Copyright © 1997, Neal McEwen
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Most of us have seen stationary spark gaps and the spectacular rotary gaps. But, in the photo to the right is a wireless artifact seldom seen and less often recognized; it is a "quenched" spark gap.
Building on the work of 19th century scientists, Marconi made wireless telegraphy practical. By the turn of the century, wireless was used to communicate with ships at sea. Although replaced by continuous wave (CW) transmitters, spark was once 'king.' Transmitters as large as 100 KW were built using spark technology.
One of the basic components of wireless transmitters was the spark gap, often just called a gap. To better understand the characteristics of gaps and in particular quenched gaps, it is helpful to examine spark transmitters. In the diagram to the right, a simplified schematic is shown. Several hundred volts at approximately 500 hertz is applied to the primary of the transformer. A potential of 15,000 volts was common on the secondary. The condenser charges for each cycle, until the breakdown voltage of the gap is reached. The condenser discharges and an oscillation is set up in primary of the oscillation transformer. This in turn sets up an oscillation in the secondary of the oscillation transformer. The primary does not radiate because it is a closed circuit oscillator. However, the secondary, being an open circuit, does radiate.
The function of the spark gap itself is to present a high resistance to the circuit to allow the condenser to charge. When the breakdown voltage of the gap is reached, it then presents a low resistance to the circuit so the condenser will discharge.
The primary and secondary of the oscillation transformer are closely coupled, mutually exciting each other. This results in a broad wave and inefficient use of energy and the radio spectrum. It is desirable to keep the oscillations in the primary as short as possible so that the oscillations in the secondary can seek its own natural period. How can oscillations in the primary be stopped? By use of a spark gap that returns to its high resistance state after the first few cycles of the condensers discharge. Not only are oscillations in the primary stopped, but the secondary cannot couple energy back into the primary, because the high resistance of the gap essentially opens the circuit.
Gaps came in three varieties, open gaps, rotary gaps and quenched gaps. The open gap consists of two electrodes made of Zinc or similar material. This type of gap gets very hot quickly. In some cases, the transmitter was fitted with a blower motor to remove the hot gases, thereby removing the low resistance path. Needless to say, the open gap does not function well in damping out the oscillations. Some open gaps used fins on the electrodes to aide in cooling the gap. Due to the difficulty in cooling, the open gap was found only on low power transmitters.
The rotary gap was fitted to end of the transmitter's AC generator supplying primary voltage to the spark transmitter. The rotary gap provides a path for the spark at predetermined periods in sync with the alternator voltage. It also rapidly quenches the gap because the electrodes are pulling away from one another.
The quenched gap was the most effective in cooling the gap and therefore extinguishing primary oscillations. The quenched gap consists of a number of special plates shown in the accompanying diagram. The plates were arranged in a rack like device as shown in the photo at the top. As many as sixteen plates were used. The plates were separated by mica washers. The plates were generally made of a hard bronze like metal. To force the spark to stay in the middle of the discs, the disc is slightly thicker at the center; electricity takes the shortest path.
The separation between the plate is less than 0.01 inches. Each spark is very small and therefore does not produce much heat. Therefore, the oscillations are quenched very quickly. On larger transmitters, the gap is equipped with a blower. Typically 1,200 volts per gap is allowed. Notice in the photo above, that with the tap, the gap can be made to use the appropriate number of discs. This type of gap is relatively noiseless in operations. In contrast, the open gap and rotary gaps are very noisy. It also has no moving parts! Because the oscillations are damped so quickly, the oscillation transformer can be more tightly coupled, making a more efficient transmitter. Quenched gaps were used on transmitter up to 35 KW.
The advantages of the quenched gap do not come for free, however. The assembly must be taken apart at frequent intervals to clean the sparking surfaces of the plates. Continuous duty transmitters using quenched gaps need service on two week intervals. Notice in the photo above the handle on the right end of the assembly. This releases compression on the plates and washers such that it could be disassembled and put back together.
Notice in the photo above that the gap sits on top of a baking tin like object. This is actually the condenser for the primary circuit. It is a mica condenser made by Dubilier. On top of the condenser are two small copper spheres about 1/2" in diameter. This is a safety gap such that the mica in the condenser is protected from over voltage and hence puncture.
My friend Puck, a key collector in Virginia, picked up this quenched gap at a ham fest and was not sure what it was. After he described it to me, I thought it was a very early home brew condenser. Puck decided to send it to me. Was I ever surprised!!! Unfortunately, the gap itself is not marked. I would dearly love to know who the maker was. Yet, I consider this a nice asset to my collection and it is a real conversation piece among visitors.
Bucher, Elmer E. Practical Wireless Telegraphy. London; 1921
Morgan, Alfred P. Wireless Telegraphy and Telephony. New York: Henley, 1920.
Nilson, Arthur R. & Hornung, J. L. Practical Radio Telegraphy. New York: McGraw-Hill, 1928