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Re: Tesla Coil RF Transmitter



Original poster: "Gary Peterson" <gary@xxxxxxxxxxxx>


Original poster: William Beaty <billb@xxxxxxxxxx>

> Original poster: "Gary Peterson" <gary@xxxxxxxxxxxx>

> I think we can all agree that a grounded Tesla coil without a
> Marconi-type antenna is a poor source of radio waves, i.e.,
> electromagnetic waves that have closed back upon themselves and are
> no longer associated with the antenna.

Not necessarily.

If we keep making a transmitting antenna smaller, but we also keep
stepping the voltage up which drives that antenna, then the antenna
doesn't act smaller.  As long as the strength of the EM field at
1/4-wavelength distance is not decreasing, then we can keep making the
antenna physically smaller.  We only pay for this in wasted power, since
step-up transformers use coils which get hot. . . . Or in other words,
it's hard to judge how well a small transmitting antenna works.

While a theoretician might find it difficult to determine how well an electrically short antenna might work, in practice it's a simple matter. In using this term "antenna" I'm referring to a physical structure intended for the launching of radio waves, i.e., electromagnetic waves that have closed back upon themselves and are no longer associated with said launching structure.


Even the shortest antenna must always radiate SOME
electromagnetic waves, and if an impedance-matching network is involved,
then far more energy gets out than one might expect.  To make your small
antenna act larger, just step up the drive voltage while stepping down the
current.  Don't forget: if a superconducting Tesla coil was used, with
superconductor primary and superconductor capacitor plates, hooked to a
superconducting ground plane, then the Tesla coil would be just as good as
a humongous quarter-wave antenna.  Of course the voltage would get a bit
high, and you might have to bury the thing inside a block of solid Teflon
a mile across.

In using the phrase "a grounded Tesla coil without a Marconi-type antenna" I was referring to a Tesla-coil transmitter such as might be assembled by someone on this list. I tend to agree with Chuck when he asserts that the "Effective Radiated Power (ERP) would still be miniscule, probably undetectable (electromagnetic waves [that have closed back upon themselves]) beyond about 500 to 1000m distance I would think."


MIT has a couple of very cool antenna animations which demonstrate
part of the idea that short antennas aren't necessarily "short":

quarter-wave antenna animation, 2megs
http:/web.mit.edu/8.02t/www/802TEAL3D/visualizations/light/QuarterWaveAntenna/QuarterWaveAntenna.htm

short antenna animation, 2megs
http:/web.mit.edu/8.02t/www/802TEAL3D/visualizations/light/dipoleRadiationReversing/DipoleRadiationReversing.htm

The caption associated with the second animation doesn't say anything about the antenna being electrically short. It looks to me like the thing is just drawn to a smaller scale than previously. Also, these animations represent the operation of a dipole "antenna" (in the sense used above) in free space. This is an inaccurate representation of the launching structure of a Tesla-coil transmitter, which is grounded and by my definition is NOT an "antenna."


. . . There is *always* some radiation from a short transmitting
antenna, . . . The question is, how much power gets out?  Or
more accurately: what's the SNR [Signal-to-Noise Ratio] at a
great distance from the TC?  If your Tesla coil's radio emissions
are far weaker than the natural VLF noise at that frequency, then
you might as well just collect the natural EM waves as a power
source, since your TC doesn't add much.

I notice that you're talking about the Tesla-coil transmitter as if it was intended as a source of "radio waves." The Tesla system does not work by the propagation of what are called "radio waves" in the narrowest sense of the term, i.e., far-field electromagnetic waves that have closed back upon themselves and are no longer associated with the launching structure. Operating a 20 watt SSTC transmitter at 108 kHz a couple of years ago, I found that noise is all that you hear with an LF receiver at 1,000 meters. Bring a tuned receiving transformer with the secondary removed close to the receiver's antenna and the electrostatic noise goes away, to be replaced by "dead air" created by the CW Tesla-coil transmitter.


> 2) If so, at what distance from the TC transmitter can the electrical
> disturbance be detected using a receiving transformer of similar size?

That's a signal-to-noise issue, no?   If there were no noise, then it
would be easy to detect the signal by just cranking up the receiver gain.

Yes it is, only you're using the term "signal-to-noise ratio" as it applies to "radio wave" reception. The propagation of electrical energy between a Tesla-coil transmitter and a Tesla receiving transformer is not by "radio waves." It is by electrical conduction between the ground terminals, and direct and indirect electrostatic induction between their respective elevated terminals. In a fully developed Tesla transmission-reception system the signal-to-noise radio is so great that electrostatic noise is essentially nonexistent; it's a "static eliminator."


William J. Beaty

Gary Peterson