|
|
最好的天线基础教程
Antenna basics
VSWR
VSWR is 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., that which 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 (both are present on an antenna) at any particular point of the antenna. This ratio of voltage to current varies on different parts of the antenna, which means that the impedance is different on different spots on the antenna if you could pick any spot and measure it.
As stated before, the impedance for the entire chain from the radio to the antenna must be the same, and almost all radio equipment is built for an impedance of 50 ohm.
If any 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 the wave is reflected back down the line. The amount of the wave reflected back depends on how bad the mismatch is.
The combination of the original wave traveling down the coaxial cable (towards the antenna or opposite during receive) and the reflecting wave is called a standing wave. The ratio of the two above described waves is known as the Standing Wave Ratio.
The result is presented as a figure describing the power absorption of the 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
One common way of visualizing the VSWR is a polar plot called Smith chart. From this plot the VSWR value, the return loss and the impedance for the different frequencies can be derived. Therefore it is an important instrument for understanding antennas. To learn more about the SMITH chart, see e.g. http://sss-mag.com/smith.html
Retrun Loss
This is basically the same thing as VSWR.
If 50 % 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).
When studying a graph showing Return Loss/VSWR, a deep and wide dip of the curve is good since this shows an antenna with good bandwidth (spreadband). Consequently, the narrower the dip is, the bigger risk that also desired channels will be reflected away (narrow band).
Return Loss Chart
Note: To be able to compare figures from different manufacturers, you must be aware of the conditions under which the measurement was made. Was impedance 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
Normally a radio needs to work on multiple frequencies. For example, the 2.4 GHz ISM band used by Bluetooth/Wi-Fi/Zigbee/WiMedia devices has a range from 2400-2483 MHz. In this band PAN communication uses 78 channels for its frequency hopping technique, 1 MHz 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 that is 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 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 is a figure showing the ratio of the total radiated power to the total input power . Efficiency has no unit and the ideal figure is 1.
Efficiency =radiated power /input power
It is 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 or was the transmission line included? Often, the figure for efficiency will 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
Antenna gain is a measure of directivity. In order to explain this better, we must first have a look at the different 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 of these types (although some say that there is only one - the dipole). 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
This is similar to gain but the heat 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 3D gain
The gain can also be presented as a 3D gain. The radius of the spheriod is proportional to the antenna gain.
Gain in 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 can reach 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 be expressed as dBi or dBd. The first is gain compared to the isotropic radiator and the second gain is compared to a half-wave dipole in free space (0 dBd=2.15 dBi).
It may 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
This is similar to gain but the heat losses (i.e. the efficiency) are disregarded. We will then get a pattern as the dotted line shown in the figure. Point "c" refers to directivity, point "a" to gain and point "b" to the isotropic reference.
Polarization
Radio waves 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 field to the other - This is what is known as "oscillation".
The position and direction of the electric field with reference to the earth’s surface (the ground) determines wave polarization. In general, the electric field 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 wave travels it spins, covering every possible angle. It can either be righthanded or lefthanded circular polarization depending on which way its spinning.
Note: Small antennas have no clear polarization.
Polarization chart
Impedance matching
An ideal 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. The VSWR 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 circuit elements associated with its radiating elements such that one or more of the antenna properties are controlled by the received signal.
Antenna polarization: In a specified direction from an antenna and at a point in its far field, is the polarization of the (locally) plane wave which is used to represent the radiated wave at that 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 a coaxial line and a radiating sleeve which in effect is formed by folding 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 the reference polarization ellipse, the polarization orthogonal to a specified reference polarization.
Directional antenna: An antenna having the property of radiating or receiving electromagnetic waves more effectively in some directions than others.
Effective radiated power (ERP): In a given direction, the relative gain of a transmitting antenna with respect to the maximum directivity of a half-wave dipole multiplied by the net power accepted by 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 antenna where the angular field distribution is essentially independent of the distance from a specified point in the antenna region.
Frequency bandwidth: The range of frequencies within which the performance of the antenna, with respect to some characteristics, conforms to a specified standard.
Front-to-back rati 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 of a lobe, the angle between the two directions in which the radiation intensity is one-half the maximum value.
Half-wave dipole: A wire antenna consisting of two straight collinear conductors of equal length, separated by a small feeding gap, with each conductor approximately 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 that its impedance and radiation characteristics repeat periodically as the logarithm 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 given plane 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 side lobes or to reduce the degradation of the intended antenna system performance resulting from the presence of side lobes. |
|
IP卡
狗仔卡
发表于 2007-3-16 12:44:32
提升卡
置顶卡
沉默卡
喧嚣卡
变色卡
显身卡