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*109m/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
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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
-
ro=2.0 mm, ri=1.1
mm -> Z = 25 Ω
-
ro=3.3 mm, ri=1.0
mm -> Z = 50 Ω
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Updated 03.04.2010
