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On 11/21/14, 5:25 AM, John Cooper wrote:
Can a spark gap tesla be modulated by cw (morse key)? Not too cause
interferance or actually transmit rf but just to play with. Back in
early days of radio this was done with a spark gap transmitter. But
I dont think they actually had high voltage on the key itself? If I
broke the 120v with the key powering the nst wouldnt it weld the
contacts eventually? I think I could do it using a 12v relay but not
sure bout the lag between key closure and relay activation. Has
anyone heard of this or has messed with spark gap tranmitters?
Keying 120VAC is no big deal. Tube rigs that key in the cathode circuit
often have hundreds of volts on the key, if they're operating
grounded-grid. I had a tube based code practice oscillator that would
give you a fairly healthy shock.
A typical shipboard transmitter might only have used 500 or 1000 W, so
you're switching 5-10 Amps on the key, which isn't all that unreasonable
with decent contacts.
If you look at the design of shipboard radios of that era (lots of
exposed components with HV), and imagine what it would be like to be a
radio operator in a heavy sea, with the ship pitching and rolling, I'm
surprised that electrocution wasn't a regular occupational hazard.
Maybe it was.
(BTW, that is the origin of the 1000W input power limitation for amateur
licenses in the days of yore; ships and hams had comparable powers for
spectrum management purposes)
I believe that the spark transmitter at W1AW uses a relay for keying.
Here's part of a description of a system in a museum in Australia:
High powered system - Stage 2:
High powered systems use alternating current at around 500 Hz from a
rotary converter via a high voltage transformer to charge the main
capacitor.
ST 021405, Rotary Converter - G. Weymouth, Radio, circa 1915
High powered system - Stage 3:
The signal then passes through a keying relay, which protects the morse
operator (and the key) from the high voltages and currents required by
the transmitter. The morse key could not stand up to the high energies
involved in controlling the sparks.
The Marconi system became available as a series of spark transmitters,
the smallest of which was a 0.5kW set, the type T17, which was installed
on many small and medium-sized cargo ships. A synchronous rotary
sparkgap, which could be operated as a plain spark gap in an emergency,
was coupled to the primary of a tuned aerial transformer, as shown in
Figure 6.10. The power source was generally a motor alternator providing
100V a.c. at 500 H, giving the transmitted signal a characteristic sound
when detected at the receiver. A larger version, the type T18, providing
1.5 kW, was available for use on board larger ocean-going ships and gave
the operator the choice of four standard wavelengths, 450,600,700, and
800m. Keying these high-power spark transmitters meant breaking a
fairly heavy current and using a wide gap setting for the key, which had
substantial contacts. A shipboard wireless operator of the time
recounted the difficulties this caused:
"I recall the old key which was used ot break the heavy current for
telegraphic signalling purposes. It was actually pounded and was
equipped with giant platinum contacts. The maximum speed we could
handle was about ten words a ....