[Prev][Next][Index][Thread]

Re: This phase shift stuff...



Tesla List wrote:

>         A long thin secondary with no top load may be the best example to be
> studied.  I worry about a probe disturbing the fields around the coil and
> causing false measurements.
> A physically large coil will be less
> susceptible to this however.  Fiber optic probes made for the task (small)
> may have very negligable efects.  A continuous sine wave may not be the
> signal of choice.  Perhaps a pulse generator or arbitrary signal generator
> would be the best thing to use even though that is on of the few toys I
> still don't have ;-(

Some extra comments:
There is no need for a long thin secondary. The effects appear in any
form. I used a secondary with 12.5" of length and 3.5" of diameter.
The excitation of the primary with shorted gap by a slow square wave
current source allows extremely sensitive measurement of resonances,
because perfect beats (observed over the low-impedance primary) 
appear only at these frequencies, and they are practically not
affected by losses (they decay faster as the frequency increases, of
course). Losses cause the same measurements made with sine waves
far less clear.
I made the measurements using a simple 555 oscillator driving a
current source made with two transistors in cascode configuration.
No special signal generator was used.
And note also this: There are high-order resonance modes in coils
with large top loads too. It happens, however, that these modes
"see" the large top capacitance as a practical short-circuit to
ground, and put a voltage zero close to the top of the coil instead
of a voltage maximum. The frequencies of these resonances are
close to 2x, 4x, 6x, ... the main resonance frequency of the coil
without top load.
But do these high-order modes have influence in the operation of
normal coils? Ideally no, but they can explain why some coils show 
sparks along their lenght. If the primary is tuned to about twice the
correct frequency, this can cause a voltage maximum close to the 
middle of the secondary. Maybe sparks can also excite these modes.

Antonio Carlos M. de Queiroz