Re: 1/4 wave theory/cite the variance?

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "Jim Lux by way of Terry Fritz <twftesla-at-qwest-dot-net>" <jimlux-at-earthlink-dot-net>



> >         One infers it is the freq at which the thing goes 1/4 wave
> >         resonance.  relatively easily determined.  First (and lowest)
> >         of the self resonant modes.
> 
>         Lowest of the self-resonant modes is clear enough.  For an isolated
> wire that occurs at a length of a half wavelength, and for a grounded
> vertical wire at a quarter wavelength, due to fact that there's an image
> antenna below the ground.  If I remember my mechanics, you could
> substite isolated bar and clamped bar.


To flog the dead equine a bit... resonant (in the sense of having no
reactive impedance component) lengths are a bit short of the multiple of
1/4 wavelength (and the amount short varies by diameter of conductor,
relative to length, the medium, and which multiple you are talking a bit).

However, if one is speaking of the frequency at which the current, voltage,
charge, or whatever,  forms a nice sinusoid along the wire, that IS at the
actual appropriate fraction of the free space wavelength. (and, the antenna
has a fair amount of reactance at this point..)

The latter is what the open/closed tube or clamped bar, or string, etc.,
would be comparable to.  I wonder what the equivalent impedances are in
both cases (assuming that the amplitude is small enough that the
nonlinearities are negligible)