IONOSPHERE-HOLES AND RADIO PROPAGATION By Robert H. Welsh For those of us that have been around radio for some time, many of us began our interest in short-wave radio by listening to the various short-wave broadcast stations. These stations have been operated by many different nation-states and commercial enterprises. The larger, nationally funded transmitters used AM modulation, powerful transmitter stations, and directional antennas. They operated in the 60, 27, and 25 meter bands. One can still copy some of these international stations in the high end of the 40 meter band. I began listening to these stations on my grandparents PHILCO AM, FM, SW radio near the New Jersey shore. Occasionally, I would listen to amateur radio operators AM transmissions as well. I found all those transmissions exciting, and I tried to understand how those radio waves could travel such long distances. Later in my life, while stationed with the U.S. Army Security Agency at an electronic intercept site in northern Turkey, I installed a Collins R-390 receiver in the tracking van where I worked. My operator and I regularly listened to the BBC from London and Radio Hilversum from the Netherlands. These transmissions provided us with news and music. Of course, stations such as the Voice of America or Radio Moscow provided interesting sources of propaganda from their respective governments. For amateurs, these international broadcast stations could be used to measure radio propagation from different parts of the globe; but, there have been other uses of international short-wave stations. I will focus on these other uses in this article.
MISSILE EXHAUST PLUMES SHORTWAVE RADIO SIGNALS During a different assignment at a U.S. Army Security Agency electronic intercept site in northern California, I maintained and operated a short-wave monitoring system. Inside a communications van located outside the main operations building, we connected a large number of Collins R-390 receivers to several rhombic antenna systems that were directed toward Australia and Asia. Signals copied by these different AM broadcast stations were detected and coupled to recording and analysis equipment. During each experiment, the U.S. Navy launched missiles from their site at Point Mugu California. We monitored how the high-temperature exhaust plume from the missile engine changed the ionization of the D, E and F layers of the ionosphere as we copied the various stations. The typical exhaust plume temperature of these missiles is on the order of 2300 Kelvin (almost 3600 degrees Fahrenheit). A radar located at Stanford University provided missile altitude data via an HF radio
Figure 1. Potential atmospheric effects due to space-power rocket launch. (Courtesy Advisory Group for Aerospace Research & Development, Conference Proceedings, Seine France, Number 295, April 1981, p. 45)
link just outside the 20 m amateur band. The data gave us knowledge of when the missile passed through different layers of the ionosphere. See Figure 1. We measured the receiver’s AGC voltage as the missile passed through the F-layer of the ionosphere at altitudes of 160 km (100 miles). Note that the F-layer has the highest concentration of free electrons and is considered the most important layer for long-distance HF communications. As the missile passed through the F-layer, we observed that the short-wave broadcast signals varied. The AGC voltage of the receivers indicated a decrease in signal strength. After the missile passed through the F-layer, we observed a slow increase in signal strength until the signal strength returned to the level before the missile passed through that layer. What then is the effect of the exhaust plume on the ability of the ionosphere to maintain the propagation of short-wave radio signals? Experiments have shown that the exhaust plume of a missile chemically induces a change in the ionosphere. Water and hydrogen in the
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