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Re: And what of the FCC?
---------- Forwarded message ----------
Date: Thu, 2 Oct 1997 20:11:13 -0500
From: Geoff Schecht <geoffs-at-onr-dot-com>
To: Tesla List <tesla-at-pupman-dot-com>
Subject: Re: And what of the FCC?
<snip>
>
> All of these posts regarding RFI etc have caused me to wonder:
> What is the difference between a rotary gap TC and the prehistoric
> rotary gap CW transmitters? Is it just a matter of tuning a TC to
> deal with an antenna instead of a toroid or sphere as the "load"?
> I have heard recordings of these signals from way back when, and I
> can tell you that if the local birds sounded so melodious, we would
> all spend our spare time outside blowing our feathered friends the
> way of the Passenger Pigeon. If you have ever heard the Russian
> Woodpecker on HF (shortwave, if you must), then I can tell you the
> Woodpecker was much more agreeable of a sound. The Woodpecker was either
> Soviet OTH radar, or a Soviet mind control device, depending who you
> want to believe.
> I dont know much about sparkgap transmitters, but I am pretty sure they
> contained no active device, if you will, such as a vacuum tube.... so
> mustn't they have been pretty much a TC? They were about the same freqs,
> as I recall.
> BTW, 500kHz is still very much an int'l distress frequency, the last I
> heard. More of a ship freq than anything else, to my knowledge. So is
> 2182 kHz. Correct me if I am wrong tho.
> Randy
>
Randy:
The idea behind a sparkgap (or any other radio transmitter) is to
efficiently couple the RF energy from a source to an antenna. The antenna
is then supposed to transfer this energy into free space. Sparkgap
transmitters don't have the big secondaries (in terms of the number of
turns) that are typical of a TC. Any transmitter must contain a
tuning/matching network to allow optimal transfer of energy to the
radiating system. The antenna itself usually has an input impedance between
25 to around 1000 ohms, ideally with nothing but a real (resistive)
component.
I may raise some hackles with this comment, but a Tesla coil is really just
an easy way to generate high voltages at moderate frequencies (in relation
to the 2-30MHz, radio spectrum, that is). They have been used to power
X-ray machines and they found limited use in early particle accelerator
work but they have little other commercial value. Small, handheld TC's are
handy for finding pinhole leaks in glass vacuum systems, however, and a
number of quack medical devices used those little units to energize UV
generator tubes that were supposed to cure everything.
Other than that, Tesla coils emit very impressive sparks and are fun to
build and play with.
The original Marconi radio station on Cape Cod, MA has a few of its
artifacts on display in a museum on the site (which is now under the
auspices of the US Parks Dept). It was a gargantuan affair and I believe
that it may have used a rotary SG. There's a much smaller amateur SG
transmitter on display at the ARRL headquaters in Newington, CT. The last
time I was there, it was operable and they'd give you a demo (it isn't
connected to an antenna, of course).
An early SG transmitter advantage was that you didn't need a
superheterodyne receiver with a BFO to receive the signals. Coherers and
galena-based crystal sets were adequate for detection; that's all they had
until the early 1920's anyway. Tuning of those receivers was rudimentary,
at best, so you'd hear just about everyone within range when they were
key-down whether you wanted to or not.
Stationary sparkgap transmitters had a very buzzy, ratty note as far as
received tone goes. Rotaries have been described as being slightly
"musical" in tone and skilled operators could frequently identify each
other by both the sound of particular transmitters as well as by an
operator's "fist" or sending cadence.
The problem with the very broadband emissions from spark gap transmitters
became intolerable as the world became more dependent on radio for both
messaging and entertainment. SG transmitters were in use in the US until
the 1930's. By that time, the good folks at RCA, EiMac and the various
other tube manufacturing concerns had advanced the art far enough to permit
the construction of vacuum tubes capable of many kilowatts of (coherent)
power output at really useful shortwave communication freqs (2-30MHz).
I've read about SG's being used as the power supplies in some diathermy
machines and induction hardening apparatus at least through the Second
World War. Vacuum tubes were pretty much 100% war-critical materiel and
sparkgap RF generators didn't tie up those valuable devices when they were
needed the most. Sparkgaps aren't particularly efficient but they're cheap,
easy to build and they don't produce that much interference as long as
they're not attached to antennas.
Anyway, sparkgaps and Tesla coils are inextricably linked. The old Russian
Woodpecker was a pulsed (I believe,spread-spectrum) OTH radar that was used
to detect missile launches in the Western Hemisphere. It certainly didn't
employ a spark gap.
Geoff Schecht