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Discoveries in science usually begin with something in the world around us that we would like to understand. The phenomenon comes first, then we work out a theory to explain it.

But as the American Association for the Advancement of Science reports, sometimes the discovery process goes the other way. A theory may successfully explain one phenomenon — but the same theory also implies the existence of other phenomena that no one has ever yet observed or perhaps even imagined.

In the late 1800s, a German physicist named Heinrich Hertz confronted a challenge of just this sort. Some years earlier, in 1865, British physicist James Clerk Maxwell had set forth a theory that linked electricity and magnetism. These early discoveries would lead their successors to creating some of the technologies we rely on every day.

Maxwell’s theory was the first of what are now called field theories. It not only showed that electricity and magnetism were related; it also demonstrated that light waves resulted from an interplay of electric and magnetic fields. It was the first big step from the billiards-ball universe of Sir Isaac Newton to the universe of fields as we understand it today.

But Heinrich Hertz had an additional insight. He realized that Maxwell’s theory didn’t just explain familiar visible light. The theory also implied that a similar interplay of electric and magnetic fields — what we now call electromagnetic radiation — should take place on other scales, producing waves of electromagnetic radiation that would be vastly longer than light waves.

With only Maxwell’s equations to guide him, Hertz came up with a simple device, combining an electrical induction coil and a Leyden jar (an early form of capacitor) that — in theory — could produce invisible electromagnetic waves. He paired this with another simple device, two brass spheres with a spark gap between them, to detect the waves.

And they worked! As Hertz wrote in his 1889 report of his discovery, the sparks “are microscopically short, scarcely a hundredth of a millimeter; they last only about a millionth of a second. It almost seems absurd and impossible that they should be visible; but in a perfectly dark room they are visible to an eye which has been well rested in the dark.”

Heinrich Hertz had built the world’s first radio transmitter and a receiver to pick up its signal. Physicists were, well … electrified by his achievement, which not only confirmed Maxwell’s theory but also showed that light was only one small portion of a much broader spectrum of electromagnetic radiation.

At first, to the wider world, Heinrich Hertz’s work was simply a cool scientific discovery with no practical applications. But in the early 1890s, per Columbia University, “Hertzian waves” as they were then called, caught the attention of a technology-minded young Italian, Guglielmo Marconi.

For decades, people had been looking for some technology that would allow a telegraph link without wires — avoiding the cost of stringing lines and the constraint of fixed routes. But none of the early attempts at “wireless” communications worked out.

By 1894, Marconi had improved on Hertz’s purely experimental apparatus — and by the next year, he could send and receive signals across a distance of more than a mile, even with a hill in the way. Two years later, he applied for a British patent and demonstrated his technology to officials from the British post office and observers from the military.

In 1897 Marconi launched a company called the Wireless Telegraph and Signal Company. Communication from and to ships at sea was one of the first applications Marconi pursued, and a Marconi device was used as early as 1899 to summon rescue after a ship collision. Then, in 1912, the Titanic disaster showed the world the critical role that wireless communications could play in rescue at sea.

So, what are radio waves used for? The most common use is for communication. From the pioneering wireless telegraph to the cell phone, sending and receiving messages has been the most widespread and familiar application of radio-frequency technology. But other uses were also soon found for radio-frequency (RF) waves.

All electromagnetic waves, including RF waves, carry energy and can be used to heat materials. For example, microwave ovens are an offshoot of radar technology, but according to the book Manufacturing Techniques for Polymer Matrix Composites (PMCs), the idea of radio-frequency heating has been used for resin and composite curing since the early 1900s.

Radio waves can also be used for direction finding, a technique closely tied to Marconi’s early insight about radio and maritime safety. Radio waves, like visible light, stream outward from a source — and with the right type of receiver, you can determine what direction they are coming from. Thus, radio beacons can serve as the equivalent of lighthouses but are detectable through fog.

Since the 1920s, radio direction-finding has been particularly important in aviation. But today, a variant technique — using precise timing of satellite transmissions rather than direction of the transmission — is the basis for GPS location technology.

If a radio transmitter can function as the equivalent of a lighthouse, it can also function as the equivalent of a spotlight. Per the National Weather Service, Heinrich Hertz himself had found that objects could reflect the electromagnetic waves he detected. In 1904, a German inventor even patented a method for ships to radio-detect hazards to navigation, though his device did not prove usable enough for service.

In 1922, experimenters at the U.S. Naval Research Laboratory near Washington D.C. found that passing ships on the Potomac and Anacostia rivers disrupted transmissions from their experimental high-frequency radio equipment. The research team recognized that the same principle might be used to detect enemy ships.

Incremental but steady progress continued in the years that followed, and in 1937, the British government began development of the world’s first radar early warning system. A high-power transmitter technology, called the magnetron, provided a crucial element of the system, which would be decisive in the Battle of Britain.

Radar was the most dramatic wartime application of RF technology, but World War II gave an enormous boost to every aspect of the technology. The sheer amount of RF equipment built and used was impressive. Unsurprisingly, new RF technologies such as television — still semi-experimental before the war — took off in the postwar era.

The American Physical Society reports that, as researchers in the early 1930s were exploring the technology that would become radar, an engineer at Bell Labs was assigned to track down the sources of static that sometimes interfered with radio transmissions.

The engineer, Karl Jansky, found that one major source of static was thunderstorms. But another source of static was more elusive, though curiously rhythmic. It slid across the sky every day and showed subtle shifts in the course of a year. Jansky finally determined that the signal came from the direction of the constellation Sagittarius — the same direction as the center of the Milky Way galaxy.

We now know that the static Jansky detected is produced by turbulent gas swirling around the black hole at the center of our galaxy. From this, a new branch of science had been launched: radio astronomy. It would come fully into its own in the postwar years, powered by the new generation of ultra-sensitive receivers originally developed for wartime use.

Another postwar development, the transistor, allowed development of far more compact RF equipment of all sorts. This also allowed high-powered and sophisticated radio and radar systems to be packed into missile nose cones and satellites. In time, the transistor would also make RF technology far more reliable and cheaper, setting the stage for the enormous proliferation of RF devices of all sorts, including smartphones and RFID tags for identifying lost pets.

In spite of all this progress, there was a time in the late 20th century when RF technology seemed to be falling a bit out of fashion, at least in the popular culture. Broadcast TV networks were losing viewers to cable, while music tapes, CDs and wired internet downloads encroached on the popularity of radio.

Even on the defense applications side, the hottest topic was what might be called “anti-RF” technology — better known as stealth technology, at the heart of which are shapes and materials that make aircraft and other vehicles less visible to enemy radar.

The outward sidelining of RF technology was more about pop-culture image than reality, but it had some real consequences. Technology-minded kids were more likely to fiddle with computer equipment than radio equipment, as kids in the midcentury era had done. The downstream consequence was a shortage of RF engineers once RF technology became “in” again.

In the last couple of decades, the apparent sidelining of RF technology has turned around in a big way.

The most obvious sign of this is our phones. Mobile radiotelephones have existed for many decades, but earlier models were fairly bulky and expensive, and remained specialty devices. But cellphones were small, cheap and handy, and then compact computer technology turned them into smartphones. Suddenly wireless, which had over time become an archaic word for “radio,” became the hottest thing going.

While the sheer number and variety of RF devices used in everyday life has continued to proliferate, the defense sector also remains a leader in pushing the RF technology frontier. This has been the case since the dawn of the technology, when British army and navy observers witnessed Marconi’s demonstration of radio in 1896.

In a crisis or in combat, timely and reliable information is always precious. Highly mobile modern warfare has made the wireless information battlefield more critical than ever, while posing the toughest challenges for technology systems as well as their operators. At the same time, defense research is exploring entirely new RF technologies.

Every RF receiver since Heinrich Hertz’s experimental rig, for example, has used some form of antenna to pick up the signal. But recent works with quantum effects that produce vastly bloated “Rydberg atoms,” highly sensitive to RF frequencies, could provide an entirely new way to detect RF signals, from radar beams to radio transmissions, with capabilities no conventional antenna system can match.

Conventional antennas, for example, must be scaled in proportion to the RF waves they detect, meaning that low-frequency signals can only be detected by bulky antennas. Quantum RF technology, however, allows compact scanners to detect even long-wave signals — denying a spectrum “hiding place” for hostile transmissions. At the same time, quantum technology can allow friendly forces to stay a jump ahead of enemy attempts to jam their communications.

Between the proliferation of RF applications and the development of entirely new technologies, the great pioneering age of RF engineering, it seems, may still lie ahead of us.

<p class="Are you interested in science and innovation? We are, too. Check out Northrop Grumman career opportunities to see how you can participate in this fascinating time of discovery.

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