Re: Frequency of Flat Spiral Coil

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

Original poster: "Terry Fritz" <twftesla-at-qwest-dot-net>

Hi Dave,

At 11:30 AM 3/15/2002 -0600, you wrote: 
>
> Today I hooked up a flat plate antenna through a 20dB attenuator to an HP
> 5328A universal counter.  My flat spiral secondary measured 260KHz as its
> best resonance frequency when tested with a signal generator and voltmeter. 
> The frequency counter shows resonance in a range of 24KHz to 32KHz while the
> coil is actually operating.  Does anybody know what this means?



If the coil is being run from a spark gap, it will have a small duty cycle over
one second.  Each pulse only lasts maybe 300uS at say 120 BPS.  So the total
actual operating time in 1 second is 120 x 0.0003 = 0.036 seconds.  96% of the
time, the coil is not oscillating.  A frequency counter simply counts the
number of osciallations over 1 second.  Thus, a counter is not useful for
measureing the resonanat frequency of a disruptive coil.  If the coil is CW,
then it should work fine.  Best to use a scope triggered on the 120 BPS (or
whatever breakrate of the coil and measure the time for each cylce.  F0 = 1/Tc 
The actual frequency the counter measures with a disruptive coil depends an
many things that cannot normally be controlled.

>
>  
> Also, using the same antenna system and feeding into an oscilloscope, a nice
> pulse appears on the screen.  However, there are clearly two traces in the
> pulse.  I can manipulate the traces by adding the 30" flat plates to the flat
> spiral secondary terminal connection.  Adding a 30" plate 10" above the coil
> causes the positive trace to ring longer.  Moving the 30" plate 20" away from
> the coil makes the positive pulse ring longer yet.  Having a plate on both
> the bottom and the top causes both traces to ring longer.  But I haven't yet
> made the pulses ring in unison.  I'm going to add some degrees of tuning by
> winding a bare copper primary and connecting an 8 point spark gap.  



Not sure what the second trace is.  If you slow the sweep rate of the scope,
you should be able to get just one trace and figure out where the second is
coming from.  I assume if it is a dual trace scope, that the second channel is
disabled.  You may be seing the 60Hz from the NST charging super imposed on the
bursts as the gap fires.  You may be able to work with the trigger settings to
eliminate this.  Hard to say without being there.

>
>  
> If anyone knows why the coil appears to be running at a lower frequency than
> the tank circuits are tuned, I'd like to learn why.


The counter is great for checking the resoant frequency with a signal generator
or other constant source.  But for bursts of frequency, it is not any good. 
Use a scope for that.

>
>  
> Dave