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Re: slow-wave helical resonator
Original poster: "Ed Phillips by way of Terry Fritz <twftesla-at-qwest-dot-net>" <evp-at-pacbell-dot-net>
Tesla list wrote:
> Original poster: "Gary Peterson by way of Terry Fritz
> > > > . . . what is meant by a 'slow-wave' resonator? . . .
> I would appreciate the list's critical comments regarding the accuracy of
> the following definition:
> slow-wave helical resonator
> A resonant circuit consisting of a single-layer cylindrical inductor wound
> in the form of a helix, usually including capacitive end loading. The speed
> of a wave disturbance along the axis of the helix is significantly less than
> the propagation of an electromagnetic wave in free space. A Tesla coil's
> secondary and a magnifying transmitter's base-driven extra coil can both
> behave as a slow-wave helical resonators, the latter propagating the wave at
> around 17% the speed of light in free space.
In my experience High-Q helical resonators are designed with an L/D of
the order of 1/2 to 2, shielded to improve the Q, and the only end
loading is some probe or capacity disk for fine-tuning. The propagation
velocity is a function of the geometry and the frequency. [A helical
resonator with capacitive end loading gets pretty close to a regular L/C
circuit, depending of course on the wire length and the operating
frequency.] My ITT handbook (REFERENCE DATA FOR RADIO ENGINEERS) makes
a distinction between "helical resonators" and "quarter-wavelength
helical resonators". The conductor length length for a
"quarter-wavelength resonator" is "of the order" of a quarter
wavelength, with the actual value depending on the geometry.
TC's seem to me to be closer to helical antennas, whose effective (V/c)
varies from around 4 for ND^2/lambda of 0.003 to 1 at ND^2/lambda of
about 0.4. Since ND^2/lambda is close to zero for most coils, they are
extremely poor radiators.
P.S. I suspect a Google search on "helical resonators" and "helical
antennas" would generate a lot of hits.