It is the figure traced out in time by the instantaneous electric field vector associated with the radiation field produced by an antenna. Electromagnetic waves in free space travel in a direction that is perpendicular to the direction(s) of oscillation of the electric and magnetic fields. For example, if an RF wave is traveling in the z-direction, the electric field could be oscillating in either (or both) the x- and y-directions (referred to as the horizontal and vertical directions).
Examples: (1) RF waves can be separated into vertical-only and horizontal-only polarized waves, both of which are a type of linear polarization. (2) Satellite signals are often circularly polarized, having both vertical and horizontal components rotating around the z-direction of wave propagation.
The difference between the power input to and the power reflected from a discontinuity in a transmission circuit. This parameter is often expressed as the ratio in decibels of the power incident on the antenna terminal to the power reflected from the terminal at a particular frequency or band of frequencies.
Example: Antennas often are designed to have a return loss of -10dB or less, meaning that at least 90% of the electrical energy generated by the radio is actually transferred into electromagnetic wave energy.
Voltage Standing Wave Ratio (VSWR)
Is another way to measure return loss. It is a ratio of the maximum to minimum amplitude (or the voltage or current) of the corresponding field components appearing on a line that feeds an antenna.
Examples: Antennas are designed to have a VSWR of 2.0:1 or less, meaning that at least 90% of the electrical energy generated by the radio is actually transferred into electromagnetic wave energy. Return loss of -9.6 dB is equivalent to a 2.0:1 (VSWR).