The radio frequency antenna is the main component of the entire signal chain, and if the quality of the antenna is determined, it is described by the antenna parameters.
The main parameters of the antenna are:
The directivity of the antenna The directivity of the antenna is to describe the different characteristics of the antenna in different directions. For the transmitting antenna, it radiates different energy of electromagnetic waves in different directions, and for the receiving antenna, it has different relative ability to receive electromagnetic waves with the same intensity from different directions in space. Different forms of antennas have different directivities. The directivity of an antenna can be represented by a directivity pattern, a directivity coefficient, or a lobe width (radiation width).
(2) Input impedance
The input impedance of the antenna is defined as the ratio of the high-frequency voltage to the high-frequency current at the two feeding points of the antenna (ie, the antenna input). The input impedance of different antennas is generally different. When connecting the antenna with a feeder, the characteristic impedance of the feeder should be the same as the input impedance of the antenna, also called matching, so as to effectively transmit the signal energy received on the antenna.
(3) Voltage standing wave ratio
The VSWR of the antenna is not only related to the properties of the antenna itself, but also to the properties of the cables and impedance matchers used. The better the antenna system matching, the closer the VSWR is to 1.
The gain of an antenna is different from that of an amplifier. It does not amplify the input signal, but describes the degree to which a directional antenna has a greater ability to radiate or receive electromagnetic waves than a standard antenna.
Gain definition of directional antenna: Under the condition of equal electromagnetic wave field strength, the useful signal power output by the directional antenna to the matched load in the maximum receiving direction (ie, the direction of the maximum value of the main lobe) is the same as that of the lossless omnidirectional antenna (standard) placed there. The ratio of the useful signal power output by the antenna) to the matched load, or the difference between the two levels. The symbol G is often used to indicate that the greater the gain of the directional antenna, the stronger its directivity.
Efficiency of the antenna Generally speaking, part of the power that the antenna gets from the feeder is radiated by the antenna; the other part is lost by the conductors and dielectrics that constitute the antenna body. Antenna efficiency is equal to the ratio of transmit power to input power. The same pair of antennas have the same efficiency value in both receiving and transmitting states.
The frequency bandwidth of the antenna refers to the difference between the two frequencies corresponding to when the electrical characteristics of the antenna, such as gain, directivity coefficient, and input impedance, meet the specified requirements, and the power transmitted to the feeder is 1/2 of the output power at the center frequency. Also known as the passband of the antenna.