[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]
Re: SSTC As a transmitter.
Original poster: "Paul Nicholson by way of Terry Fritz <twftesla-at-qwest-dot-net>" <paul-at-abelian.demon.co.uk>
Gary Peterson wrote:
> what impedance does the lower end of a magnifying transmitter's
> extra coil see, looking towards ground?
Anything from several 10s of ohms up to several hundred ohms.
> Tesla liked to use proportionately tall helical resonators when
> an application required sharpness of tuning. This would also
> help get the isotropic cap up into the air without having to use
> an excessively long vertical conductor, which would be a source
> of that undesirable electromagnetic radiation.
So the long vertical conductor would radiate but not the tall
vertical coil :)) In fact the EM radiated from the TC is
proportional to the amount of charge displaced into the topload
during each RF half cycle, and the height of the topload.
Radiation is very little unless the topload height is a non-
negligible fraction of a free-space wavelength. At say 150kHz,
a topload at 100 metres elevation would give you 1/20th of a
wavelength and you would surely need a license at that point.
> It would be nice to have a lower frequency allocation, say in
> the area of 73 kHz
If you're not using EM radiation, you don't need the license. You
just need to convince the inspector that you're using a non-EM
During the '70s some radio amateurs were keen on what they called
'earth mode' communications, which basically involved pumping lots
of LF power circa 20kHz into the ground, using two electrodes set
into the ground quite some distance apart. The receiver uses
a similar pair of electrodes to pickup up the differential voltage
due to the current field in the earth excited by the transmitter.
As you can imagine the signal strength falls off very rapidly with
> I'm interested in seeing the results of real-world propagation
> studies that involve the use of slow-wave helical resonators and
> elevated capacitances.
If you're interested in power transmission, try measuring the
coupling coefficient between two TCs as a function of range. The
coupling will fall away very rapidly (about the same as the earth
current mode mentioned above). As the topload is raised, the
coupling falls away less rapidly, and eventually you end up with
the topload a quarter-wave above ground and a coupling proportional
to 1/range^2 at long distances, in which efficient radiation is
And what is meant by a 'slow-wave' resonator? Does the adjective
> ...that undesirable electromagnetic radiation.
Undesirable? If you want propagation over long distances, then
you need to be encouraging radiation, surely?
> The lowfers use a technique called coherent CW which I think
> uses WWV to synchronize transmitters and receivers.
Yes, WWV, and in UK, the MSF, can be used to sync both the
carrier frequencies and modulation phase at each end. Extremely
narrow bandwidths can be used to enhance signal/noise ratio, and
the limitation here tends to be phase distortion during
propagation, which sets a limit to the minimum reciever bandwidth
in any given situation.
On the whole, a non self-oscillating CW TC should make quite a
good exciter for a short (wrt wavelength) wire antenna. The
high impedance at the top of the coil is appropriate for the
high radiation resistance of the short wire. A low frequency
antenna tuner correctly set up to match a transmitter into a
short wire would look a lot like a base-fed TC.