最好的天线基础教程

Antenna basics

VSWR
VSWRis a measure of impedance mismatch between the transmission   line andits load. The higher the VSWR, the greater the mismatch. The   minimumVSWR, i.e., that which corresponds to a perfect impedance    match, isunity.
To understand thedefinition above we must understand what impedance   is. Impedance inantenna terms refers to the ratio of the voltage to   current (both arepresent on an antenna) at any particular point of the    antenna. Thisratio of voltage to current varies on different parts of the   antenna,which means that the impedance is different on different spots   on theantenna if you could pick any spot and measure it.
Asstated before, the impedance for the entire chain from the radio tothe   antenna must be the same, and almost all radio equipment isbuilt   for an impedance of 50 ohm.
Ifany part of this chain fails to show a 50 ohm impedance due to e.g.bad   connections, incorrect antenna length, etc., the maximum power  will not be radiated from the antenna. Instead part (or all) of thewave is   reflected back down the line. The amount of the wavereflected back   depends on how bad the mismatch is.
Thecombination of the original wave traveling down the coaxial cable  (towards the antenna or opposite during receive) and the reflectingwave   is called a standing wave. The ratio of the two abovedescribed   waves is known as the Standing Wave Ratio.
Theresult is presented as a figure describing the power absorption ofthe   antenna. A value of 2.0:1 VSWR, which is equal to 90 % power  absorption, is considered very good for a small antenna: 3.0:1 is  considered acceptable (-6dB) which is equal to 75 % power absorption.

Smith Chart
Onecommon way of visualizing the VSWR is a polar plot called Smith  chart. From this plot the VSWR value, the return loss and theimpedance   for the different frequencies can be derived. Therefore itis an important   instrument for understanding antennas. To learn moreabout the SMITH   chart, see e.g.http://sss-mag.com/smith.html
Retrun Loss
This is basically the same thing as VSWR.
If50 % of the signal is absorbed by the antenna and 50 % is reflected  back, we say that the Return Loss is -3dB. A very good antenna might  have a value of -10dB (90 % absorbed & 10 % reflected).
Whenstudying a graph showing Return Loss/VSWR, a deep and wide dip   of thecurve is good since this shows an antenna with good bandwidth  (spreadband). Consequently, the narrower the dip is, the bigger riskthat   also desired channels will be reflected away (narrow band).
Return Loss Chart

Note: To be able to compare figures from differentmanufacturers, you   must be aware of the conditions under which themeasurement was   made. Wasimpedance matching used or not?
Conversion table VSWR / Return Loss
Performance
VSWR
Return Loss (dB)
1.01
-46.1
Better
1.05
-32.3
1.1
-26.4
1.2
-20.8
1.3
-17.7
1.4
-15.6
1.5
14.0
1.75
-11.3
2.0
-9.5
2.5
-7.4
3.01
-6.0
5.85
-3.0
Worse
8.72
-2.0
17.4
-1.0
Bandwidth
Normallya radio needs to work on multiple frequencies. For example, the   2.4GHz ISM band used by Bluetooth/Wi-Fi/Zigbee/WiMedia devices   has arange from 2400-2483 MHz. In this band PAN   communication uses 78channels for its frequency hopping technique, 1 MHz between eachchannel.
This means that the antenna mustperform well over a range of   frequencies. So, the goal must be tomake it resonant in the middle of   that band. The term that isimportant here is bandwidth or how much   band your antenna works wellover. One method of judging how well   (efficiently) your antenna isworking is by measuring VSWR.
Typically, bandwidth is measured by looking at SWR, i.e., by finding the   frequency range over which the SWR is less than 2.

Efficiency
Efficiency isa figure showing the ratio of the total radiated power to the   totalinput power . Efficiency has no unit and the ideal figure is 1.
Efficiency =radiated power /input power
Itis essential to know how the measurement was performed before  comparing figures from different manufacturers: was a matching network  used? Was the measuring point as close to the antenna as   possible orwas the transmission line included? Often, the figure for   efficiencywill dramatically decrease when the antenna is built into a   device.
Note: This is a good figure of merit, especially for small antennas.
Efficiency

Gain & 3D Pattern
Antennagain is a measure of directivity. In order to explain this better,   wemust first have a look at the different antenna types and theirradiation patterns.
Basicallythere are only two types of antennas: The dipole antenna   (Hertzian)and the vertical antenna (Marconi). All antennas can be   broken downto one of these types (although some say that there is only   one - thedipole). In addition to this we have a theoretical perfect   antenna(non-existent) that radiates equally in all directions with 100%  efficiency. This antenna is called an isotropic radiator.
Basic Antenna types

Antenna Radiation Patterns

Thisis similar to gain but the heat losses (i.e. the efficiency) are disregarded. We will then get a pattern as the dotted line shown inthe   figure. Point “d” refers to directivity,point “a” to gain andpoint “b” to the   isotropic reference.
Gain presented as 3D gain

The gain can also be presented as a 3D gain. The radius of the spheriod is   proportional to the antenna gain.
Gainin theory Since all real antennas will radiate more in some directions  than in others, you can say that gain is the amount of power   you canreach in one direction at the expense of the power lost in the  others. When talking about gain it is always the main lobe that is  discussed.
Gain may beexpressed as dBi or dBd. The first is gain compared to the   isotropicradiator and the second gain is compared to a half-wave dipole   infree space (0 dBd=”2″.15 dBi).
Itmay be worthwhile considering the fact that instead of doubling your  amplifier output, you could alternatively use an antenna that has 3db  more gain than your current antenna and achieve exactly the same effect.
Note: Small antennas usually have low gain, often between 0 and 2dBi.
Note: Regarding efficiency and radiation patterns - what is  true for  transmission is generall also true for  reception.
Directivity
Thisis similar to gain but the heat losses (i.e. the   efficiency) aredisregarded. We will then get a pattern as   the dotted line shown inthe  figure. Point “c” refers to   directivity, point “a” to gain andpoint “b” to   the   isotropic reference.

Polarization
Radiowaves are built by two fields, one electric and one magnetic. These  two field are perpendicular to each other. The sum of the fields is  the electromagnetic field. Energy flows back and forth from one fieldto   the other - This is what is known as “oscillation”.
Theposition and direction of the electric field with reference to the  earth’s surface (the ground) determines wave polarization. Ingeneral,   the electric field is the same plane as the antenna’sradiator.
Horizontal polarization —— the electric field is parallel to the ground.
Vertical polarization — the electric field is perpendicular to the ground.
Thereis one special polarization known as Circular polarization. As the  wave travels it spins, covering every possible angle. It can eitherbe   righthanded or lefthanded circular polarization depending on whichway   its spinning.
Note: Small antennas have no clear polarization.
Polarization chart

Impedance matching
Anideal antenna solution has an impedance of 50 ohm all the way from  the transceiver to the antenna, to get the best possible impedance  match between transceiver, transmission line and antenna. Since ideal  conditions do not exist in reality, the impedance in the antenna  interface   often must be compensated by means of a matching network,  i.e. a net   built with inductive and/or capacitive components.   TheVSWR result is   optimized by choosing the proper layout and  component values for the   matching net and the maximum potential of  the antenna is shown.
dB units
Decibel (dB) is a mathematical expression showing the relationship   between two values.
The RF power level at either transmitter output or receiver input is   expressed in Watts, but it can also be Expressed in dBm. The relation   between dBm and Watts can be expressed as follows:
P dBm = 10 x Log P mW
For example: 1 Watt = 1000 mW; P dBm = 10 x Log 1000 = 30 dBm
100 mW; P dBm = 10 x Log 100 = 20 dBm
Conversion table dBm / Watt
dBm
Watt
0
0,001
10
0,01
20
0,1
30
1
40
10
The following definitions are taken from IEEE Standard Definitions of   Terms for Antennas, IEEE Std 145-1983.
Adaptive (smart) antenna: An antenna system having circuitelements   associated with its radiating elements such that one or moreof the   antenna properties are controlled by the received signal.
Antenna polarization: In a specified direction from anantenna and at a   point in its far field, is the polarization of the(locally) plane wave which is   used to represent the radiated wave atthat point.
Antenna: That part of a transmitting or receiving system which is   designed to radiate or to receive electromagnetic waves.
Coaxial antenna:An antenna comprised of a extension to the inner   conductor of acoaxial line and a radiating sleeve which in effect is   formed byfolding back the outer conductor of the coaxial line.
Collinear array antenna: A linear array of radiating elements, usually  dipoles, with their axes lying in a straight line.
Co-polarization: That polarization which the antenna is intended to  radiate
Cross-polarization: In a specified plane containing thereference  polarization ellipse, the polarization orthogonal to aspecified reference  polarization.
Directional antenna: An antenna having the property ofradiating or  receiving electromagnetic waves more effectively in somedirections  than others.
Effective radiated power (ERP): In a given direction, therelative gain of a  transmitting antenna with respect to the maximumdirectivity of a  half-wave dipole multiplied by the net power acceptedby the antenna  from  the connected transmitter.
E-plane: For a linearly polarized antenna, the plane containing the  electric field vector and the direction of maximum radiation.
Far-field region: That region of the field of an antennawhere the angular  field distribution is essentially independent of thedistance from a  specified point in the antenna region.
Frequency bandwidth: The range of frequencies within whichthe  performance of the antenna, with respect to some characteristics, conforms to a specified standard.
Front-to-back ratio: The ratio of the maximum directivity of an antenna  to its directivity in a specified rearward direction.
Half-power beamwidth:In a radiation pattern cut containing the  direction  of the maximum ofa lobe, the angle between the two  directions in which  the radiationintensity is one-half the maximum  value.
Half-wave dipole:A wire antenna consisting of two straight collinear  conductors ofequal length, separated by a small feeding gap, with each   conductorapproximately a quarter-wave length long.
H-plane: For a linearly polarized antenna, the plane containing the   magnetic field vector and the direction of maximum radiation.
Input impedance: The impedance presented by an antenna at its   terminals.
Isolation: A measure of power transfer from one antenna to another.
Isotropic radiator: A hypothetical, loss less antenna having equal  radiation intensity in all directions.
Log-periodic antenna:Any one of a class of antennas having a structural  geometry such thatits impedance and radiation characteristics repeat  periodically as thelogarithm of frequency.
Major/main lobe: The radiation lobe containing the direction of maximum  radiation.
Microstrip antenna: An antenna which consists of a thin metallic  conductor bonded to a thin grounded dielectric substrate.
Omnidirectional antenna:An antenna having an essentially non-directional pattern in a givenplane of the antenna and a directional  pattern in any orthogonal plane.
Radiation efficiency: The ratio of the total power radiated by an antenna  to the net power accepted by the antenna from the connected transmitter.
Side lobe suppression:Any process, action or adjustment to reduce the  level of the sidelobes or to reduce the degradation of the intended  antenna systemperformance resulting from the presence of side lobes.