The Art of Amateur Radio by Ron Hashiro, AH6RH and the (Emergency Amateur Radio Club) Wireless Dispatch.
This is the last of three articles on the art of amateur radio. In the first article, we took a look at the essence of radio waves. Last month, we looked at the similarity of radio waves and light and looked at ways to use reflections to enhance VHF/UHF radio communications. This month, we examine refraction.
Refraction occurs when a wave passes through materials that vary the speed of propagation. In effect, materials with more or less “densities” will slow down or speed up the wave. This causes the wave front to bend or refract. We see this when light passes from air to glass or from air to water. When the light wave encounters a material that is more “dense” the wave will bend towards the side that first contacts the denser material. The dense material “slows down” that side of the wave, causing the direction of the wave to shift.
An everyday example of this is a magnifying glass. The thick center causes light traveling down the middle to slow for quite a bit before it resumes normal speed, while the light traveling near the edges slow down only for a moment. This causes the waves to bend and come together at a focus.
Radio waves also bend. Recall our discussion that water absorbs and affects radio waves? Variations in water vapor in the air can bend radio waves and plays an important part in a phenomenon called tropo duct openings.
Normally, air temperature continuously decreases as one goes higher in the atmosphere. Occasionally, a weather phenomenon called a temperature inversion occurs that traps a pocket of warm air at higher altitudes. The warm air is sandwiched between two layers of cooler air.
Since warm air can hold more moisture, it has more water vapor than the surrounding cooler, dry air. Radio signals travel slower in the warm, moist air than in the cooler air. This causes the cool air to bend or refract radio signals towards the warmer air. In effect, the two layers of cool air now form a wave guide.
If a strong temperature inversion forms between Hawaii and California, it will form a natural wave guide over 2,000 miles long. This happens occasionally during the summer months of June to August and may last for a few minutes to a few hours. It was especially prevalent during July 1994 when an extra strong high pressure system dumped cool dry air from above and a lack of trade winds caused strong inversions to form below, causing the tropo duct to open for several days.
Another example of bending radio wave called knife-edging occurs near the tops of mountains. If the mountain top has a sharp ridge, the radio wave will bend down as it passes the top of the ridge, away from its original direction. In this setting, an antenna with low gain and a higher angle of radiation would work better than a high gain, low angle antenna.
This effect is especially pronounced and observable along the Koolau ridge line, where the top is often only a couple of feet wide topped with only grass and no trees. Vegetation such as trees and shrubs absorb the signal and ruin the effect. And a narrow ridge line would cause more of the signal to bend without impacting on a broad mountain top.
At the EARC meeting, Bob Hlivak, NH6XO brought out an interesting concept. An old-timer noted to Bob that trans-Koolau propagation is enhanced when there is good cloud cover and diminishes when skies are clear. Apparently, the clouds reflect the signal over the Koolau Mountains more than occurs through the knife-edging effect.
That evening, a quick test between myself and Bev Yuen, AH6NF showed the existence of radio paths that appeared to be enhanced by the complete cloud cover overhead. The signals could not otherwise be explained by the usual knife-edge model. This effect was observed that evening on VHF, and subsequently on UHF. Further tests are being conducted to further explore the concept.
The art of amateur radio seeks to explore and improve our knowledge of radio communications. This series of articles shows that amateur radio is an art built on science with practical applications. QSOs ranging from everyday contacts to critical emergency radio communications can be enhanced by leveraging basic knowledge of radio waves and creatively using any and all available opportunities that surround us in daily life.
I hope you’ll make time to exercise the art of amateur radio and explore the fun world of VHF and UHF simplex radio communications.