LESS LINE, STANDING WAVES, NODES, STANDING WAVE RATIO
If the transmission is not terminated in its
characteristic impedance ,then there will be two waves traveling along the line
which gives rise to standing waves having fixed maxima and fixed minima.
The ratio of the maximum to minimum magnitudes
of current or voltage on a line having standing wave is called the standing-wave ratio S. That is, S= E max
= I max; Emin =I min.
For coaxial lines it is necessary to use a
length of line in which a longitudinal slot, one half wavelength or more long
has been cut. A wire probe is inserted into the air dielectric of he line as a
pickup device, a vacuum tube voltmeter or other detector being connected
between probe and sheath as an indicator. If the meter provides linear
indications, S is readily determined. If the indicator is non linear, corrections
must be applied to the readings obtained.
The relation between standing wave ratio S and
reflection co-efficient k is, S =1+ k/1-k.
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