[600MRG] TM1LY and TM100LY Lafayette Stations on the Air.

[D.J.J. Ring, Jr.]

Arc transmitters couldn't be keyed for on/off keying because it took a
moment to strike the arc, so frequency shift keying was used.

See below,
73
DR

See this article from Wikipedia
<https://en.wikipedia.org/wiki/Arc_converter#Keying>.
Keying[edit
<https://en.wikipedia.org/w/index.php?title=Arc_converter&action=edit&section=3>
]

Since the arc took some time to strike and operate in a stable fashion,
normal on-off keying <https://en.wikipedia.org/wiki/On-off_keying> could
not be used. Instead, a form of frequency shift keying
<https://en.wikipedia.org/wiki/Frequency_shift_keying> was employed.[8]
<https://en.wikipedia.org/wiki/Arc_converter#cite_note-8> In this
*compensation-wave
method*, the arc operated continuously, and the key altered the frequency
of the arc by one to five percent. The signal at the unwanted frequency was
called the *compensation-wave*. In arc transmitters up to 70 kW, the key
typically shorted out a few turns in the antenna coil.[9]
<https://en.wikipedia.org/wiki/Arc_converter#cite_note-9> For larger arcs,
the arc output would be transformer coupled to the antenna inductor, and
the key would short out a few bottom turns of the grounded secondary.[10]
<https://en.wikipedia.org/wiki/Arc_converter#cite_note-10> Therefore, the
"mark" (key closed) was sent at one frequency, and the "space" (key open)
at another frequency. If these frequencies were far enough apart, and the
receiving station's receiver
<https://en.wikipedia.org/wiki/Receiver_(radio)> had adequate selectivity
<https://en.wikipedia.org/wiki/Selectivity_(electronic)>, the receiving
station would hear standard CW when tuned to the "mark" frequency.

The compensation wave method used a lot of spectrum bandwidth. It not only
transmitted on the two intended frequencies, but also the harmonics of
those frequencies. Arc converters are rich in harmonics. Sometime around
1921, the Preliminary International Communications Conference[11]
<https://en.wikipedia.org/wiki/Arc_converter#cite_note-11> prohibited the
compensation wave method because it caused too much interference.[4]
<https://en.wikipedia.org/wiki/Arc_converter#cite_note-Little_1921_125-4>

The need for the emission of signals at two different frequencies was
eliminated by the development of *uniwave methods*.[12]
<https://en.wikipedia.org/wiki/Arc_converter#cite_note-12> In one uniwave
method, called the *ignition method*, keying would start and stop the arc.
The arc chamber would have a *striker* rod that shorted out the two
electrodes through a resistor and extinguished the arc. The key would
energize an electromagnet that would move the striker and reignite the arc.
For this method to work, the arc chamber had to be hot. The method was
feasible for arc converters up to about 5 kW.

The second uniwave method is the *absorption method*, and it involves two
tuned circuits and a single-pole, double-throw
<https://en.wikipedia.org/wiki/Contact_terminology>, make-before-break key.
When the key is down, the arc is connected to the tuned antenna coil and
antenna. When the key is up, the arc is connected to a tuned dummy antenna
<https://en.wikipedia.org/wiki/Dummy_antenna> called the *back shunt*. The
back shunt was a second tuned circuit consisting of an inductor, a
capacitor, and load resistor in series.[13]
<https://en.wikipedia.org/wiki/Arc_converter#cite_note-13>[14]
<https://en.wikipedia.org/wiki/Arc_converter#cite_note-14> This second
circuit is tuned to roughly the same frequency as the transmitted
frequency; it keeps the arc running, and it absorbs the transmitter power.
The absorption method is apparently due to W. A. Eaton.[4]
<https://en.wikipedia.org/wiki/Arc_converter#cite_note-Little_1921_125-4>

The design of switching circuit for the absorption method is significant.
It is switching a high voltage arc, so the switch's contacts must have some
form of arc suppression. Eaton had the telegraph key drive electromagnets
that operated a relay. That relay used four sets of switch contacts in
series for each of the two paths (one to the antenna and one to the back
shunt). Each relay contact was bridged by a resistor. Consequently, the
switch was never completely open, but there was a lot of attenuation.[15]
<https://en.wikipedia.org/wiki/Arc_converter#cite_note-15>

On Fri, Dec 18, 2020 at 9:37 PM J Mcvey <ac2eu at yahoo.com> wrote:

> Wow that must have been LOUD. I suppose all the plumbing was to vent the
> ozone?
> How did they key it?
> That setup is a lost art for sure.
>
>
>
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