Re: Tesla Coil RF Transmitter

["Tesla list" <tesla@xxxxxxxxxx>]

Original poster: William Beaty <billb@xxxxxxxxxx>

On Mon, 3 Oct 2005, Tesla list wrote:

> 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.  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.

:)

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

If we keep making our small antenna shorter, then the fields near the
antenna have to be more and more intense in order to keep radiating the
same level of power.  But ideally it doesn't take wattage to create high
field intensity.  Just use an ideal step-up transformer to crank up the
voltage.  Or use a non-ideal transformer where you have to throw away lots
of wire-heating wattage for each watt of RF that you transmit.


> In my mind the questions to be answered are these:
>
> 1) Is a well-grounded non-sparking toploaded Tesla coil operated in a
> CW mode at, say, 35 kHz capable of producing locally a periodic
> disturbance in the earth's electrical charge?

Yes, since it would violate the laws of physics if it did not.  There is
*always* some radiation from a short transmitting antenna, and radio waves
always disturb the earth's charge a tiny bit.  There is no wall around a
Tesla coil, and some of the fields escape as RF waves.

The question is, how much power gets out?  Or more accurately: what's the
SNR 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.


> 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.


> 3) If the disturbance can be detected at a multiple wavelength
> distance from the launching structure, does this distance increase
> with an increase in transmitter power?

It had better.   If it didn't, then radio physics has a problem.


> 4) If the transmission-reception distance does, in fact, increase
> with an increase in transmitter power, what is the mathematical
> relationship between the two?

For antennas in free space it's inverse square, so if you move the
receiver twice as far out, then you have to crank the power up by 4x.

But in a two-D waveguide with the ground and ionosphere reflecting the
waves, the relationship would be improved: somewhere between inverse
proportion and inverse square.  The RF doesn't spread as a cone, instead
it spreads as a pie-slice.

And finally, if the Earth cavity has a high-Q resonance at your TC
frequency, then the distance limitation gets all weird.  It acts like a
microwave oven, where the distance to the magnetron isn't as important as
the losses in the waveguide walls, and the diffraction pattern in the
resonant cavity.



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William J. Beaty                            SCIENCE HOBBYIST website
billb at amasci com                         http://amasci.com
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Seattle, WA  206-789-0775    unusual phenomena, tesla coils, weird sci