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Re: Arc Transmitter

Subject:      Re: Arc Transmitter
       Date:  Sun, 11 May 1997 20:55:27 -0700
       From:  Bert Hickman <bert.hickman-at-aquila-dot-com>
Organization: Stoneridge Engineering
         To:  Tesla List <tesla-at-pupman-dot-com>
 References: 
            1


Tesla List wrote:
> 
> Subject:  Arc Transmitter
>   Date:   Fri, 09 May 1997 20:58:25 -0700
>   From:   Gary Weaver <gweaver-at-earthlink-dot-net>
>     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?  Or is it CW only?  This makes me wonder if a Tesla
> Coil can be modulated?  What if you modulate the 120 VAC line into
> the neon transformer?  What if you modulate a 3rd coil on the
> ground end of the secondary coil?  What if you modulate a TC with
> a carrier frequency and voice?  Has anyone ever tried to use a
> Tesla Coil as a transmitter?
> 
> Gary Weaver
Gary,

In the old days, the only way to generate high power RF signals was with
spark gap oscillators or specially built high speed alternators. As
arc-driven power levels grew, various schemes were developed to get
greater signal purity using improved quenching. This took the form of
multiple closely spaced copper plates (a static quenched gap), rotating
plate gaps, rotary-break gaps (like TC rotaries) or using very strong
magnetic fields and a high current DC arc (Paulsen arc converter). The
last device was the "king of the hill" for high power RF generation
before high power vacuum triodes came along. 

Unlike our spark-gap driven disruptive Tesla Coils, these devices used
the negative resistance characteristic of an arc powred from a DC source
to excite a series LC circuit  - this provided high power _undamped_ (or
Continuous Wave - CW) RF. These Paulsen Arc Transmitters were built in
sizes up to 1000 KW for transcontinental radio-telegraphy. The 2nd
edition of Morecroft's "Principles of Radio Communication" have pictures
of one of these behemoths, a 500 KW version looks to be about 8 feet
high and 10 feet in diameter. These typically broadcasted on 3000 -
18,000 meters (100 kHz - 17 kHz), although some went as low as 12 kHz!
When high power triodes were developed, these rapidly became obsolete,
and many of the huge magnets found later use as cyclotron magnets in
early atomic research in many universities. Even though they were CW
transmitters, they still had rather broad (by today's standards)
bandwidth.

These were modulated by turning them on and off - sending Morse code -
at up to 50 words/minute. Some primitive experiments in voice modulation
were also tried (with lower power equipment) by modulating current going
into the tank circuit in conjunction with quenched and rotary spark
gaps, or by modulating the ground path of a coupling transformer going
to the antenna. However, these were only marginally successful. Once
linear modulation using vacuum tubes became popular, the days of spark
were numbered. Once high power tubes became available, the days of spark
were over. 

A tube-powered tesla coil can also be easily modulated. The audio signal
can be clearly heard in the corona discharge, especially if the plate
supply is running off filtered DC. A MOSFET-driven coil can also be
modulated, and the signal clearly heard in the corona. Unfortunately you
are also broadcasting this signal to the rest of the world... something
the FCC (in the USA) frowns upon, especially if you're running at
typical coiling power levels. A Tesla coil IS a transmitter - but one
that's normally tuned to be a lousy radiator (i.e., having a very high
VSWR). However, because the coil DOES radiate, running a high power
modulated Tesla Coil could result in your equipment being confiscated
and a nasty fine. This may be one experiment you'd want to skip. :^)

Safe coilin' to you!

-- Bert --