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