British 60-Pounder at Ypres, Early War
The Story Begins Here
James Patton
When the Race to the Sea was over, and the opposing forces were bogged down in their fixed positions, the artillery truly became the King of Battle. For maximum effectiveness, the guns need to be corrected by forward observation. Everybody had airplanes, so it was obvious that a way should be found to use them for this job. For the British, there was also the lurking U-boat threat, even in the English channel, and the Royal Navy was keen to use their aerial assets to spot the submarines as well.
The British solution was the Sterling Spark Gap wireless transmitter, also referred to as the No. 1 Aircraft Transmitter Spark. It was designed in late 1914 and first brought into use at Gallipoli in 1915 by the Royal Naval Air Service (RNAS). They were operating off of the seaplane carrier HMS Ark Royal (later re-named as the eighth HMS Pegasus) to support naval gunfire, which was not very successful, due to the limited trajectory of the guns and the lack of appropriate shells.
The Sterling device was based on a design by then-Lieutenant Basil Binyon (1885- 1977) OBE MA RNVR, an experienced wireless engineer who was serving in the RNAS. After the war Binyon was a director of the Marconi Wireless Telegraph Co. (MWTC) and a founding director of the BBC. In 1939, at age 62, he was appointed a group commander in the Royal Observer Corps.
The Sterling was state-of-the-art in 1914, simple but robust, employing a principle discovered by Heinrich Hertz (1857-94) in 1886. The tuning was straightforward, the aim was to create a 'good fat white spark' for maximum antenna current. Unlike its successor, the continuous wave (CW) transmitter, the Spark Gap sent the dots and dashes of Morse Code as bursts of radio frequency (RF) energy on an otherwise silent frequency, whereas the CW transmitter broadcast a steady RF signal (wave) which was briefly interrupted by the Morse Code dots and dashes. CW technology enabled voice transmission when they figured out how to modulate the wave.
Around 1,300 Sterling No. 1 sets were made during the First World War. The first 100 or so were made by the Sterling Telephone and Electric Company Ltd. (hence the name) and the remainder by the War Department (WD) Wireless Factory. Subsequent variants models were manufactured by MWTC as well as the WD totaled 3,898. In 1918 a similar device to the Sterling was produced for the U.S. Army Signal Corps by the Connecticut Telephone and Electric Company, which was called the Airplane Radio Telegraph Transmitting Set Box Type BC-15A.
In the original version, the Sterling operated on the wave lengths between 100 and 260 meters. Batteries provided up to 10 volts DC power. The transmitter's RF output power of 30 to 40 Watts, coupled to the 35 meter (or ¼ wave) long-wire antenna that trailed behind the plane and the height above ground, gave an operating range of 8-10 miles.
The set was usually mounted on a tray on the side of the aircraft fuselage, outside of the cockpit. Since both the Morse key and the transmitter produced sparks, they had to be enclosed. The transmitter was in a box with a small mica window, so the operator could see the ‘strength’ of the spark. Both of these enclosures were precautions to minimize the danger of the spark igniting the gasoline vapors that could build-up in the cockpit. The antenna was made of stranded copper wire with a 1 ½ kg weight at the end (to keep the wire trailing straightened out. After reaching the desired altitude, the wire was spooled out, either from the side of the aircraft fuselage alongside the wireless set or through an insulated grommet in the floor of the cockpit. Aircraft engaged in long-range reconnaissance used a version that powered the transmitter with airstream-driven generators giving 600 volts DC , producing a kilowatt of RF power.
At first, communication was air -to- ground only, so the sender (usually the observer), had no way of knowing if the message had been received, and frequently simultaneous transmissions occurred, causing jamming. A device known as a "Clapper break&"; was fitted to the transmitter to produce a slight variation in the sound of the received signal, so that the ground operators could differentiate between the various aircraft transmitting. There were developed one or two character codes to enable the user to identify himself and tell the batteries how to adjust their fire.
The Full Kit for Aircraft Observation
Early on, receivers had been tried out in aircraft, but with little success due to the incoming signal being masked by the engine ignition spark, as well as the ambient noise, both from the engine and the slipstream. Eventually those problems were overcome. The ignition system was shielded and flyer’s helmets were equipped with built-in Type A reed headphones, invented by Sidney G. Brown FRS (1873-1948), which could amplify the weak voltage output of the crystal receiver. Notwithstanding the deficiencies, the Sterling set was very important, especially when the users were equipped with both a transmitter and a receiver.
Perhaps the most significant development was to the wireless sets themselves: first by improving tuning over the ‘cats-whisker’ crystal receivers. The Tuned Frequency (Tt) 3vacuum tube receivers, capable of receiving either Spark or CW transmissions, were developed at RNAS Cranwell in 1917. They also developed a vacuum tube transmitter known as the TI1, which was a simple feed-back oscillator using two vacuum tubes operating in parallel with a tappable coil for tuning. Completing the rig was a windmill-type generator that produced 1200 volts DC, significantly upping the transmission power. Even radio telephony (voice communications) became possible by the end of the war; some RAF aircraft were equipped with two-way voice-capable communication with a range of over 10 miles.
Post-war versions operated on shorter wave lengths and were powered by a 150 watt wind-driven alternator. Used by the RAF into the 1930’s, these radios had ranges of 200-300 miles air to ground and 40 miles air to air. If you’re intrigued by the technology of the Sterling devices, here is a LINK for you.