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Although the principle of a spark gap transmitter may seem to be very simple, there was a lot more to the designs than might be apparent at first sight.
Looking at how a spark gap transmitter works, yields a lot of interesting design techniques. Spark gap transmitter operation was a little more complicated that many today may believe.
Operation of a spark gap
Before looking at the operation of the overall spark gap transmitter and seeing how a spark gap transmitter works, it is worth taking a look at the core element of the circuit to see how that operates.
Although the spark gap may appear as a very simple element within the spark gap transmitter circuit, an understanding of its operation is key to being able to see how the circuits work. It took early pioneers a while to understand how a spark gap worked and therefore how best to use one in a transmitter.
Essentially a spark gap acts within a transmitter as an electronic switch. When it has not fired, it forms an open circuit with a very high resistance. As the voltage across the spark gap builds up a point is reached where the air between the contacts breaks down and the spark gap fires or arcs over and a spark is seen. When this occurs the air within the 'spark gap' becomes ionised and a conducting plasma is formed.
Although spark gaps require a high voltage to cause the spark to form, to maintain it requires current, and in fact the resistance of spark gap once ionised and the current is conducting can be as low as about two Ohms.
Spark gap from high power transmitter dating from early 1900s.
This low level of resistance means that any spark gap transmitter circuit needs to be able to supply the required level of current without burning out. Many early developers did not understand this - they used induction coils to generate the high voltages to cause the spark gap to fire, but the induction coil was not designed to provide the current needed and often they failed or burnt out as a result.
Looking more closely at the spark itself, the actual manner of the breakdown is highly un-predictable and once spark has struck and the path has become established even this is highly variable. The level of current varies wildly.
The overall result is that the spark generates wide band radio frequency energy that can be coupled in the spark gap transmitter to an antenna and radiated.
Looking at how the spark gap works, it is quite crude in its operation and therefore it is hardly surprising it is not particularly efficient and radiates a poor signal.
How does a spark gap transmitter work?
When looking at how a spark gap transmitter works, it is necessary to bear in mind that this was an area of technology that was developing very quickly around the end of the 1800s and the early 1900s. As a result many ideas were being developed, some good and some not so good, and there was a large variety in the ways the different spark transmitters worked.
In concept, a very basic spark gap transmitter consists of a voltage source fed through a resistor to a capacitor which has the spark gap across. The voltage across the gap rises until it sparks. The spark discharges the capacitance until it is below a sustaining voltage and the spark goes out. Then the capacitor charges up again until it sparks again, and the cycle repeats.
The spark gap is connected to the antenna which enables the signal to be radiated. Normally there are tuning arrangements to limit the bandwidth of the signal.
The resulting high amplitude pulses from the spark gap have very sharp edges, i.e. the current rises from nothing to a high value in a short space of time. As a result they generate wide-band radio frequency energy. Something similar happens with a lightning strike producing the crack often heard across the medium wave or short wave bands.
The energy produced by spark gap transmitters was tuned to a degree by the tuned circuits of the transmitter and the antenna, but nevertheless they still radiated energy over a wide bandwidth. As a result they were removed from service as they interfered with other users using much more narrow band techniques like Morse and amplitude modulation, etc.
Spark gap transmitter quencher
When looking at how a spark gap transmitter works, one concept that was adopted early in the development was that of a spark quencher.
One of the issues that was found, particularly with operation of high power spark transmitters was that some of the energy from the antenna circuit was transferred back into the spark circuit after the first burst of oscillation.. This transferred the spark into a short period arc which lowered the overall efficiency, and under some circumstances it caused a transmission to occur on two separate frequencies.
Methods of 'quenching' the spark were investigated and implemented.
One method of quenching applied to spark transmitters was to reduce the coupling between the spark and antenna circuits.
Better methods of 'quenching' the spark gap transmitter involved introducing some methods of rapid de-ionisation of the spark gap. These started to be introduced as early as the 1890s to prevent an arc developing from the spark.
One of the first de-ionisation methods of quenching was developed by Elihu Thomson and involved what he called a 'magnetic blowout' scheme. In this a suitably timed magnetic field was applied at right angles to the direction of the spark. Other ideas included using a direct air blast to ensure any arc may be extinguished.
The idea that gained most popularity for quenching the spark gap transmitter was to use a rotary spark gap. This consisted of one of more stationary elements and a rotating element which had some projecting spokes. As the rotating spark points would only be able to support a spark for a short time, any arc being extinguished before it became established.
Spark gap transmitter improvements
One of the major issues with early spark gap transmitters was that the efficiency was very low. This resulted from the fact that the spark gaps fired continuously when the key was depressed. The issue with this was that the induction coil used to generate the back EMF to create the spark in the spark gap would only be able to drive around 100mA or so across the gap once the arc was running. This meant that only a very low level of power was delivered to the antenna.
Some early methods of increasing the power involved increasing the gap between the two electrodes of the spark gap which increased the voltage. This meant that lethal voltages appeared on antennas.
One advance in spark gap technology was relatively simple. It entailed adding a capacitor across the secondary winding of the induction coil used to generate the spark. The addition of this single capacitor to the spark gap transmitter made a large difference. It eliminated the continuous arc which dragged down the voltage from the induction coil. Placing the capacitor across the secondary of the induction coil in the transmitter enabled both the gap current and the resulting antenna current to increase, and also the fast discharge of the capacitor removed the gap resistance from the antenna circuit. Both of these attributes come as a result of the addition of the single capacitor.