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Re: Wire length,resonance, and Q (fwd)



---------- Forwarded message ----------
Date: Thu, 14 May 1998 22:39:42 -0400
From: bmack <bmack-at-frontiernet-dot-net>
To: Tesla List <tesla-at-pupman-dot-com>
Subject: Re: Wire length,resonance, and Q (fwd)



----------
> From: Tesla List <tesla-at-pupman-dot-com>
> To: tesla-at-pupman-dot-com
> Subject: Re: Wire length,resonance, and Q (fwd)
> Date: Tuesday, May 12, 1998 1:41 PM
> 
> 
> 
> ---------- Forwarded message ----------
> Date: Tue, 12 May 1998 09:04:05 -0500
> From: David Huffman <huffman-at-fnal.gov>
> To: Tesla List <tesla-at-pupman-dot-com>
> Subject: Re: Wire length,resonance, and Q
> 
>
> >
> >The most intriguing thing is the cases where the coil resonates at
> >frequency HIGHER than the wire length alone indicates!  Malcolm made
> >a passing refence to this in one of his recent posts as well. 
Preliminary
> >quick experiments indicate that the coil geometry has alot to do with
it's
> >ultimate resonant frequency apart from the length of the wire.  Really
> >bizzare things happen when the aspect ratio is below 0.1.
    
     "I didn't think this is true. The max frequency. occurs when the wire
is
> straight and in a vacuum with nothing else around it. As soon as you
begin
> to coil the wire the resonance goes down.
> Dave Huffman"

   So did I, thats why I posted it!   
   
   The experiment I ran, was basically making measuments of 
commercial (rather old surplus types) and homemade resonators.
A chart was made of all their vital statistics, including wire length used.

much to my surprise, there were two long coils where the max 
ampitude was at a frequency greater than the natural frequency of the
wire length.  These were definitly resonant peaks- not just "hot spots",
as no other frequency above or below produced anywhere near that
amplitude.

Some folks on the list seem to have the eronious impression that I
am promoting the "Length method" .  I thought the post was clear on
just how unreliable  that method can be!  Obvously there IS a wire
resonance effect, but nailing it down can prove elusive. Thats why RF
engineers are cautioned that the length of wire  used in air core coils
should always be less than 1/8 wave to avoid spurious responces.
JM

Sulaiman wrote:

"Our coils have TWO resonant frequencies; one is equivalent to the
straight wire case, quarter-wavelength; the other is determined
by self inductance and capacitance which for typical coils WILL
be at a higher frequency than the quarter wavelength frequency.
Ideally we add capacitance to bring the LC resonance down to the
same frequency as the quarter wavelength frequency to ensure
optimal performance, that is no standing-wave-type voltage peaks
before the top-load."

How did you determine this? Do you have empirical results?
Again, WHY would a higher frequency be "typical"???  JM





















> >According to conventional physics, (let me know if I missed something)
> >a charge and it's attendant feilds will propagate faster  in a straight
> >wire
> >than in a coil. It follows that the coil should always resonate lower
than
> >the wire since the velocity is less than the speed of light.
> >
> >Why then, do long space wound coils resonate at a frequency higher
> >than expected? This has nothing to do with the LC ratio either. I would
> >expect that no matter what gain or reduction of L vs C for a given
> >geometry, they should always result in a frequecy lower than that of
> >a straight wire.  Whats going on here???
> >
> >Before I go and re-invent the wheel, does anyone have an explaination
> >and/or experimental data on this?
> >
> >Curious in NY
> >Jim McVey
> >
> >
> >
> >
> >
> >
> >
> >
> >
> >
>