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Re: Halloween Coiling and the FCC



Original poster: "Paul Nicholson by way of Terry Fritz <twftesla-at-qwest-dot-net>" <paul-at-abelian.demon.co.uk>

Paul Benham wrote:
> fitting chokes ... close to the spark gap helps greatly in
> reducing down radiated emissions at low frequencies, say up
> to 30MHz,  but actually makes the radiation up at UHF in
> the TV band here in the UK worse(470-860MHz).

That's a very nice observation. Placing RF chokes either side 
of the gap would be expected to raise the frequency at which 
the most radiation is produced - the chokes mark the ends of
what is effectively a dipole.  Move them nearer to the gap and
you raise the frequency.  Move them close enough so that the
entire choke-arc-choke can be boxed in a conductive enclosure
and you have a nicely screened setup.

> It may be that I have been wasting my time with the tin
> enclosure

I doubt it.  You describe what we'd hope to see, and it's
very nice to see this demonstrated.  Not a waste of time 
because if the high frequencies are confined by the box and
chokes, you only have the relatively low frequencies in the
rest of the primary circuit.

Jim wrote:
> if the receiver was tuned to a suitable frequency up a ways,
> so atmospheric noise is less, and where radiation resistance
> of the TC itself is higher... A TC with a 15 foot ground wire
> might be a reasonably effective radiator at 10-15 MHz....even
> if it is the 100th harmonic of the fundamental (at 150kHz),
> the increased effectiveness of the antenna might help..

That's a good point, reminding us that the actual source of the
RF (ie the bit that's oscillating) need not be the same component
as the structure which is radiating it.   

Let's say you've a 1kW coil at 150kHz.  At a BPS of 100 that's
a peak power of around 50kW.   Lets say that 0.1% of the input
power finds its way into a higher mode of the primary at say
10Mhz. That's 100 pulses per second at a power of 50 Watts. 

If 10% of this energy then propagates along the secondary
(a reasonable guess, I think) that's around 5 W to a radiator
which is perhaps 1/20th of a wavelength.  The proportion of
this which is actually radiated depends on the ratio of the
secondary's radiation resistance to its loss resistance at 10Mhz.
With some reasonable guesses, we could get a uV/m or so at 1km,
which could be quite detectable, especially as from this source
the energy could be confined to quite a narrow bandwidth.

> Indeed, a spectrum analyzer plot from a few hundred meters away
> would be an interesting thing to look at...

Very.  Would have to do a sweep with the coil off so that the
background could be subtracted.  Perhaps a synchronous demodulator
locked to either the firing rate or the secondary current.  Then
you'd be able to identify where on the TC the radiated RF was
being created.

The nice thing about RF coming from breakout, etc, is that it
would be fairly well spread across the spectrum, so that even if
the total power was quite high, the amount appearing within any
normal comms bandwidth would be quite low.

For this reason, a communications receiver may not be as good
as a spectrum analyser for picking up 'breakout noise'.
The voltage controlled tuner and IF stages from an old video
recorder or TV make a great wideband spectrum analyser for UHF.
Disable the agc and take the video output to scope-y, sweep to
scope-x.

I hope the OP can give us a little more info about what freqs
were involved, power levels, etc.  Obviously an area ripe for
some investigation. I'm sure that in principle it should be
possible to operate a TC in the most sensitive areas, and its
just a case of identifying and suppressing unwanted radiation
before your neighbours do it for you!  

But watch out, if the FCC/RA/nanny state, etc get their teeth
into coiling, then we'll be up the creek, well and truly.  If
ever TCs were legally declared to be 'spark transmitters' then
coilers would all have to retreat inside screened rooms. No more
outdoor TC displays.

Wonder if the commercial chaps have any regs they are already
required to meet re radiation?   Perhaps their gear has to be
FCC approved before use?  Let's try to avoid all that by not 
making enough 'noise' in the first place to attract attention!
--
Paul Nicholson
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