Voltage Standing Wave Ratio
(VSWR or SWR)
1 + |G|
1 - |G|
VSWR =
Vmax Vi + Vr
Vmin Vi - Vr
=
=
Vmax occurs at points where the impedance is purely real ñ
where the incident and reflected voltage waves have constructive interference.
VSWR is an index of how badly the impedances are mismatched.
A VSWR of 1 means the impedances are matched. Higher values
of VSWR indicate increasing mismatch.
The VSWR is associated with a traditional instrument used for
microwave measurements ñ the SLOTTED LINE.
The SLOTTED LINE is a length of transmission line with a narrow
slot which is an opening for a small probe. The probe can be a
short piece of wire to detect the electric field ñ proportional to the
voltage...or a loop of wire to measure the magnetic field, proportional
to the current. The probe is movable, and is used to find the maxima.
Until recently, the SLOTTED LINE was the workhorse of microwave
laboratories. Now, it generally sits on the shelf, replaced by the
more modern and easier to use Network Analyzer.
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