Re: secondary harmonic suppre

Hello coilers,
Richard Craven said:
>In a standard helical resonator the device is a quarterwave 
>long and can be link or loop coupled (see the ITT databook 
>for a section). The problem is that such a resonator "throws 
>back" at harmonics by which I mean that a band pass filter, 
>for example, will have good out-of-band attenuation except 
>at harmonics of f (2f particularly so in elliptic designs).
>The point is that there is a technique where the top third 
>of a coil is wound in the opposite direction to the first 
>two thirds. It struck me that it might be interesting to 
>wind a TC resonator such that the top third is reversed. 
>Apparently, if you do this, the harmonic response at 2f and 
>3f is dramatically suppressed and the first significant 
>harmonic occurs at 4f, by which time the amplitude is well 
>down cf. the fundamental.
>My suggestion is that there will be less out-of-phase 
>currents flowing in the resonator; the secondary output will 
>be more coherent and the power in the fundamental will be 
>higher. If someone has access to a coil winding setup, it 
>would be interesting for that person to copy and existing, 
>proven resonator, but reverse the top third (i.e. if you 
>have 600 turns, make the top 200 go the other way). I would 
>imagine the sudden change in direction could be anchored 
>with a suitable adhesive blob.
        The idea sounds very interesting, with several caveats.  First, the
distributed capacitance will probably be about the same, since it is mostly
determined by the surface area of the coil for a closewound coil.  Second,
the total inductance of the coil will be reduced somewhat since some of the
flux linkages of the windings will now be in opposition.  This effect could
be reduced by leaving some space between the two windings.  Finally, when
used as a conventional tesla coil, the change in direction should be placed
at least one coil diameter (and preferably two) above the base of the coil,
since most of the mutual inductance between the primary and secondary occurs
in the bottom of the coil.
Mark S. Rzeszotarski, Ph.D.