Signals corps and radio engineers learned a lot about radio propagation during WW2 solar cycle 17 – much of it the hard way. 

World War 2 coincided with Solar Cycle 17. It began in 1934, peaked as the war started in Europe, and then declined through 1944. Solar activity impacts radio communications. This had been studied scientifically and empirically, but there was much to be learned.

Part of the challenge was frequency allocation. Different branches of all the armed services struggled to find the right frequencies for different needs. Typically, lower HF was used for all sorts of purposes. These bands filled up fast. And, although they are more predictable during sun spot variations, they are still subject to increasing signal absorption during the day, and lots of long distance interference at night.

In some cases, planning assumptions were just plain wrong. For example, mainstream doctrine said that everything over 30 MHz was line of sight only. Early in the war, peak sun spot activity drove up the Maximum Usable Frequency. Everyone was surprised to find low-power VHF signals traveling a thousand miles or more. Radio signals on various frequencies could be heard from approaching aircraft long before they showed up on radar.

With everyone using radio on all fronts, lots of interference unintentional interference occurred simply because there was not enough information to allocate channels that would not interfere with each other, often over long distances.

On the other hand, the cycle was good news for wireless intercept. During WW2 solar cycle 17, well positioned monitoring stations could tune in to entire theaters of war at from far away. For example, the listening station just outside Winnipeg, Manitoba was responsible for monitoring German and neutral shipping, as well as the Japanese Navy. Monitors in New Zealand did a pretty good job of monitoring traffic during the Battle of the Atlantic.

Going into the war, much of the signals planning was geared towards transmitter power and receiver sensitivity. Actual experience showed that propagation effects and frequency selection was more important. Both Allies and Axis did a good job of organizing their monitoring stations to take advantage of propagation in the collection of signals intelligence. The Allies just did a better job of using the information.

Many of the radio techniques developed in the early 1930’s had to be reconsidered as the solar cycle strengthened in the early years, and then again as it waned. Another discovery was the effect of topography on the selection of wireless intercept sites. Mountains, forests and jungles blocked coverage, while deserts and salt water increased it, at least on the higher frequencies. Operation Market Garden illustrated what could go wrong when these factors weren’t addressed.

WW2 Solar Cycle 17 – Discovering Propagation Forecasting

By 1939, scientists had developed a fairly good understanding of the ionization and recombination processes that created atmospheric layers and radio propagation. Telephone companies and a few broadcasters had put together empirical charts for estimating quality of service. Unfortunately, these were only available for a few popular communications circuits, such as between New York and London.

However, with the explosion of military radio communications during the war, engineers and leaders recognized they needed much better tools for allocating frequencies and estimating signal path performance. This was especially important in many areas of the world where there was no data on ionospheric patterns. Various committees addressed this problem. New organizations sprang up, such as the Interservice Radio Propagation Laboratory reporting to the U.S. Joint Chiefs of Staff.

In short order, IRPL created new tools for evaluating and forecasting propagation, many of which are still in use. New methods were developed to short-term warnings of about radio disturbances. These included ionospheric and geomagnetic storms that disrupt communications, especially in polar regions. Technicians also developed an improved understanding of atmospheric noise and its impact, especially in tropical areas.

In 1943, IRPL published its famous Radio Propagation Handbook detailing these discoveries and techniques, also updated to reflect wartime experience in 1948.