Keywords
co-axial cable, transmission line, characteristic impedance,
power rating, dielectric breakdown, dielectric hysteresis,
loss
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Abstract
The physical basis for the operation
of co-axial transmission lines is ancient, well-understood
and fundamental to radio frequency electronics. However,
some aspects of the link between the underlying physics
and the practical implementation appear rather obscure;
for example, the reasons for the widespread use of two
particular characteristic impedances, 50 and
75.
Also,
interest is gathering in the topic of transmitter power
ratios for switch-over from analogue to digital terrestrial
television, and the capacity of the co-axial ‘feeders’ to
handle the requisite average and peak powers features amongst
a long list of critical factors that will affect the cost
and practicality of switch-over.
This White Paper provides
some of the necessary background about the electrical
characteristics of co axial cables.
It is demonstrated that there are optimum characteristic
impedances and they are evaluated, and it is shown that
practical co-axial cables exhibit a fundamental peak-to-mean
ratio; they can withstand short-term peaks of voltage
considerably greater than the voltage corresponding
to the long-term
maximum power rating.
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