Re: SSTC - experiments

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "Jim Lux" <jimlux-at-earthlink-dot-net> 

Many comments interspersed below..

 > Original poster: "Mccauley, Daniel H" <daniel.h.mccauley-at-lmco-dot-com>
 > Dave,
 >
 > One thing *you* are forgetting is that at 200-300kHz, a 4" electrode rod
 > placed above the top load is an
 > extremely inefficient radiator.  Its probably negligibly more than what
 > a SSTC radiates by itself.

Probably true.. since the SSTC is bigger than the rod.

 >To get efficient transmission of RF power, you need an efficient antenna.
Ooops (since this is my business, and I have to explain it all the time at
work in connection with things like HF radar sounders)... you need an
efficient SYSTEM.  The antenna is just the antenna, and it has some loss and
radiation characteristics. The efficiency is more determined by how you
couple the power to it. There is an efficiency aspect: radiation
resistance/(radiation resistance+loss resistance), but, even for physically
very, very small antennas, one can make the loss resistance small in
comparison to the radiation resistance.

Coupling power to it efficiently can be another matter, since small antennas
are typically very reactive and have very small radiation resistances (a ohm
or less), so you have to supply a lot of reactive power at high currents,
which makes the IR loss thing more and more of a problem.

If you slavishly design with 50 ohm output amplifiers, 50 ohm transmission
lines and so forth, then, sure, you're going to want an antenna that is
"resonant" (really, non-reactive) and has a feedpoint impedance close to 50
ohms, if only to reduce losses in the coupling networks that will transform
whatever you've got to the nice 50 ohms.  There's nothing saying, though,
that this is inherently more or less efficient than, say, a physically small
loop antenna with a radiation resistance of 2 ohms, coupled to a transistor
pushpull pair with an output impedance of 2 ohms, and the reactive power
sucked up or synthesized by the amplifier.

It's only if you're doing a system partitioning: Amplifier: matching
network: antenna, and requiring particular interfaces between the subsystems
that you wind up with potentially low efficiencies (after all, a 10 dB
attenuator makes a fine matching network, 20 dB return loss right out of the
box, no adjustment or pruning required)


   A
 > 1/2 dipole antenna would have to be 2340 feet for 200kHz.  There are other
harmonics, but these are buried
 > quite deeply.

A halfwave doublet is just a convenient, easy, non reactive antenna.  Of
some interest is that most AM broadcast station antennas are non-resonant
(at least the elements by themselves), and they are mightily concerned about
system radiation efficiency, since they are transmitter power limited.  Low
loss tuning networks and narrow band operation (which helps with the low
loss) are the key.
 >
 > Also, SSTCs radiate quite a bit of RF in general.  When I audio
 > modulated my coils, I can pick up the transmissions
 > on my friends HAM set-up (with his huge tower antennas) almost 10 miles
 > away (flat terrain, no hills, mountains, etc...)
 >
 > Dan
 >
 >  > Hi Dan,
 >  >
 >  > One thing you missed is that the rods may constitute an
 >  > antenna, and may
 >  > bring Uncle Charley for a visit.  Coils are classified as scientific
 >  > equipment and not transmitters due to no antenna.  Add that
 >  > antenna and then
 >  > your in trouble.
 >  >
 >  > David E Weiss

Tesla coils would be considered as "unintentional radiators" and subject to
the usual "must accept interference, must not cause interference, must shut
down if notified you're interfering" kinds of rules.  To fit in the
intentional radiator/transmitter category, you'd need some sort of
substantial, obvious, antenna.  A 4 foot rod attached to the top probably
wouldn't fit in that category.  A feed line to a 100 foot tower with a
matching network probably would.

If you're planning on selling/manufacturing SSTCs, then you've got a host of
other regulatory issues as well.