A new Tesla coil and k measurements

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

Original poster: "Antonio Carlos M. de Queiroz by way of Terry Fritz <teslalist-at-qwest-dot-net>" <acmq-at-compuland-dot-com.br>

Hi:

I have set up a page showing my Tesla coil with a flat primary,
where I tried to predict the coupling coefficient in advance:

http://www.coe.ufrj.br/~acmq/tesla/tefp.html

The prediction of the coupling coefficient didn't work very
precisely (designed: 0.105; measured 0.117), and I am thinking
that a reason may be the nonuniform current distribution in the
secondary coil. Errors in inductance calculations would move the
error in the opposite direction. Do programs as Fantc,
Acmi, and Mandk take the current profile into account?

The coil worked -very- well, producing impressive streamers that
almost reach the length of the secondary coil (but to the sides,
specially when I cover the tip of the antenna with a half sphere),
even with the relatively low power of the power supply (5 kV/30
mA NST and 5 nF of primary capacitance). Consequence of the good
coupling obtained, I imagine. The weather was also good and dry
during the tests.

An interesting observation is that visible improvement in performance
was obtained when I increased even more the effective coupling, by
connecting the bottom of the secondary coil to the central end of
the primary, while grounding the external turn. The mode passed from
8:9 to 6:7 (k=0.15). This is the "Oudin coil", in this case made
with two separate coils.

I tried too a solenoidal primary, designed to cause operation in
the very fast mode 3:4 (k=0.28). But I relied in the 3 cm clearance
between primary and secondary, and fixed the primary coil to a
15 cm PVC tube section with screws crossing the tube. What I got
was an arc between the secondary coil and the upper screw holding
the primary. A curious arc, spreading in a large plume when
touching the secondary coil ("racing sparks?"). Apparently,
it didn't perforate the varnish insulation, but just connected
capacitively to the coil. I should have taken a picture.

I will try to take better pictures in the next days.

Antonio Carlos M. de Queiroz