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- Please see Television interference (electrical interference) for a different view of the spark transmitter
History
The spark gap transmitter was the product of many people, often working in competition, in the history of radio.
James Clerk Maxwell predicted the propagation of electromagnetic waves through a vacuum in about 1862 and physicist Heinrich Hertz was the first to verify the predictions of Maxwell. In 1888 using a tuned spark gap transmitter and a tuned spark gap detector (a loop of wire with a small gap in the diameter) located a few meters away, Hertz verified with a series of UHF experiments that electromagnetic waves produced by the transmitter when it sparked caused sparks to appear in the spark gap of the receiver.
Nikola Tesla pursued the application of his high voltage coil technology to radio. By tuning a receiving coil to a specific frequency he showed that the radio signal could be greatly magnified through resonant action. Tesla's U.S. Patent 447920, "Method of Operating Arc-Lamps" (March 10, 1891), describes an alternator that produces high-frequency current for that time period, around 10,000 cycles per second (later to be known as hertz). His patentable innovation was to suppress the disagreeable sound of power-frequency harmonics produced by arc lamps operating on frequencies within the range of human hearing. The frequency produced was in the longwave broadcasting range (VLF band). Tesla continued research into higher frequency devices and, by early 1896, he attained the means to produce undamped (or "continuous") waves around 50,000 cycles per second for radio transmission.
Guglielmo Marconi inspired by the various experimenters, began developing wireless telegraphy using spark transmitters. Marconi was able to establish a successful commercial wireless telegraph service that served the United States and Europe, and he received patents for radio technology in the US and abroad. When Marconi made the reported 1901 Trans-Atlantic transmission, the power portion of his system was based on Tesla's findings. Tesla and Marconi remained in legal battles for patent priority even after both men died. In 1943 the US Supreme Court invalidated one of Marconi's patents in favor of Nikola Tesla.
Reginald Fessenden's first attempts to transmit voice employed a spark transmitter operating at approximately 10,000 sparks/second. To modulate this transmitter he inserted a carbon microphone in series with the supply lead. He succeeded but experienced great difficulty in achieving intelligible sound. Spark gap transmitters generate fairly broad signals. As the more efficient transmission mode of continuous waves (CW) became easier to produce and band crowding and interference got worse, spark-gap sets and damped waves were legislated off the new shorter wavelengths by international treaty, and replaced by Pousen arc transmitters and high frequency alternators. Later these too yielded to vacuum tube technology and the 'electric age' of radio would end.
In 1905 a "state of the art" spark gap transmitter operated at around 400 meters (750 kHz) and generated a signal from about 250 meters (1.2 MHz) to 550 meters (545 kHz). The receiver was simple unamplified detectors, generally coherers (small quantity of metal filings lying loosely between metallic electrodes). This later gave way to the famous and more sensitive galena crystal sets. Tuners were primitive or nonexistent. Early amateur radio operators built low power spark gap transmitters using the spark coil from Ford Model T automobiles. But a typical large commercial station in 1916 might include a 1/2 kW transformer that supplied 14,000 volts, an eight section condenser, and a 500 amp rotary gap.
Operation
sparktrans2.jpg|thumb|right|333px|A typical spark transmitter circuit.
Legend:
capacitor - C1 and C2;
resistor - R;
inductor - L.
]] The spark transmitter is very simple in operation, but it presented significant technical problems mostly due to very large induced EMF when the spark struck, which caused breakdown of the insulation in the primary transformer. To overcome this the construction of even low-power sets was very solid. The damped wave output was very wasteful of bandwidth, and this limited the number of stations that could communicate effectively without interfering with each other.
In its simplest form, a spark-gap transmitter consists of a spark gap connected across an oscillatory circuit consisting of a capacitor and an inductor in series or parallel. In a typical transmitter circuit, a direct current source charges a capacitor through a resistor until the spark gap discharges, then a pulse of current passes through the capacitor. The inductor and capacitor after the gap form a resonant circuit. After being excited by the current pulse, the oscillation rapidly decays because energy is radiated from the antenna. Because of the rapid onset and decay of the oscillation, the RF pulse occupies a large band of frequencies.
The function of the spark gap is to present initially a high resistance to the circuit to allow the capacitor to charge. When the breakdown voltage of the gap is reached, it then presents a low resistance to the circuit causing the capacitor to discharge. The discharge through the conducting spark takes the form of a damped oscillation, at a frequency determined by the resonant frequency of the LC circuit.
Spark gaps
Construction
The spark gap varied in construction, depending on the power to be handled. Some were fairly obvious types of fixed gap, others were more complex. Wear and cooling were constant problems. As the power of units increased, the problem of quenching arose.
Quenching refers to the act of extinguishing a previously established arc in the gap. This is considerably more difficult than intiating the breakdown of the gap. A cold, non-firing spark gap contains no ionized gases. Once the voltage across the gap reaches its breakdown voltage, gas molecules in the gap are very quickly ionized along a path, creating a plasma that consists of ions and free electrons between the electrodes. The hot plasma also heats part of the electrodes to incandescence. The incandescent regions contribute free electrons via thermionic emission, and (easily ionized) metal vapor. The mixture of ions and free electrons in the plasma is highly conductive, so that a sharp drop in the gap resistance results. Without this highly conductive channel in the gap, efficient tank circuit oscillation would be impossible. However, the current discharge sustains the plasma and, until it is extinguished, the capacitors cannot recharge for the next pulse.
Quenching the arc
Several methods were applied to quench the arc.
- Jets of air that would literally 'blow out' the plasma,
- multi-plate discharger of Max Wien, (though possibly invented by Nikola Tesla) used in medium power spark sets which was known as the "whistling spark" for its distinctive signal,
- an electromagnetic field at right angles to the gap.
Rotary gaps
This need to extinguish the spark lead to the development of the rotating spark gap. These devices were used with an alternating current power supply and produced a more regular spark and could handle more power than a conventional spark gap. The inner rotating metal disc had a number of studs on its outer edge. A discharge would take place when two of the studs lined up with the two outer contacts which carry the high voltage.Rotary gaps are run in two modes, synchronous and asynchronous. A synchronous gap runs at a fixed speed and is constructed so that the gap fires in direct relation to the waveform of the A.C. line feed to the capacitors. The point in the waveform where the gaps are closest were changed by adjusting the rotor position on the motor shaft relative to the stator's studs. By running a properly set up synchronous gap, it is possible to have the gap fire only at the voltage peaks of the input current. This technique allows the tank circuit to fire only on the maximum voltage peaks and delivers the pulse from a fully charged capacitor each time the gap fires. If properly engineered, synchronous spark gap systems will deliver the largest amount of power to the antenna. They are however very temperamental, and difficult to maintain.
Asynchronous gaps are more common. They work quite well and are much easier to run. Break rates commonly were in excess of 400 bit/s. Since the gap could be fired more often than the input waveform switches polarity, more power can be fed into the tank circuit, as the capacitors can be charged and discharged more rapidly.
These rotary gaps also served to alter the tone of the transmitter since changing the number of studs changed the sidetone frequency - this was a means of enabling operators to distinguish different transmitters on the same (nominal) frequency. A typical high-power multiple spark system (as it was also known) was a rotating commutator with six to twelve contacts per wheel, 9 inches to a 24 wide inches, driven by about 2000 volts The output of rotary spark gap transmitter is turned on and off by the operator using a special kind of telegraph key, designed with large contacts to carry the heavy current often in excess of 20 amps.
External links
- Alternator, Arc and Spark
- Fessenden and the Early History of Radio Science
- Brief history of spark
- Massie Spark Transmitter The new England Wireless and Steam Museum
"Spark-gap transmitter"