propagation velocity, wire length was 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>

But in coax (for which velocity factor is an issue), the entire EM field is
contained in the dielectric, hence the noticeable change in velocity
dependent on the dielectric (proportional to 1/sqrt(epsilon)).  In an
insulated conductor in air, with no shield around it, most of the field is
in air, so the velocity change is quite small.  If you have significant
capacitance (compared to the inductance), that can also slow the
propagation, accounting for the small phase shift in the current (or
voltage) between top and bottom of the secondary (the magnitude of which
has been both measured and theoretically predicted, so we don't get the
quarter wave thread going again...) 

There are two effects going on here. One is the EM propagation, the other
is the transmission line effect. The former is affected by dielectric
constant (epsilon) which is essentially related to the index of refraction
(light being, after all, just another form of EM radiation). 
There is also the propagation velocity in a transmission line, which is
dependent on the distributed L and C along the line. Coax with air (or
vacuum) insulation also has a propagation velocity less than free space
(inversely proportional to sqrt(LC)), as do "synthetic transmission lines"
used for delay lines and pulse forming networks. There is this really weird
coax used for delay lines with a spiral inner conductor creating very high
L per unit length, and a correspondingly low propagation velocity (it's
also real high impedance (1000 ohms?) since impedance goes as sqrt(L/C)).
Changing the dielectric in coax changes the propagation speed for two
reasons: a) it changes the C per unit length and b) it changes the medium
in which the EM wave propagates.  The former is a stronger effect than the
latter, for what it's worth.

Finally, waveguide propagation (just another transmission line, really,
with a different mode of propagation) is also slower than free space.



Tesla list wrote:
> 
> Original poster: "rheidlebaugh by way of Terry Fritz
<twftesla-at-qwest-dot-net>" <rheidlebaugh-at-zialink-dot-com>
> 
> Your statement is well made. I was not looking at the size of an antenna,
> This is a good posting. I was looking at the propagation velosity of an AC
> signal on any conductor.  All coax venders list the velosity difference
of the
> coax they sell.You nead this to cut your cable.  A conductor is not free
space
>       Robert  H
> 
> > From: "Tesla list" <tesla-at-pupman-dot-com>
> > Date: Thu, 14 Feb 2002 16:53:41 -0700
> > To: tesla-at-pupman-dot-com
> > Subject: Re: Need Formula for length of spiral
> > Resent-From: tesla-at-pupman-dot-com
> > Resent-Date: Thu, 14 Feb 2002 17:17:15 -0700
> >
> > 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.
> >
> >
> >