A 1/4 wave Tesla coil

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

Original poster: "Skip Greiner by way of Terry Fritz <twftesla-at-uswest-dot-net>" <sgreiner-at-wwnet-dot-com>

Hi All
 
I thought that some of you might be interested in a true 1/4 wave coil.
Following is a complete description. This was designed and built to be 1/2 half
of a twin. The second secondary is now under construction and will be added
soon.
 
Secondary:
Skeleton form made of 12 polystyrene 1/2" rods and acrylic spacers
Size: 14" x 30"
Turns: 240
Wire: #16 stranded PVC covered machine tool wire, 1/8" in diameter
Wire length: 880'. This corresponds to about 280khz
Free air resonance: 400khz
Ctop: toroid 8" x 25" dryer duct with aluminum foil coating
Ctop capacitance: ~22pf
Secondary resonance with Ctop: ~280khz
 
Primary:
Diameter: 19"
5 turns 3/8" copper tubing on 5/8" centers
Capacitor: .021uf
Primary resonance with cap: ~280khz
 
Primary/secondary system resonance: ~280khz 
System Q: over 150 but measurement limited by my instrumentation
K: Not measured at this time but expected to be .21 or higher
Best coupling is obtained with top of primary about 1/2" above bottom of
secondary
 
Power: 15/120 NEST
Break: SRSG, 1800 rpm and 120 BPS
Contact spacing: 0.2" total, so far but still working up to larger spacing
 
Performance:
Operating frequency: 280khz
Spark length with no breakout tab: multiple arcs 48"(+)
Spark length with breakout tab: single arcs 60"(+), very, very thick
Interesting note: With no Cop, arcs are close to 42" from top terminal, but
coil is obviously poorly tuned and much corona around top turns of secondary.
 
This coil operates at the design frequency. There is no indication that any ion
cloud adds capacitance to the system that might lower the operating frequency. 
This coil was designed basically using Medhurst and Wheeler formulae. No tuning
was required after assembly except to phase the SRSG and to increase the
coupling between primary and secondary.
 
Changing the primary turns only reduces the output length of sparks. Question
to those on the list with experience: The output sparks are very thick and fat
but only reach 60"(+). The coupling is as close as possible, just before racing
sparks start, and changing the SRGS phasing only decreases the length. I would
appreciate any suggestions to increase the spark length. John F., I know you
have done much in this area and I would appreciate your comments in particular.
Note also that the above indicated operating characteristics occur with about
100 volts input and additional voltage up to full line voltage seems to make
little if any difference in spark length.
 
Skip Greiner