Radar’s ancestors: From sound mirrors to modern detection technology

Radar has become a well-settled technology today, especially in the field of navigation. 

But the big crude dishes made of wood or concrete that dotted the English coast before the Second World War, which you might think were the predecessors of radar dishes were nowhere close to radar technology. They did remind me of the Easter Island statues. 

The pre-radar beginnings were so primitive you might wonder if it wasn’t prehistoric cave-dwellers who invented it. But of course not. The first conceptual studies related to radar began in the late 19th and early 20th centuries after scientists began studying electromagnetic waves (a type of energy that travels through space in the form of vibrating electric and magnetic fields). 

In 1886, German physicist Heinrich Hertz demonstrated the existence of radio waves, building on the theoretical predictions of James Clerk Maxwell. Hertz’s experiments showed that radio waves could be reflected by metallic objects, a principle crucial to radar technology.

It took many more years for this knowledge to be translated into the technology that radar came to represent.

The period between the two World Wars witnessed a dramatic transformation in warfare. With the invention of the airship and the aeroplane, the skies, once the exclusive domain of birds, became a battleground for noisy propeller-driven flying machines. 

As threat perceptions grew, nations scrambled to develop and deploy new offensive and defensive technologies. One of the defensive technologies was early warning systems that generated alerts of approaching enemy aircraft. 

Among these was a seemingly outlandish idea – using giant parabolic sound mirrors to detect the distant hum of aircraft engines. If you happened to see remnants of these sound mirrors, and didn’t know what they were, you might well have compared them with the Easter Island statues!

The concept behind the sound mirrors was a basic principle of acoustics. Parabolic reflectors, such as those used in telescopes and satellite dishes, can focus sound waves just as they reflect light waves. By constructing a large, concave dish, sound waves from a distant source would be collected and concentrated at a focal point. Here, a sensitive microphone could pick up faint sounds that indicated the presence of approaching aircraft before they became visible.

Birth of listening ears

Early experiments related to the sound mirror concept are said to have taken place in the years leading up to World War I, when France and Germany are thought to have dabbled in the technology. A pair of brass parabolic reflectors used for demonstrations still exist in museum collections.

However, it was Britain that went whole hog for the idea during the inter-War period. Vulnerable as it was as an island nation that was facing growing threats of aerial bombardment, it saw sound mirrors as a potentially low-cost and effective early warning system that alerted defense forces of approaching aircraft.

The sound mirrors that were operational were built between 1916 and the 1930s along England’s south and east coasts. These early models, relatively small and made of wood or concrete, resembled giant ears perched on hill cliffs. The sound waves they collected were channeled to horn-shaped receivers at the focal points of the sound mirrors, where observers listened intently for the telltale thrumming of approaching aircraft engines.

The system was far from perfect. First of all, distinguishing aircraft engine noise from other environmental sounds, like wind and waves, proved difficult. Not just that, the effectiveness of the mirrors depended greatly on weather conditions. Strong winds could disrupt the sound waves, and rain and fog further hampered detection capabilities.

And, worst of all, these mirrors had a very limited range. They could detect the sound of powerful engines from some miles away, but by the time an aircraft was located in the sky, it was likely to be within visual range already, and that made the mirrors redundant.

A few more years passed before radar was properly developed and used. And when it was used, it affected the fate of battles in dramatic ways. Neither distance nor cloudy skies could hinder detection of enemy aircraft or ships. 

It was yet another technology developed for military used, which is now deployed in a wide range of civilian applications. A technology developed by a young and ambitious German engineer named Christian Hülsmeyer in 1904, which took some more decades to be pressed into service.

Today radar technology is used in a wide range of industries – from motion sensors in automatic doors and speed measurement in sports to soil moisture management and building structural health monitoring. Could Hülsmeyer have imagined it all?