Journal For Combination Flat Spiral And Solenoid Secondary Coil

March 25, 2002

Today I tested a combination of flat spiral and solenoid coils consisting of a flat spiral:

  • Diameter - 25.25"
  • Wire Gage - 12
  • Turns - 144 (counted)
  • Center Post - 1/4" brass threaded rod
  • Coating - 3 layers polyurethane and 2 layers Envirotex Lite polymer coating
  • Inductance - 4.42mH (measured)

And a solenoid coil:

  • Diameter - 1.3125"
  • Length - 27.5"
  • Wire Gage - 21
  • Turns - 947 (calculated at 98%)
  • Coating - 3 layers of polyurethane and the top half covered with one layer of Envirotex Lite
  • Inductance - 2.03mH (measured)

The total inductance measured with the solenoid coil connected to the center winding of the flat spiral coil was 6.93 mH.  The frequency was measured with a frequency generator, voltmeter, and oscilloscope at 428 KHz.  While measuring the frequency, I noticed many unusual wave patterns.  Most of the waveforms did not look like a sine wave at all.  There would be a sudden peak followed by a partial, gradual decline and then a sudden drop to the opposite polarity peak.  It was more like a sharp pulse than a sine wave.

Total secondary capacitance was back-calculated at 19.95 pF.  This included the 4" diameter copper ball seen below.

The primary capacitor is .009 uF and the primary coil can be tapped from 7 uH to 22 uH and was tapped at 15 uH for this test run.  The primary capacitance and inductance were measured.

The spark gap is an air-cooled 8-point series gap with only two gaps used.

Beyond the coating on the flat spiral coil, there are also two 1/4" thick Plexiglas disks between the primary and flat spiral coils.  This was found to be necessary due to arcing through the Plexiglas.  Now there is no arcing directly between the flat spiral and primary coils.

The bottom lead of the solenoid coil is connected to the center lead of the flat spiral coil.  The outer lead of the flat spiral coil is connected to the ground.

Below is a picture of the secondary setup...


The solenoid coil was wound to be 3 times the wire length of the flat spiral coil.  It wasn't clear to me before starting the winding whether the resonance would be two separate waves or one continuous wave.  Also, my goal was to have the flat spiral be the main resonator, and then hopefully, there would be 3 of the flat spiral waves in the solenoid coil.

During the calculation and tuning process, however, I decided to start with the resonance of the total system.  Before doing more exotic tests, I felt it important to apply standard Tesla coil operating techniques to get a benchmark.  Using the above parameters and tuning for 428KHz, I got the output as seen below...


There was nothing particularly striking with the output of this setup other than the pattern of electric jets (I incorrectly called these streamers at first) from the copper ball.  The electric jets were straight and occurred consistently from the same positions.  They did not dance around as streamers on a solenoid-only coil behave.  The output of the flat spiral/solenoid combination resembled that of a plasma ball under high pressure but without the rarefied gas effect.  The electric jets were not long and varied from 2 to 4 inches. 

Another interesting observation is that even though the spark gap ran continuously, the length of the electric jet output varied.  After making a few adjustments for closer resonance, the electric jets dropped back to less than an inch and expanded to 5" in length.  There was no particular rhythm to this expansion and contraction effect.  It almost seemed the coil was responding to the electric field in the ground connection, surrounding atmosphere, or both.

There were no corona discharges in the section of the solenoid covered with polymer coating.  But in the lower part of the solenoid coil, specific locations emitted a jet of ions and generally glowed with corona. 

I was shocked (literally) to find out that a solenoid coil could hold a substantial charge for several minutes after the coil was turned off and even removed from the system.  In one case, I stroked my fingers along the coil and was shocked about midway down.  Another time I removed the solenoid from the flat spiral to adjust.  The connection between the two is via a copper alligator clip.  When I touched the alligator clip, I was shocked while removing the coil.  About 30 seconds after receiving the shock and with the coil disconnected, I touched the alligator clip again and received another shock.  Not being amused, I moved the alligator clip in contact with a ground wire to drain any remaining energy.  After touching the alligator clip to the ground wire for a couple of seconds, I touched the clip with my hand again and received another shock.  All three shocks were of considerable charge. 

After running the tests above, I connected a 30" flat disk to the bottom of the spiral coil center lead.  The rest of the coil remained in the same configuration.  The lower disk did not diminish performance much.  But there were ion leaks around the disk, which slightly reduced the copper ball output.  With the copper ball removed, the discharge was still in the same pattern but emanated from the 1/4" brass threaded rod.  I will disassemble the coil tonight and coat the disks, copper ball, and solenoid coil with another layer of polymer coating.  I'll also try to find something to use for immersing the capacitors in oil. 

When I start the coil up again tomorrow night, I'll try to tune to the resonant frequency of the flat spiral coil alone and see if I can get four wavelengths in the combination system.  [I never got around to this experiment as I began looking into the physics involved.]