From G3PLX:
The nearest thing still around that must sound like spark, is LORAN. After
all, it is generated in exactly the same way. The only difference is that
the LORAN megatron is driven with 6 pulses at 10uS intervals - that is, a
burst of 6 cycles of 100kHz - whereas the spark transmitter only had a
single spark per emission. In both cases the RF envelope would have risen
more rapidly than the rise-time of the maximum audible frequency, so the
signal must have sounded very similar in the headphones. Indeed, the
envelope waveform of the experimental spark transmitter shown on the VE2CV
website is almost identical to the waveform of a LORAN pulse shown on the
Loran website.
If you listen on 100kHz with the receiver on AM, apart from the
low-frequency pulse-rate at the basic p.r.f. between 10 and 20Hz, you will
hear a 'squeekiness' to the sound, which results from the fact that the
basic pulse is actually a burst of 8 or 9 'sparks' at a 1kHz rate. If the
basic spark transmitters were 400-600Hz, they would have sounded lower in
pitch than this 1000 Hz squeek, but the harmonic content would have sounded
exactly the same. LORAN is effectively transmitting strings of 9mS spark
transmission dots.
A simple spark transmission on LF would have had a bandwidth determined
almost entirely by the antenna resonance, but I imagine that as
transmissions were taken higher in frequency they would have become rapidly
wider and wider, unless steps were taken to narrow-down the emission by
means of extra tuned-circuits. Whether this was ever done or not I don't
know. There would have been good reasons to keep the bandwidths of
transmitters and receivers roughly the same.
73
Peter
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