Читать книгу Antenna and EM Modeling with MATLAB Antenna Toolbox - Sergey N. Makarov - Страница 43

Along with the reflection coefficient Γ or S₁₁, another measurable quantity of significant interest is the voltage standing‐wave ratio or VSWR. On a transmission line connected to non‐matched antenna, both waves – incident and reflected V⁺ and V⁻, respectively – form a prominent standing wave. At every point in space, this standing wave has a certain amplitude as a sinusoidal function of time. The VSWR is given by the ratio of maximum and minimum standing wave amplitudes on the line. It may be shown that [1–3]

(1.38a)

For the matched antenna, the VSWR is exactly one; for a non‐matched antenna, it is always greater than one, and may even approach infinity for a short‐ or open‐circuited antenna.

The VSWR may be used instead of the reflection coefficient to determine and plot the impedance bandwidth. In this case, the criterion of

(1.38b)

corresponds to

(1.38c)

with a sufficient degree of accuracy.