Spark Gap Transmitter: EM Wave Generation & Transmission

[cnh1995]

I recently read about the early spark gap transmitter..But I don't understand how the EM waves are generated and transmitted..Induction coil creates sparks in the gap but what produces the waves and how are they transmitted?? Need I study electromagnetic waves in detail??

 

[tech99]

I recently read about the early spark gap transmitter..But I don't understand how the EM waves are generated and transmitted..Induction coil creates sparks in the gap but what produces the waves and how are they transmitted?? Need I study electromagnetic waves in detail??

The transmitter is a generator of high frequency alternating current, which is supplied to an antenna. The transmitter is based on an LC circuit, which has a resonant frequency, like a bell. The capacitor is connected to a high voltage DC power supply, and as it charges up, the voltage across it gradually rises. When it reaches a critical value, the spark gap breaks down, forming a low resistance path and connecting the capacitor in series with the inductor. This creates the LC circuit, and several cycles of damped oscillation then takes place, gradually falling in amplitude as the energy is lost. Most of the energy is radiated from the antenna but some is wasted in the losses in the circuit. The transmitter produces bursts of oscillation and cannot produce a continuous wave. Operation is possible up to tens of GHz.

 

[davenn]

..Induction coil creates sparks in the gap but what produces the waves and how are they transmitted??

The thing to realize is that the spark isn't a single pulse discharge across the gap, but that it is an oscillation of arcing back and forwards across the gap
The frequency of that oscillation is covered in tech99's LC circuit description

Dave

 

[Baluncore]

The spark gap has a non-linear VI characteristic with high frequency components.
The spark gap is part of, or is tightly coupled to a resonant LC “tank” circuit.
The magnetic field of the tank inductor current is a magnetic dipole antenna.
An external antenna can be lightly coupled to the tank coil.

A “spark” transmitter “plucks” the LC tank at a regular rate which provides a damped oscillation. It generates a rough audio modulated tone.

An “arc” transmitter has a continuous arc across a gap. It produced a continuous wave. I have seen instructions on how to adjust the drip rate of alcohol into the arc chamber to get it running smoothly.

 

[tech99]

The spark gap has a non-linear VI characteristic with high frequency components.
The spark gap is part of, or is tightly coupled to a resonant LC “tank” circuit.
The magnetic field of the tank inductor current is a magnetic dipole antenna.
An external antenna can be lightly coupled to the tank coil.

A “spark” transmitter “plucks” the LC tank at a regular rate which provides a damped oscillation. It generates a rough audio modulated tone.

An “arc” transmitter has a continuous arc across a gap. It produced a continuous wave. I have seen instructions on how to adjust the drip rate of alcohol into the arc chamber to get it running smoothly.

The spark gap has a non-linear VI characteristic with high frequency components.
The spark gap is part of, or is tightly coupled to a resonant LC “tank” circuit.
The magnetic field of the tank inductor current is a magnetic dipole antenna.
An external antenna can be lightly coupled to the tank coil.

A “spark” transmitter “plucks” the LC tank at a regular rate which provides a damped oscillation. It generates a rough audio modulated tone.

An “arc” transmitter has a continuous arc across a gap. It produced a continuous wave. I have seen instructions on how to adjust the drip rate of alcohol into the arc chamber to get it running smoothly.

My feeling is that the tank coil will radiate much less than the antenna, due to its small dimensions.

 

[sophiecentaur]

My feeling is that the tank coil will radiate much less than the antenna, due to its small dimensions

But of course. The sort of wavelengths that are used with the old spark transmitter were pretty long* and a correspondingly long radiating element is needed in order to match the RF signal adequately and to get a half decent efficiency.
*Largely because of the design of the receiving system.

An “arc” transmitter has a continuous arc across a gap. It produced a continuous wave. I have seen instructions on how to adjust the drip rate of alcohol into the arc chamber to get it running smoothly.

Brilliant bit of history there!

 

[berkeman]

And another bit of history...

The ARRL (American Radio Relay League) got its name because in the early days of radio, it took, a number of relays to get a message across long distances. The early spark transmitter and mostly passive receivers did not offer enough gain for long-range transmission...

http://www.arrl.org/ham-radio-history

 

[sophiecentaur]

Spark transmissions brings a new dimension to the idea of Spread Spectrum !
Shannon limit eat your heart out.

 

[tech99]

Spark transmissions brings a new dimension to the idea of Spread Spectrum !
Shannon limit eat your heart out.

Quite agree. However, if we look at the waveform on a CRO, it is an amazingly clean, damped sine wave. Of course, spreading occurs because the modulation envelope is a sawtooth. It is also likely that oscillation could occur at higher resonances in the antenna and tuning coils. Regarding operating frequencies, J Chandra Bose at Cambridge demonstrated results with millimetre waves, using the "loose contact" type of detector. (http://en.wikipedia.org/wiki/Jagadish_Chandra_Bose).

 

[Baluncore]

My feeling is that the tank coil will radiate much less than the antenna, due to its small dimensions.

With a “spark” transmitter there are high frequency harmonics across the spectrum. The tank coil is better at radiating the higher harmonics. Hertz was first to verify Maxwell's theoretical work in the laboratory, at UHF wavelengths using a "spark" transmitter. Marconi's first transatlantic communication in 1901 is now believed to have actually employed a harmonic of his spark coil resonant frequency. It appears he was using HF not LW, but did not realize it. What a missed opportunity. Authorities then allocated all their useless SW bands with wavelengths below 100m to the amateurs, while LW was used for unreliable commercial long distance communications.

Higher harmonics are missing from the “arc” transmitter coil so an external antenna is needed to increase the radiated signal at the resonant frequency. That gave the early “arc” an advantage over the “spark” transmitters for frequency channelisation. From 1898 when Sir Oliver Lodge patented “syntonic” tuning, the bandwidth of signals was narrowed to distinct frequencies. It has taken almost a century to Spread the Spectrum again.

No teacher knew anything about the spark coil we had in the high school lab. I was listening to it from 10m away with some old WW2 headphones, just a diode across each earpiece coil. When I reached up with both hands to remove the headphones I received a shock from the ends of the spring band on the headphones. The Bakelite was a good insulator. I have not repeated the experiment.