Introduction to main parameters of antenna

One 、 Input impedance and matching
The input impedance of the antenna refers to the impedance of the antenna at the feed port . Generally, the input impedance of the antenna is designed as 50 o . When the feeder and antenna impedance match , The reflection of the feed port is minimum , The energy on the feeder can be effectively transmitted to the antenna . therefore , The feeder needs to match the impedance of the antenna , That is, the feeder characteristic impedance value is designed as 50 o . therefore , Hardware PCB The characteristic impedance of the microstrip line connecting the antenna on the should be 50 o . To facilitate the adjustment of impedance matching , The built-in antenna model generally needs to reserve one near the feeding point π The location of the shape matching network . The input impedance of the external antenna is generally close 50 o , Reserve matching network according to actual needs .
Two 、 Return loss 、 Reflection coefficient and standing wave ratio
Return loss calculation formula ：$returnwavedamageConsumption=\frac{Enter\; intoshootworkrate}{backshootworkrate}=（workratebackshootrate{）}^{-1}$
When the feeder and antenna conjugate match , All the energy is absorbed by the load , There is only incident wave on the feeder , No reflected wave .
When the antenna and feeder do not match , That is, when the input impedance of the antenna is not equal to the characteristic impedance of the feeder , The load can only absorb part of the energy .
Part of the energy of the incident wave is reflected back to form a reflected wave .

chart 2.1 Energy conversion of antenna
As shown in the figure above , When the feeder is mismatched with the antenna , Assume that the antenna has no loss , Input 10W The power of 9.5W Radiate through the antenna ,0.5W The power is reflected back . Return loss here RL=-10log(0.5/10)=13dB .
In case of mismatch , There are both incident waves and reflected waves on the feeder . The ratio of the voltage amplitude of the reflected wave to the incident wave is called the reflection coefficient .
$backshootsystemCount\Gamma =\frac{backshootelectricPressure}{Enter\; intoshootelectricPressure}$
Where the phase of the incident wave and the reflected wave is the same, the sum of the amplitudes is the largest , Form antinodes ; Where the phase of the incident wave and the reflected wave is opposite, the amplitude subtraction is the smallest , Form nodes . The ratio of antinode voltage to nodal voltage amplitude is called standing wave coefficient , Also called voltage standing wave ratio (VSWR).
$StationedwaveThanVSWR=\frac{waveabdomenelectricPressure}{wavesectionelectricPressure}$
The closer the terminal load impedance is to the feeder characteristic impedance , The smaller the reflection coefficient , The closer the standing wave coefficient is to １, The better the match .
Basic requirements of antenna ： In Bobbie VSWR ≤2; namely ： Return loss RL ≥9.6dB .（ notes ：VSWR=2 when ,RL=9.54dB
Approximate 9.6 dB, Reflected power / Incident power =11.11%）

3、 ... and 、 efficiency
Antenna efficiency refers to the ratio of antenna radiation power to input power , Calculation formula ：
$GodLineeffectrate\eta =\frac{Spokeshootworkrate}{transportEnter\; intoworkrate}$
The efficiency of the external antenna is generally 70%-85%, The efficiency of the built-in antenna is generally 50-80% .
Part of the energy not radiated is reflected back , Part is the loss of the antenna itself . As shown in the figure below , Radiation efficiency =9W/10W=90% .

Four 、 Directivity and gain
Directivity of antenna refers to the ability of antenna to radiate electromagnetic waves in a certain direction . For the receiving antenna , Directivity indicates whether the antenna is aligned
The radio waves transmitted in the same direction have different receiving capabilities . The directivity of an antenna is usually represented by a pattern . The direction map can be used to say tomorrow
The ability of a line to transmit or receive electromagnetic waves in all directions of space . A single symmetric oscillator has “ Doughnuts ” Shaped
Directions ：

chart 4.1 Symmetrical dipole pattern
Gain refers to the condition that the input power is equal , Actual antenna and ideal radiation unit （ Point source antenna ） At the same point in space
The square ratio of the generated field strength , That is, the ratio of power .

The commonly used omni-directional antenna generally refers to the horizontal plane （ The plane perpendicular to the antenna axis ） Omnidirectional antenna . Its pattern is a circle in the horizontal plane , It means that the gain of each direction is equal . Directional antenna refers to the antenna whose energy is concentrated in a certain direction .

Upper figure ： Omnidirectional antenna （ Left ） And directional antenna （ Right ） Directions
The antenna gain reflects the concentration of the energy radiated by the antenna . The greater the gain , The more concentrated the energy . According to the conservation of energy , The more energy distributed in one direction , Naturally, the less energy distributed in other directions . The maximum gain of directional antenna is large , But its vertical plane covers a smaller area .

Upper figure ： Comparison of antenna patterns with different gains
5、 ... and 、 bandwidth
Antenna bandwidth refers to the working frequency range in which one or some electrical properties of the antenna meet the requirements . Usually , When working at the center frequency, the antenna can transmit the maximum power , When it deviates from the center frequency, the power it transmits will be reduced . Simply speaking ： Standing wave less than 2 For the working band width is the bandwidth .
6、 ... and 、 Polarization
When radio waves travel in space , Its electric field direction changes according to a certain law , This phenomenon is called the pole of radio waves
turn . The electric field direction of radio waves is called the polarization direction of radio waves .
Usually , If the electric field direction of the electric wave is perpendicular to the ground , We call it vertically polarized wave . If the electric field direction of the radio wave is the same as the ground
parallel , It is called horizontally polarized wave . The corresponding antennas are called vertical polarization antenna and horizontal polarization antenna .

Besides vertical polarization and horizontal polarization , And circular polarization 、 Elliptical polarization and other polarization methods . According to different rotation directions, circular polarization can be divided into left
Rotating circular polarization and right rotating circular polarization .
As shown in the figure below , If the electric field intensity on the section perpendicular to the transmission direction is decomposed into x Weight and y component , namely ：
$E_x=E_{1}sin(wt-\beta z)$
$E_x=E_{1}sin(wt-\beta z+\delta )$

Upper figure ： Polarization diagram
When Ex And Ey Phase difference  by 0 or 180 When the degree of , The polarization mode is linear polarization .
When E1=E2, And Ex And Ey Phase difference  by 90 When the degree of , The polarization mode is circular polarization .
When E1≠E2, And Ex And Ey Phase difference  Not for 0 Degree or 180 When the degree of , The polarization mode is elliptical polarization .
When the polarization direction of the incoming wave is inconsistent with that of the receiving antenna , Polarization loss usually occurs in the receiving process , for example ： When receiving any linearly polarized wave with a circularly polarized antenna , Or when receiving any circularly polarized wave with a linearly polarized antenna , Will produce 3 DB polarization loss , That is, it can only receive half the energy of the incoming wave .
When the polarization direction of the receiving antenna （ For example, horizontal or right-handed circular polarization ） And the polarization direction of the incoming wave （ Corresponding to vertical or left-handed circular polarization ） When completely orthogonal , The receiving antenna will not receive the energy of the incoming wave at all , At this time, it is said that the transmitting antenna and the receiving antenna are polarization isolated .