Why Choose that Frequency?

You may have wondered how different devices came to use the RF frequency bands they use. This is not a trivial question. It usually depends on what people can imagine, what can be built, how urgently it’s needed, and what the market will bear.

Let’s start at the beginning. James Maxwell formalized electromagnetic theory with mathematics in 1864. Heinrich Hertz came along and verified Maxwell’s work a few decades later, demonstrating that light and heat were electromagnetic waves, later called radio waves. Guglielmo Marconi successfully transmitted a wireless signal across the Atlantic late in 1901, and the wireless age was born.

Marconi technology found its first commercial application in wireless telegraphy. It spurred the first International Radiotelegraph Conference in 1906 in Berlin, where a convention on wireless communication for ships at sea was signed by 29 countries. This convention

“… laid down the first international rules for wireless telegraphy — today known as the Radio Regulations — which became a cornerstone of ITU’s work. Very soon after its invention, this technology had been adopted as a way for ships in distress to summon help. It was necessary to set a standard for communication that everyone, from whatever nationality, would be able to understand, especially in an emergency.”

You can read more about this on the International Telecommunications Union (ITU) webpages: https://www.itu.int/itunews/manager/display.asp?lang=en&year=2006&issue=06&ipage=pioneers&ext=html

While its intentions were good, the convention was insufficient. During the night of April 14-15, 1912, the British luxury ocean liner RMS Titanic struck an iceberg and sank less than three hours later.  How could this happen?

In 1912, ships did not all use the same telegraphic call for distress. Marconi operators on British ships like Titanic used “CQD” while German ships had adopted “SOS”. There was also no requirement for a 24 hour-a-day watch officer to be on duty in the onboard telegraph office. If a distress message came in and no one was on duty, a ship could be delayed in coming to the rescue. Furthermore, the Titanic telegraph officers on duty the evening of the disaster were preoccupied, not with safety, but with sending the personal telegrams of wealthy clients onboard.

What happened? Even though Titanic operators received warning messages about hazardous icebergs in their vicinity, they ignored them to attend to passenger messages. After the Titanic struck the iceberg, the distress message “CQD” was sent out. This confused operators on other ships who used “SOS”. Rescue ships were further delayed because they did not all have telegraph operators on duty to hear or respond to Titanic’s call for help. Although Titanic telegraphers began sending the “SOS” distress call later, the luxury liner’s fate was sealed. The telegraph saved over 700 souls that night. Inattentiveness to and confusion about the urgent messages sent contributed to the number (more than 1500) that perished.

A few months later, at the Second International Radiotelegraph Conference in London, the international community agreed to use a 500 kHz frequency for ships’ radio distress calls. Furthermore, ships were to observe three minutes of radio silence a quarter after and a quarter before the hour to listen for distress calls. In the United States, Congress passed the Federal Radio Act of 1912, establishing its legal right to regulate radio frequencies in the United States.

Marconi’s wireless technology was finally harnessed for broadcast purposes with commercial AM radio after World War I. In the United States, the first licensed AM radio station was Pittsburgh’s KDKA station in 1920. AM broadcasting took off in the 1920s, and the AM spectrum became a mess of interfering stations. In response, the government reasserted and expanded its spectrum authorities in 1927, allocating AM frequency bands designed to avoid interference. The Federal Communications Act of 1934 established a new regulatory body – the Federal Communications Commission (FCC) – for frequency allocation. Today, the FCC licenses the use of all spectrum in the US, including that of AM radio stations operating in 10 kHz bands from 540 kHz to 1700 kHz.

Frequency Modulation (FM) radio was invented by Edwin Armstrong in the 1930’s. Its improved sound (e.g., in music broadcasts) came at the expense of higher bandwidth (200 kHz) requirements and more complex technology. Entities invested in AM technology sought to strangle FM before it could emerge on the broadcast stage. US entry into World War II further delayed its commercial success. FM would finally come into its own in the 1950s and overtake AM in the ensuing decades. And the FCC licensed (and still licenses) spectrum to FM radio stations in the 88 MHz to 108 MHz frequencies.

We already saw in a past blog that the first operational radar system in Britain used old-fashioned HF (25 MHz) instead of newfangled microwave (1 GHz to 100+ GHz) technology. The Brits used HF because they didn’t know enough about microwave technology to build their urgently-needed radar early warning system. Many remarkable advances in radar technology were made during World War II that extended operational radars into the GHz frequencies. Today, the ITU designates frequency bands for radar from 1.5 GHz to 40 GHz (see https://www.itu.int/dms_pubrec/itu-r/rec/v/R-REC-V.431-8-201508-I!!PDF-E.pdf)

We’ve seen how different technologies came to use different frequencies. Wireless telegraphy and AM radio leveraged the frequencies that inventors understood. They captured market share by granting people the ability to communicate, receive news, and be entertained. They were also used to implement safety, distress, and rescue measures. Competition for spectrum usage led to frequency regulation in national and international jurisdictions. Urgent requirements during World War II spurred development of new applications, broadening spectrum usage from well-understood frequencies to experimental bands. Today, radio frequency usage continues to follow the arc of what can be imagined, what can be built, the urgency of the capability, and what the market will bear.