本帖最后由 myvioletchen 于 2013-1-6 16:46 编辑
Antenna basics
file:///C:/Users/CHENLI~1/AppData/Local/Temp/msohtmlclip1/01/clip_image002.jpg VSWRis a measure of impedance mismatch between the transmission line and its load.The higher the VSWR, the greater the mismatch. The minimum VSWR, i.e., thatwhich corresponds to a perfect impedance match, is unity.
To understand the definition above we must understand what impedance is.Impedance in antenna terms refers to the ratio of the voltage to current (bothare present on an antenna) at any particular point of the antenna. This ratioof voltage to current varies on different parts of the antenna, which meansthat the impedance is different on different spots on the antenna if you couldpick any spot and measure it.
As stated before, the impedance for the entire chain from the radio to theantenna must be the same, and almost all radio equipment is built for animpedance of 50 ohm.
If any part of this chain fails to show a 50 ohm impedance due to e.g. badconnections, incorrect antenna length, etc., the maximum power will not beradiated from the antenna. Instead part (or all) of the wave is reflected backdown the line. The amount of the wave reflected back depends on how bad themismatch is.
The combination of the original wave traveling down the coaxial cable (towardsthe antenna or opposite during receive) and the reflecting wave is called astanding wave. The ratio of the two above described waves is known as theStanding Wave Ratio.
The result is presented as a figure describing the power absorption of theantenna. A value of 2.0:1 VSWR, which is equal to 90 % power absorption, isconsidered very good for a small antenna: 3.0:1 is considered acceptable (-6dB)which is equal to 75 % power absorption. file:///C:/Users/CHENLI~1/AppData/Local/Temp/msohtmlclip1/01/clip_image003.jpg
Smith Chart
One common way of visualizing the VSWR is a polar plot called Smith chart. Fromthis plot the VSWR value, the return loss and the impedance for the differentfrequencies can be derived. Therefore it is an important instrument forunderstanding antennas. To learn more about the SMITH chart, see e.g. http://sss-mag.com/smith.html
file:///C:/Users/CHENLI~1/AppData/Local/Temp/msohtmlclip1/01/clip_image005.jpg Thisis basically the same thing as VSWR.
If 50 % of the signal is absorbed by the antenna and 50 % is reflected back, wesay that the Return Loss is -3dB. A very good antenna might have a value of-10dB (90 % absorbed & 10 % reflected).
When studying 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 risk that also desiredchannels will be reflected away (narrow band).
Return Loss Chart
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Note: To be able to compare figures fromdifferent manufacturers, you must be aware of the conditions under which themeasurement was made. Was impedancematchingused 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 |
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Normally a radio needs to work onmultiple frequencies. For example, the 2.4 GHz ISM band used byBluetooth/Wi-Fi/Zigbee/WiMedia devices has a range from 2400-2483 MHz. In thisband WPAN communication uses 78 channels for its frequency hopping technique, 1MHz between each channel.
This means that the antenna must perform well over a range of frequencies. So,the goal must be to make it resonant in the middle of that band. The term thatis important here is bandwidth or how much band your antenna works well over.One method of judging how well (efficiently) your antenna is working is bymeasuring VSWR.
Typically, bandwidth is measured by looking at SWR, i.e., by finding thefrequency range over which the SWR is less than 2.
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Efficiency file:///C:/Users/CHENLI~1/AppData/Local/Temp/msohtmlclip1/01/clip_image012.gif is a figure showing the ratio of the totalradiated power file:///C:/Users/CHENLI~1/AppData/Local/Temp/msohtmlclip1/01/clip_image013.gif to the total input power file:///C:/Users/CHENLI~1/AppData/Local/Temp/msohtmlclip1/01/clip_image014.gif. Efficiency has no unit and the ideal figureis 1.
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It is essential to know how the measurement was performed before comparingfigures from different manufacturers: was a matching network used? Was themeasuring point as close to the antenna as possible or was the transmissionline included? Often, the figure for efficiency will dramatically decrease whenthe antenna is built into a device.
Note: This is a good figure of merit, especially for small antennas.
Efficiency
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file:///C:/Users/CHENLI~1/AppData/Local/Temp/msohtmlclip1/01/clip_image018.jpg Antenna gain is a measure ofdirectivity. In order to explain this better, we must first have a look at thedifferent antenna types and their radiation patterns.
Basically there are only two types of antennas: The dipole antenna (Hertzian)and the vertical antenna (Marconi). All antennas can be broken down to one ofthese types (although some say that there is only one - the dipole). Inaddition to this we have a theoretical perfect antenna (non-existent) thatradiates equally in all directions with 100% efficiency. This antenna is calledan isotropic radiator.
Basic Antenna types
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Antenna RadiationPatterns file:///C:/Users/CHENLI~1/AppData/Local/Temp/msohtmlclip1/01/clip_image020.gif This is similar to gain but theheat losses (i.e. the efficiency)
are disregarded. We will then get a pattern as the dotted
line shown in the figure. Point "d" refers to directivity,
point "a" to gain and point "b" to the isotropic reference.
Gain presented as 3Dgain
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The gain can also be presented as a 3D gain. The radius of the spheriod isproportional to the antenna gain.
Gain in theory Since all real antennas willradiate more in some directions than in others, you can say that gain is theamount of power you can reach in one direction at the expense of the power lostin the others. When talking about gain it is always the main lobe that isdiscussed.
Gain may be expressed as dBi or dBd. The first is gain compared to theisotropic radiator and the second gain is compared to a half-wave dipole infree space (0 dBd=2.15 dBi).
It may be worthwhile considering the fact that instead of doubling youramplifier output, you could alternatively use an antenna that has 3db more gainthan 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 fortransmission is generall also true for reception.
This is similar to gain but theheat losses (i.e. the efficiency) are disregarded. We will then get a patternas the dotted line shown in the figure. Point "c" refers todirectivity, point "a" to gain and point "b" to theisotropic reference.
Radio waves are built by twofields, one electric and one magnetic. These two field are perpendicular toeach other. The sum of the fields is the electromagnetic field. Energy flowsback and forth from one field to the other - This is what is known as"oscillation".
The position and direction of the electric field with reference to the earth’ssurface (the ground) determines wave polarization. In general, the electricfield is the same plane as the antenna's radiator.
Horizontal polarization —— the electric field is parallel to the ground.
Vertical polarization -- the electric field is perpendicular to the ground.
There is one special polarization known as Circular polarization. As the wavetravels it spins, covering every possible angle. It can either be righthandedor lefthanded circular polarization depending on which way its spinning.
Note: Small antennas have no clear polarization.
Polarization chart
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An ideal antenna solution has animpedance of 50 ohm all the way from the transceiver to the antenna, to get thebest possible impedance match between transceiver, transmission line andantenna. Since ideal conditions do not exist in reality, the impedance in theantenna interface often must be compensated by means of a matching network,i.e. a net built with inductive and/or capacitive components. The VSWR resultis optimized by choosing the proper layout and component values for thematching net and the maximum potential of the antenna is shown.
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Decibel (dB) is a mathematicalexpression showing the relationship between two values.
The RF power level at either transmitter output or receiver input is expressedin Watts, but it can also be expressed in dBm.The relation between dBm and Watts can beexpressed 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
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发表于 2013-1-6 16:43
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