Principle, classification and requirements of antenna

One 、 Antenna action and reciprocity
An antenna is a device that radiates and receives radio waves .
Common antennas are mobile base station antennas 、 Broadcast antenna 、 radar antenna 、WIFI The antenna 、 Mobile phone antenna, etc .

The role of the antenna
1、 Energy conversion ： The antenna mainly completes guided wave （ Or high frequency current ） Energy conversion with free space waves , Therefore, the antenna is called energy converter . The transmitting antenna converts guided traveling waves into free space waves ; The receiving antenna converts the free space wave into guided traveling wave . The signal output by the hardware circuit is transmitted to the antenna in the form of high-frequency current , It is transformed into free space wave by antenna , Send signals into free space in the form of radio waves . The receiving antenna converts the received free space wave into high-frequency current , Then the signal is recovered through the processing of hardware circuit .
2、 Directional radiation ： The antenna has certain directivity , Generally, the transmitting antenna only radiates electromagnetic waves in the required direction as far as possible , The receiving antenna only receives incoming waves in the specified direction , Minimize interference and noise from other directions .

Reciprocity of antenna
When any type of antenna is used for transmission and reception , Its impedance 、 Directions 、 Polarization 、 The effective length and other characteristics are the same . therefore , Generally, there is no need to design the transmitting and receiving antennas separately , Can send and receive the same antenna . for example , The router antenna is the common antenna for sending and receiving .
Take the router for example , For antennas , There is no difference between its uplink and downlink performance . In the test of some routers of our company, there will be differences in uplink and downlink speeds , This is not caused by antennas , It has something to do with software processing .
Two 、 Radiation principle of antenna
When there is alternating current flowing on the conductor , Electromagnetic radiation can occur . If two wires are very close , The induced field produced by the two wires is counteracted in reverse , So the radiation is very weak ; Spread the two wires , The induced electric field generated by the two wires is superimposed in the same direction , Therefore, the radiation is strong .
The ability of radiation is related to the length and shape of the wire , When the length of the wire L Much smaller than the wavelength λ when , The radiation is weak ; Length of wire L When it increases to be comparable to the wavelength , The current on the wire will increase greatly , Thus, strong radiation can be formed . When the wire length is an integral multiple of half wavelength , The antenna is in a resonant state , Strong radiation .
The frequency range of radio waves is 300MHz～300GHz The radio waves of are called microwaves , Microwave is subdivided into different bands , Different wave bands have different propagation characteristics .
Radio waves are similar to light waves , Its propagation speed is related to the propagation medium . The propagation speed of radio waves in vacuum is equal to the speed of light .
We use it $C=3\ast 10^8$m/s Express . The propagation speed in the medium is ：$V=C/({\epsilon }_r{)}^{\frac{1}{2}}$ , In style ${\epsilon }_r$ Is the relative dielectric constant of the propagation medium .
Radio waves are like ripples on a pond , The wave will gradually weaken as it propagates .

chart 2.1 Water wave attenuation diagram
Radio waves are a form of energy transmission , In the process of communication , Electric and magnetic fields are perpendicular to each other in space , At the same time, both of them
Are perpendicular to the direction of propagation .

chart 2.2 Radio wave propagation model
The commonly used antenna is a vibrator with equal length of two arms, which is called a symmetrical vibrator （ Also known as dipole ）. Symmetric oscillator is a classic 、 The most basic 、 By far the most widely used antenna . The symmetrical oscillator can be regarded as formed by a double-wire transmission line with an open end . A single symmetrical vibrator can be used simply and independently , Multiple symmetrical oscillators can also be used to form an antenna array . The length of each arm is one quarter of the wavelength 、 A vibrator with a full length of one-half wavelength , Say half wave symmetric oscillator （ Also called half wave dipole ） .

3、 ... and 、 Antenna classification
In order to adapt to different uses , There are various forms of antennas . There are also different classification methods for these antennas . Including classification by frequency band 、 Classification by use 、 Classification by polarization 、 According to the classification of gain . At present, the classification based on principle is widely used , The antenna is divided into line antenna and area antenna . The wire antenna is composed of wires , The length of the conductor is much larger than the cross section . The surface antenna is composed of a whole metal plate or wire grid .

Four 、 Basic requirements of antenna
high efficiency ：
The antenna acts as an energy converter , We hope it can radiate as much input power as possible , That is, the higher the radiation efficiency of the antenna, the better .
Directional ：
A lot of times , We don't need electromagnetic wave energy to be evenly distributed in the whole space , And only hope to have strong energy in the direction of need , And the energy is weak in the direction that is not needed . This requires the antenna to have certain directivity , Radiate energy in the desired direction .