Subject: Re: Arc Transmitter
Date: Sun, 11 May 1997 12:20:02 -0700 (PDT)
From: "Edward V. Phillips" <ed-at-alumni.caltech.edu>
To: tesla-at-pupman-dot-com
"Has anyone every heard of an arc transmitter? An artical says,
in the early days of radio they used high voltage arc transmitters.
An arc transmitter transmits on a very wide band of frequencies.
What is an arc transmitter? Could it be something like a modulated
Tesla Coil?"
Arc transmitters were widely used CW generators, based on
the principle that a DC arc between two electrodes (frequently but
not necessarily copper and carbon) exhibits a negative resistance
characteristic. When such an arc is fed (through RF chokes) from
a DC source and a suitable SERIES LC circuit connected to its
terminals, an oscillation develops which is capable, under the
right conditions, of supplying hundreds of kilowatts of RF. Think
the largest arc transmitter ever built put out 2 megawatts. These
were primarily used in the frequency range from 15 to 50 kHz, but
are usable over a much wider range. I personally have built a
very small arc oscillator which worked up to 2 MHz, putting out
essentially zero power.
There's lots of literature on the subject. If you are
really interested I can send you some references which you should
be able to locate. Arc oscillators could indeed be modulated (
usually with a lossy device such as a carbon microphone) and
were used for the first voice broadcasts starting circa 1906.
As for Tesla coil use, they are bulky (quite complicated
in practice, requiring use of magnetic quenching circuits and
special hydrocarbon atmospheres), and still are just CW generators.
To run a CW tesla coils takes lots of power, and tubes are much
more convenient, more efficient, etc.
Ed