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In a spark gap transmitter, is that a "real" radio wave that is produced? I know you can hear it on an AM radio, if you are close to the gap, but is it a radio wave the same as from a radio station? Except from a radio station or modern transmitter the "gap transmitter" would be very, very fast?

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In a spark gap transmitter, is that a "real" radio wave that is produced?

It's the antenna that converts the spark induced carrier wave (controlled by an LC tuned circuit) voltage to a radio wave. It does so by matching the impedance of the cyclic voltage and current oscillations (low impedance or low V to I ratio) to 377 ohms where 377 ohms is the impedance of free space or a vacuum.

All radio antennas do this and, without a proper antenna, you will create a "radio" signal that doesn't travel effectively through the medium of space or air because it won't have E and H fields in the correct proportion for an optimized transmission.

is it a radio wave the same as from a radio station?

It will be a radio wave but it will be an on-off carrier where the on and the off are determined by a morse key that is used to "code" language into dots and dashes: -

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After the spark gap circuit and with the invention of valves/tubes, proper modulation (as opposed to on/off keying) could be implemented and radio receivers became much more sensitive. That marks the big difference in tech that emerged and basically the same principles exist today.

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The transmissions from a spark gap transmitters were "real" radio waves, but very noisy, and a single transmitter would cover a wide range of frequencies.

Modern transmitters generate the signal electronically. The signal from a modern transmitter occupies a very narrow range of frequencies, so many transmitters can operate without interference.

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  • \$\begingroup\$ When you say electronically, they still use electricity like the spark gap, though, so what does you mean by electronically? Do you mean the frequency is controlled by specialized electronics unlike power, two electrodes, and a switch? \$\endgroup\$ – johnny Sep 7 '18 at 21:33
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    \$\begingroup\$ Wow, @johnny... you are about 110 years behind \$\endgroup\$ – Maple Sep 7 '18 at 21:37
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    \$\begingroup\$ I was born in 1802. Only now have I began to understand things. \$\endgroup\$ – johnny Sep 7 '18 at 21:41
  • \$\begingroup\$ Ah... give it another 200-300 years then and you'll be a pro :) But seriously, read this \$\endgroup\$ – Maple Sep 7 '18 at 21:54
  • \$\begingroup\$ By electronically, I mean using tubes, transistors, or integrated circuits. \$\endgroup\$ – Peter Bennett Sep 7 '18 at 21:59
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Radio waves are electromagnetic waves and are the same now as in the spark-gap transmission era.

The spark gap was a means of generating fast rise time current pulses that would cause EM radiation. Before electronic oscillators were available the highest frequencies of alternating current (AC) that were available were supplied by alternators that were rotated rapidly with many poles. These were large and cumbersome and had lots of practical limits to their maximum frequency. The tuned circuit spark gap and the rotating spark gap were a period solution to a problem that had no other solution.

Tuned circuits were added later to control the transmit and receive frequency selectivity to achieve better range and bandwidth utilisation.

If you were to make a rotating spark gap (that makes an arc very rapidly) and couple it across a high-Q tank circuit (using superconducting inductors say) you could generate presentable on-off (CW) EM carrier waves that look just like those generated with modern electronic oscillators and amplifiers. They would however be very limited in modulation and tuning capabilities and very large and cumbersome to fabricate. Might still be an interesting experiment.

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