Army Studies Radio Wave Curvature
September 1956 Popular Electronics

September 1956 Popular Electronics Table of Contents Wax nostalgic about and learn from the history of early electronics. See articles from Popular Electronics, published October 1954 - April 1985. All copyrights are hereby acknowledged.

Early investigations into RF signal atmospheric "ducting" was reported in this 1956-era article in Popular Electronics. Ducting effects were first noticed during World War II when Nazi broadcasts from occupied Paris were received occasionally in London. Scientists discovered that a small change in the humidity of the air near the surface has the effect of trapping radio waves, a trapping process dubbed "ducting." These waves are conducted as if they were inside of a metallic waveguide. Research by the U.S. Army Signal Corps determined a sudden temperature rise at around 50 to 200 feet above the surface appears to have the strongest effect.

Army Studies Radio Wave Curvature

Weather tests are being conducted by Army to analyze effects of temperature and humidity changes on microwave transmissions. At left, members of meteorological team prepare "Kytoon" - a helium-filled nylon-covered balloon - for ascent.

The Army Signal Corps is carefully watching weather effects on microwaves with an eye towards "seeing" beyond the horizon. Normally, microwaves - used by, u.h.f. TV stations - can only travel from transmitting points to the horizon, or within line of sight. However, certain types of weather conditions trap these radio waves and curve ,them around the horizon.

Personnel of the Aviation and Meteorological Dept., Army Electronic Proving Ground, Fort Huachuca, Arizona, are interested in determining the exact constitution of these unusual conditions. Electronic measuring instruments have been developed that will detect minute changes in the temperature and humidity of surrounding air.

Scientists have discovered that a small change in the humidity of the air near the surface has the effect of trapping the radio wave. Microwaves caught by trapping are said to be "ducted" - or passed along as if they were inside of a metallic wave guide. A sudden temperature rise some 50 to 200 feet above the surface appears to have the strongest effect.

The Signal Corps needs to know the extent of these effects to judge whether or not microwave transmissions can be intercepted by an enemy. During World War II ducting was suspected when TV pictures from the Nazi-held transmitter in Paris could be seen across the English Channel - to the military advantage of the British.