages, down at an airfield or highway blacktop, you might see blue shimmers, like water. What you are seeing is not water. Rather, it is the refraction from the blue sky above. This is called an inferior mirage, where you look down and see the image of something above.

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