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Re: Halloween Coiling and the FCC



Original poster: "Malcolm Watts by way of Terry Fritz <twftesla-at-qwest-dot-net>" <m.j.watts-at-massey.ac.nz>

Hi Dan,

On 3 Oct 2002, at 8:06, Tesla list wrote:

> Original poster: "by way of Terry Fritz <twftesla-at-qwest-dot-net>"
<dhmccauley-at-spacecatlighting-dot-com>
> 
> 
> I know a lot of people here claim that tesla coils are quite "lousy" at
> being antennas and all that, but they throw a whole huge amount of RF hash
> out in the atmosphere especially when run outside.  Especially in a very
> urbanized area, you may attract the attention of other "listens" especially
> if near an airport, etc...
> 
> My very good friend Lou, a seasoned ham operator, made a bet with me.  He
> claimed he could hear my tesla coil with his antennas at 20 miles away
> during operation.  Since "the group" here previously claimed that tesla
> coils were lousy radiators, i figured i had that bet won.  Anyways, i set my
> coil up outside and he and my friend were at his house where all his ham
> equipment was set-up.  We then synchronized our watches and he informed me
> to key at random 3 short pulses.  This was accomplished by quickly turning
> off power to my 15kv/60mA NST.  He then told me to do this at random
> intervals for the next half hour and that he would record the exact time he
> received them.
> 
> Well, sure enough he got almost all the transmissions.  He also picked up to
> the 3rd harmonics of the signal as well which were much fainter no less, but
> still detectable.
> 
> We then reversed roles and repeated the experiment.  Voila.  Same results.
> And it was blatantly obvious when that thing was on.
> 
> I never would have thought it could be such an efficient radiator!!!!  I'm
> convinced now!!!!
> 
> 
> HOWEVER, it still wouldn't stop me from firing my coil up outside.  I'm
> going to do the same.  Of course, halloween is the perfect excuse to run the
> coil outside!!!!
> 
> Dan

The secondary is not an efficient radiator at its resonant frequency. 
It has a rather high radiation resistance being very much shorter 
than its quarter wavelength. However, it takes very little power to 
transmit a signal over miles. Cellphones are a prime example. If it 
had been an efficient radiator, he could have received the signal 
thousands of miles away. AM stations are an excellent example of 
this. 
      The worst characteristic of TCs is the wide band of frequencies 
generated by the sparks, both primary and secondary. Frequecies go 
into the tens, hundreds of MHZ and at those frequencies, just about 
any short bit of metal in the vicinity becomes an efficient parasitic 
element. The real test would be to operate one of your CW coils (if 
you have one) below breakout, and measure the field strength at Fr at 
some distance and compare that with the theoretical field strength 
you would get at the same distance if the secondary was replaced by a 
suitably long monopole.

Regards,
Malcolm