The impedance at the antenna feed point. From the voltage and current at the input feed point comes the following equation.

(input impedance) = (feed point voltage) / (feed point current)

If the feed line Characteristics exhibit an impedance of 50Ω, the input impedance will be 50 + j0Ω and you will have perfect matching.

When connected to a transmission circuit, if there is a mismatch between the transmission line impedance and the antenna impedance, the signal will reflect at the feed point. The return loss is displayed as a ratio between the power feed to the antenna and the power reflected back at the feed point. If all the power is reflected back, the return loss value will be 0dB. If all the power is absorbed by the antenna then the value will be –infinity dB.
When the return loss value is low, it means that antenna has been well matched to that frequency.

When the impedance at the feed differs the reflected wave will combine with the original wave and is called a standing wave. The ration of maximum voltage [Vmax] of the standing wave with the minimum voltage [Vmin] is called the Voltage Standing Wave Ratio [VSWR].
When there is no reflection, the VSWR value is 1. The lower the VSWR value, the smaller the reflection.
In the case of small antennas, a VSWR value of 3 or less is standard.

The strength of the signal transmitted by the antenna is a function of the angle at which it is transmitted. Directivity is calculated as a ratio between the power in the direction the antenna is transmitting and the average power (spherical) of the transmission. Directivity can be used to better understand how radio waves spread out.

The gain of an antenna is the ratio of the transmitted power in the direction of the antenna to the uniform value of a standard antenna.
For the standard antenna, if an isotropic radiator is used then it is given as the absolute gain [dBi], while a half wave dipole would be given as a relative gain [dBd] .
The relation between them is (absolute gain) = (relative gain) + 2.15dB
If there were no internal loss in an antenna, the antenna would be perfect and have equal output in all directions.

For the input power, it is show as a ratio with the transmission power with the following relationship. (Radiation efficiency) = (radiated power) / (input power) [(input power) = (radiated power) + (lost power)]
A good antenna will have a high efficiency rating and show little loss of power. When the directivity is efficient, the gain becomes uniform.

An anechoic chamber is a special type of darkroom for radio waves, in which the ceiling, walls and floors are covered with special materials to absorb radio waves to prevent them from reflecting. In this type room it is possible to test and measure the directivity of an antenna. At Mitsubishi we can quickly respond to customers requirements through the use of our in house chamber and invite customers to visit during the test. Please contact us for more information.

Our anechoic chamber