And the Beast Lives

The saga of high frequency communication


Once upon a time in the 1800s, there was a scrupulous group of scientists participating in experimentation with electricity and magnetism without being classified as alchemists or wizards. (Sorry, J.K. Rowling.)

In 1802, Gian Domenico Romagnosi suggested the relationship between electric current and magnetism, but his reports went unnoticed. Less than 20 years later, Hans Christian Orsted performed a widely known experiment on man-made electric current and magnetism where he demonstrated that a wire carrying a current could deflect a magnetized compass needle. Then there was Nikola Tesla, who determined the feasibility of wireless communications and developed means to reliably produce radio frequency currents. Tesla’s U.S. patent classified him as the “inventor of radio.”

Guglielmo Marconi, who is also often classified as the “father of radio,” eventually equipped ships with life-saving wireless radios and conducted a reported transatlantic radio communications experiment in 1901. He then established the first commercial transatlantic radio service in 1907, a mere 20 years prior to Charles Lindbergh’s oceanic crossing.

Radio waves
Speaking of oceanic, sea currents — like radio signals — travel via waves. Just like their watery counterparts, radio waves are made up of peaks and valleys. Radio waves are usually produced by electric current alternating at radio frequency, flowing in a special purpose conductor called an antenna. Antenna dimensions must be specific to wavelength and tuned to work efficiently in a selected frequency range. Very long radio waves are not practical for aviation use because of the enormous antennas needed to produce them. Radio waves do occur naturally as a result of cosmic phenomena in deep space. In fact, any kind of reciprocating motion of electric charges or magnets can produce radio waves if the interaction is fast enough.

Propagation is a term that describes the travel of electromagnetic waves and consists of three modes. The first is a straight line travel, the manner that radio waves travel through deep space. The second way is to hug the surface of the earth and consists mainly of radio waves operating at very low frequency. Skip is the third mode –- as the name implies, it involves bouncing radio waves between the surface of the earth and the ionosphere.

Frequencies between 3 Megacycles or MegaHertz (MHz) and 30 MHz are most reliable for this kind of propagation and are known as high frequency (HF) waves.

Most aircraft communication today will involve the use of “line of sight” transmission and operate in the very high frequency (VHF) spectrum. This was not always the case in aviation. When dedicated communications radios took flight they were initially of the tried and true HF variety.

Radio waves travel in straight lines (except at very low frequencies) and the earth is curved, so we should not be able to communicate with anything we can’t see. In the Titanic era, ships used very low frequencies (500 KHz) to take advantage of a bending effect called refraction, but even then the range was only about 1,000 miles. (This is called the Ground Wave mode.) Above about 3 MHz, this effect is less pronounced. At the time it was quite unexpected that these shorter wavelengths could travel across the Atlantic Ocean or worldwide. The phenomena became known as Short Wave Radio.

Soon it was realized that the reflection of the waves from the ionosphere (Sky Wave Mode) could, under certain conditions, allow reliable long-range communication. This was of great interest to maritime operators and is still in use with ships today. Marine operators could send telegrams by Morse code, tele-printer over radio systems (TOR), or phone calls by single side band (SSB), especially to set up radio stations back in their home countries.

Current technology can even make HF radio an efficient means of sending and receiving email.

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