Some coaxial cable TEM mode data:
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the propagation
velocity of the transverse electromagnetic TEM mode is independent
of frequency (non dispersive)

in this case the wavelength
is the vacuum velocity of light divided by wavelength

the wave
impedance of a TEM mode in vacuum can be calculated from vacuum permittivity
and permeability

the vacuum wave impedance can
thus be calculated to be 377 ohms

broadly speaking, this is the
free space impedance that antennas need to act as matching devices to

if the signal were propagating
in solid teflon, then the permittivity is effected
by the relative permittivity of PTFE

this means that the propagation
velocity decreases to about 2.12*10^{9}m/s, i.e. about 71 percent
of the speed of light

for this reason any desired wavelength
dependent length of PTFE insulated cable must be multiplied by 0.71

if the coaxial cable is "foam"
(approaching vacuum or ~air) insulated, the propagation velocity factor
is about 0.88

for polyethylene insulation as
the propagation medium, the propagation velocity factor turns out to be
about 0.66

the relative
permittivity of vacuum (~dry air) is one

for a TEM mode coaxial
cable using PTFE insulation we can calculate the impedance

r_{o}=2.0 mm, r_{i}=1.1
mm > Z = 25 Ω

r_{o}=3.3 mm, r_{i}=1.0
mm > Z = 50 Ω
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Updated 03.04.2010