Re: 3/4 wavelength error
Tesla List wrote:
> >From rwstephens-at-ptbo.igs-dot-netFri Jul 26 11:42:23 1996
> Date: Thu, 25 Jul 1996 21:07:56 -0500
> From: "Robert W. Stephens" <rwstephens-at-ptbo.igs-dot-net>
> To: tesla-at-pupman-dot-com
> Subject: re: 3/4 wavelength error
> Skip Greiner,
> In your inquiry about 3/4 wavength secondaries you wrote:
> >In any event I do not believe a 1/4 or 3/4 wave coil can be wound in any
> >"reasonable" configuration. In fact the resonant frequency of 1/4 wave
> >coils is always about 40% higher than the 1/4 wave wire length and a
> >toroid must be used to bring in the frequency even taking into account
> >the increase in cself due to ionization.
> In my response yesterday I wrote:
> >That statement is counter to my experience. I have found that when a
> >given freespace 1/4 wave wirelength is made into a coil, the resulting coil
> >operates at a longer wavelength (lower frequency) than its equivalent stretched
> >out wavelength. For example, I have wound 753 feet of #22 enamelled wire
> >onto a 4.1875 inch diameter form to a length of 22.675 inches. 753
> >feet of wire represents a freespace 1/4 wavelength at 327.6 kHz. The
> >actual measured Fres of this coil with no topload is 317.6 kHz. By
> >forming this length of wire into a coil the frequency dropped 3.1%.
> >Another way of looking at this is that by winding the coil into a
> >spiral, the velocity of propagation down the wire has slowed by 3.1%.
> Please disregard what I said. It turns out I am in error and don't
> know what I am talking about. When I characterized this coil and
> several others a couple of years ago I did so by hooking up a signal
> generator to the bottom and an O-scope to the top through a X10
> probe. The frequency of resonant rise was then written on each coil with
> felt pen. I've never tested them since. Well, guess what, that
> O-scope provided the capacitive loading which explains the low
> resonant frequency. The stupid part is here I've lived my life now
> for two years erroneously thinking that winding wire into a coil dropped the
> frequency. All seemed well and I was at peace with what I thought I
> understood about radio physics. It certainly hasn't hurt the highly
> successful coils I've been turning out! Anyhow, I thought I'd
> better admit my mistake here before all the flame mail starts. I
> just hope nobody threw away their Tesla coil software after reading
> what I said.
> I've just re-tested this small coil and find that it does indeed
> resonate at 151% of its freespace 1/4 wavelenth actual wirelength.
> This is much more in line with your stated experience.
> You also wrote:
> >I would certainly appreciate any comments to this and I would especially
> >like to hear from anyone who has built a 1/4 or 3/4 wave coil that comes
> >in close to the correct frequency.
> I replied:
> >I cited an example above in 1/4 wavelength resonance that was just
> >3.1% off. Now that I think I understand your definition of 'correct
> >frequency', I suspect that this may not actually be possible. As
> >soon as you wind a straight wire into a coil you increase its total
> >inductance, and that combined with self capacitance will lower the
> >self resonant frequency of that piece of wire.
> Please also disregard this portion of my reply as erroneous.
> Prior to my somewhat recent work with Tesla coils all my RF experience had
> been in microwave with waveguides, and cavity resonators. At these
> high frequencies coils are not used and the technology is a completely different
> Still learning, rwstephens
Thanks for ther feedback. I was about to go out to buy some 22 guage wire
to start a new coil since I wind all of my coils with #12 to # 16 guage.