# Some coaxial cable TEM mode data:

• 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
• 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