Re: Need Formula for length of spiral

["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>



Tesla list wrote:
> 
> Original poster: "rheidlebaugh by way of Terry Fritz
<twftesla-at-qwest-dot-net>" <rheidlebaugh-at-zialink-dot-com>
> 
> correction: The quarter wove length of a conductor is not the free space
> quarter wavelength.IT is 6 to 15% less than free space controled by Z of the
> coil or conductor.


More properly, the dielectric constant (epsilon) of the surroundings. For
all intents and purposes, air is epsilon=1. A thin coating on the conductor
will slightly load it, but the effect is more one of loss (sigma) than
speed (epsilon).

Also, don't confuse this with the "resonant half wave dipole length being
shorter than free space halfwavelength" so favored by antenna builders, and
alluded to in the 6-15% mentioned above.

Cutting an antenna slightly shorter than a half wave (or quarter wave for
ground planes) is an expedient technique to get the feed impedance
resistive (which improves the VSWR when feeding from the presumed resistive
source).  An idealized halfwave has a feed impedance that is mostly
resistive, but has a small reactive component.  The actual feed impedance
is a function of not only length, but diameter, as well.

Any standard antenna textbook, like Kraus or Balanis, will cover this, and
the formal derivation of it, in more detail than anyone could conceivably need.