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Re: Primary Coil Design

>>From Harry Adams -> adams-at-intranet.on.ca Thu Mar 28 01:19 MST 1996
<snip>
>As I understand (?) it, the Q of a coil is given by Q = (2 x PI x F x L)/R 
<liberal snip>
>So now you have the Q of the coil at some frequency F.  From the formula for
>Q the easiest way to increase Q is to decrease R
<snip>

Hello Harry and other interested coilers,
        This posting addresses the above and another posting which came
across this morning, which is similar, but relates to the secondary coil.
        The above formula is correct if you ALSO include proximity effects,
which cause the A.C. resistance to increase by another factor of 2-4 or so,
depending on many factors.  Proximity effect is the "bunching" of current in
a conductor due to electric/magnetic fields produced by nearby wires (and
objects like toroids).  As a result, the net resistance of the wire includes
both skin effects and proximity effects, and may be many times the D.C.
resistance of the wire.  Unfortunately, proximity effects are difficult to
estimate.  In a well designed primary with wire spacing of at least one
diameter between wires proximity effects should not cause significant effects.

        For close-wound secondaries this is another matter.  There appears
to be a net reduction in proximity effect for close-wound secondaries due to
the presence of a large toroid above a helical resonator.  As we all know,
the voltage between adjacent turns in a secondary coil becomes greatest near
the top of the coil due to 1/4 wave standing waves.  As a result, proximity
effects due to interactions between adjacent turns are most damaging near
the top of the coil.  
        A simple analysis of Malcolm Watt's much earlier posting of toroid
size/position demonstrates that the Q of a secondary coil can be optimized
by varying the size and position of the toroid above the top turn of the
secondary.  Note that it is dependent on both parameters.  So make sure you
vary BOTH the position of the toroid above the secondary AND the size of the
toroid if you can.
          What appears to be happening is that the electric field produced
by the toroid reduces proximity effects in the coil, thereby raising the Q
of the secondary.  This is probably due to the linearization of the 1/4 wave
voltage rise.  What I mean by this is that the capacitive top hat reduces
the phase angle by which the secondary must operate at to achieve 1/4 wave
resonance.  We need 90 degrees of phase shift for 1/4 wave, but the toroid
reduces this to perhaps 50-60 degrees along the coil, if a large toroid is
employed to make up the difference.  As a result, the voltage rise is more
linear across the secondary (plot sin(theta) for theta = 0 to 90 degrees,
then again for 0 to 50-60 degrees, to visualize the voltage rise from the
bottom to the top of the helical resonator).  This means that the overall
secondary Q is higher, and that losses are reduced.  The energy storage of a
larger capacitance (E=1/2 x C x V^2) seems to promote more energetic
breakout as well.
Comments/flames welcomed!

Regards,
Mark S. Rzeszotarski, Ph.D.