In 1759, James Watt began experimenting with steam as a source for motive power. His efforts gave the world an efficient steam engine as well as the watt, the unit by which we measure power.
One hundred and seventy five years later, Robert Watt – a descendant of James – together with Arnold Wilkins, would develop what the world would eventually come to know as radar.
The year is 1934 and here is the context:
- WW1 has been over for only 16 years. The “war to end all wars” resulted in 16,000,000 dead and 20,000,000 wounded, yet Germany is already openly re-arming. Toward the end of WW1, Germany’s Gotha bombers – airborne bombers being a new concept at the time – were flying over England, untouchable by the crude fighters that would otherwise provide defense.
- Science fiction was in vogue – readers devoured articles about Martians, time travel and “death rays”.
- Aircraft at the time had progressed beyond the Spads, Fokkers and Bleriots of the first world war, yet they were still mostly fabric-covered skeletons reminiscent of these early planes.
So in 1934, Dr. H. E. Wimperis, Director of Scientific Research for the Air Ministry, wondered if it just might be possible to incapacitate man or machine with radio energy. After all, the memories of WW1 were still fresh and England still lacked the ability to protect itself from high-flying threats due now, not to the altitude at which they flew, but to the lack of ability to react in time.
To find out, Dr. Wimperis contacted the best man to answer such a question, Robert Watt, who at the time was the lead scientist at the National Physics Lab in Slough. Specifically, Watt’s task was to calculate the amount and frequency of RF energy needed to raise the temperature of a man’s blood at various distances from the radiator.
The disappointing answer was – “too much”. However…
Arnold Wilkins – one of Watt’s scientists – remembered having heard a report of an aircraft in flight causing distortion on an experimental VHF link a year prior. If not destruction, maybe RF – reflected RF – could be used for aircraft detection.
The Air Ministry Defense Committee gave Watt and Wilkins the go-ahead to pursue their theory and for this they were provided a Handley Page Heyford, a British bi-plane bomber composed of fabric-over-metal construction. The two scientists weren’t hopeful of a fabric-covered airplane’s ability to reflect much RF but they did note that its wingspan was a close match to what would be a 49 meter dipole.
As luck would have it, the BBC happened to have a number of 49-meter transmitters – a common shortwave band then and now.
On 26 Feb 1935, a BBC transmitter in Daventry began transmitting an unmodulated carrier on the 49m (6 MHz) band. A receiver and the scientists were located a mile away in a van, along with a new and expensive piece of test equipment on which they would monitor the signal – an oscilloscope.
The Heyford’s pilot, Lt. Blucke, was instructed to fly a specific course but wasn’t told why. (Incidentally, this was the same pilot who would fly a Hallicrafters S27 and technicians to investigate Germany’s Knickebein beams 5 years later).
In the van, the scientists saw a stationary dot on the oscope’s CRT, representing the direct signal from the BBC transmitter. As the target plane progressed along its flight path, the dot started moving up and down. This is not quite what they’d expected but realized later that the phase of the reflected RF varied since the plane was in motion and that sometimes the returning RF’s phase would add to the BBC transmitter’s representation and at other times would subtract from it.
At any rate, an aircraft had, for the first time, been intentionally detected with RF.
But there were two problems:
- Any future incoming bombers wouldn’t necessarily make themselves into 49m dipoles to accomodate the enemy and
- More than the mere presence of bombers needed to be known. Their direction, range and altitude were needed as well in order to mount an effective defense.
To accomplish this a new scheme of modulation was needed. Rather than emitting a steady carrier and watching a CRT on a remote receiver, Watt proposed that short bursts of RF could be produced with any resulting echoes being measured for time delay which would then translate to range. This was not a new concept for him as his prior job was using just such a technique to measure the height of the ionosphere.
Once such an apparatus was in place, detection and ranging would be possible. With two or more set up, the variation in arrival times of the echoes could be compared in order to determine bearing. With both range and bearing known and tracked over time, altitude could be calculated trigonometrically – if they used a frequency that would provide higher resolution than was possible on 6 MHz.
With the first part of the Watt-Wilkins experiment a success the Air Ministry made 10,000 British pounds available to develop the next stage of what was then known as “range & direction finding”.
On 13 May, the team set up a pulsed transmitter and co-located receiver at an isolated military range near Orfordness, Suffolk. The transmitter and receiver had separate antennas, both of which were mounted on a 70-foot wooden mast. Over the next two weeks the team was able to detect numerous aircraft using pulsed signals from the transmitter of 10-15 microseconds.
Demonstrations to the financial backers resulted in more funding, additional stations and the training of operators – most of whom were women from the Women’s Auxiliary Air Force.
Now known as Chain Home, this system eventually began using more complex multi-element antennas that were strung between 350-foot towers. They operated on various frequencies between 22-30 MHz and had a peak power of 200 kilowatts. Maximum range was 120 miles with an elevation accuracy of 1 degree.
Since Chain Home’s frequencies included an amateur radio band, the British government “asked” the Radio Society of Great Britain (RSGB) to refrain from mentioning any recepetion of pulsed signals on 10 meters in their newsletters and other publications.