Author Topic: Making a spark gap transmitter  (Read 3435 times)

Offline Fred8328

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Making a spark gap transmitter
« on: August 29, 2010, 09:32:21 AM »

When the SHTF, could you see yourself making one of these for communication purposes if needed?

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 A spark-gap transmitter is a mechanism for producing radio signals. It has been the primary radio transmission device during the early years of radio technology. It was soon superseded by other transmitters due to its discontinuous radio wave production and widely varying frequencies.
How a Spark-Grap Transmitter Works

A spark gap transmitter is basically composed of two capacitors (electrical apparatus designed to store electrical energy). However, the switch that ultimately forms the connection between the two capacitors is ionized gas (the air gap) that facilitates the passage of an oscillating current from one side of the transmitter (from one capacitor) to the other.

In normal conditions, the air gap forms a barrier between the two capacitors. To break the gap, a spark must be induced. To this end, the capacitor that is directly connected to a power source is charged with electricity. The electrical charge does not immediately dissipate because of an in-built Glossary Link resistor that 'holds' the charge.

After the electrical charge reaches the minimum threshold voltage, the electric charge passes through the resistor to the conductive electrode touching the air gap. This electrode subsequently releases the charge towards the gap and this ionizes the air within the gap; this forms a connection between the two electrodes at the gap.

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 This initial spark makes the air gap conductive and the pulse generates an electromagnetic wave in the radio frequency range (around 350,000 Hz). This transforms the spark-gap transmitter into a resonant circuit (an inductor-capacitor setup) where alternating current is conducted. This process is very similar to what happens when the negatively charged particle from the air touches the positively charged particle on land in a lightning flash.

Very soon after the initial spark, though, the wave weakens since some of the energy escapes though the antenna found on the side of the second capacitor and some are used up due to resistance. At a certain point, the ionized gas builds up resistance once again and the flow of current stops. Since the radio wave is weakened after a while, spark-gap transmitters acquired the name 'damped oscillation' transmitters.

A typical spark gap transmitter is powered by high voltage transformers feeding AC power. The latter allows the capacitor to charge quickly and in a fixed cycle which can be controlled to allow for 'pulsing' radio signals.
History of Spark-Gap Transmitters

In a way, spark-gap transmitters may have been the key to the development of wireless transmission. In 1862, James Clerk Maxwell predicted that electromagnetic waves can travel through a vacuum; in 1888, Heinrich Hertz used a spark-gap transmitter and a spark-gap detector to prove that the transmitter was producing electromagnetic waves - every time the transmitter sparked, small sparks were observed in the receiver's spark gap.

Guglielmo Marconi, inspired by the experiments of Hertz and others (including Nikola Tesla), was able to develop practical and workable wireless telegraphy using high-powered spark-gap transmitters. Though they could not be used for voice transmission, spark-gap transmitters still became the standard equipment for the military and the maritime industry.

Sparks and Radio:

Almost everyone has heard the effect of switching a light on or off when a radio is in the room; the spark in the switch causes RF radiation which the radio picks up. The spark transmitter did the same thing, though with a modicum of tuning. Not enough, though, to enable the use of spark transmitters now, so don’t try it! They wipe out large areas of spectrum. The UK Amateur Radio Licence used to have a sentence explicitly forbidding the use of spark transmitters, but it no longer does. The regulatory authorities perhaps assume that no-one would be silly enough to try.

The discovery of radio waves is usually credited to Heinrich Hertz; but the American, Joseph Henry (whose name is given to the unit of inductance) noticed that when he was experimenting with large coils and there was a thunderstorm around, he would get unwanted sparks. His coils were picking up the radio waves from the lightning flash - a truly large-scale spark transmitter. (Hertz, incidentally, might have been the discoverer of the photoelectric effect had he not died relatively young. His receiver was a coil which gave a spark across a gap when receiving; Hertz noticed that the spark came a little more easily when the spark-gap was illuminated, but regarded this as an incidental to his main line of enquiry.)

The Spark Transmitter:

The spark transmitter is very simple, but it generated a large number of technical problems mostly due to very large induced e.m.f.’s 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 pretty hefty. Low power is a relative term. The wavelengths used were from about 50 to 6000 metres. Wavelengths shorter than 200 m (frequencies above 1.5 MHz, or perhaps I should say 1.5 Mc/s) were all allocated to amateurs even into the 1920’s, with power up to 500 W. ‘Aeroplane sets’ used 200 to 600 m, also with about 500 W; 450 to 800 m for ships with power to 10 kW; 900 to 1500 m for moderate-sized land stations with 5 - 20 kW, and wavelengths over 1500 m (frequencies below 200 kHz) for large land stations with powers up to 100 kW. Some of these transmitters had the key switching the power circuits directly (see below), with perhaps 50 A flowing; no wonder some keys needed cooling. Imagine the Health & Safety Regulators’ views on these!

The essential features of the spark transmitter were:

    * An alternator, driven by an electric motor (remember that most mains supplies were d.c. at the time) or maybe a gas or oil engine, producing 120 V a.c. at around 500 Hz;
    * The key, which keyed the alternator output which was applied to
    * A step-up transformer giving between 10 and 20 kV; the output was connected via r.f. chokes to
    * The spark gap
    * The condenser (capacitor) which was charged to be discharged through the spark gap;
    * An oscillation transformer which transferred energy to the aerial circuit;
    * A tuning coil;
    * The aerial.

When the key is closed, the condenser C1  is charged nearly to the voltage from the secondary winding of the transformer. (I use ‘condenser’ deliberately because that’s what they were called when spark sets were used.) The capacity of this was around 0.01 uF, but this was related to the number of sparks per second and to the power supplied to the condenser:

W = NCV2/2

where W = power/W,  N = no of sparks/s,  C = capacity/F,   V = voltage/V. Further, the value of C1 affected the operating wavelength, so that the frequency at which the transmitter (nominally) radiated depended on the power!  The condensers had to be big and have a high working voltage; Leyden jars were favourite for land-based stations, with mica being the dielectric for ship sets which were necessarily smaller. A unit of capacitance widely used until relatively recent years was the ‘jar’.

The inductance L1 together with the spark gap and C1 produces the high-frequency oscillations, which are transferred via L2 to the ‘open’ circuit. The coupling of L1 and L2 was fairly important but affected the tuning, so the function of L3 is to tune the output of the set. This is also the function of C3, which became important and higher frequencies.

The transmitter shown is one that employed inductive coupling. Smaller sets used a different system, called capacitive coupling: the circuit diagram is given below.

Vibroplex key of 1910

The spark gap itself varied in construction, depending on the power to be handled. There were two problems; wear, and cooling. Some were fairly obvious types of fixed gap, others were more complex and had rotating studs. These served to alter the tone of the transmitter since changing the number of studs changed the spark frequency - this was a primitive means of enabling operators to hear different transmitters on the same (nominal) frequency.


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