Saturday, January 8, 2011

【 Weak current College 】 study RFID must understand antenna knowledge 】

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1. the basic principles of electromagnetic waves generated by Maxwell's electromagnetic theory, change of the electric field around the space to produce changes in the magnetic field, and changes in magnetic fields and electric field that you want to change. In this way, changing electric field and magnetic field changes, the mutual dependence between fire, alternately and to a certain speed from near and far to spread out in space.
Periodic variation of magnetic excitation of electric field fluctuations, periodic variation of electric magnetic excitation cyclical changes.
Unlike mechanical wave electromagnetic wave, its dissemination does not need to rely on any elastic medium, it only depends on the "change the electric field generated variable magnetic field, the variable magnetic field change farm" mechanism to propagate.
When the electromagnetic frequency is low, the main subjects of the tangible to pass conductors; when the frequency increases, the electromagnetic waves will spill into the conductor, unwanted media can deliver energy to the outside, this is a kind of radiation. In the low-frequency electrical oscillation, magnetic variation between slow, almost all of their energy-back to the original circuit without energy radiated out. However, in the high-frequency electro-magnetic oscillation, is very fast exchange, energy is not possible to return the original oscillation circuit, electrical energy, the energy of electric field and magnetic field with periodic changes to electromagnetic waves propagate to the space.
According to the above theories, each segment through high-frequency current conductors will have electromagnetic radiation. Some of the conductors used for the transmission, you do not want to have too much energy electromagnetic radiation loss; a wire is used as an antenna, they hope to be as much as possible to the energy into electromagnetic emission. So there will be a transmission line and antenna. Whether the antenna or cable are electromagnetic wave theory or Maxwell's equations in the application of different circumstances.
For transmission lines, the wire's structure should be can't transfer electromagnetic energy, not to radiation; for antenna, the structure of this wire should be passed as the electromagnetic energy. Different shapes and sizes of wire in the transmit and receive a frequency of a radio signal, the efficiency comparable to many, to produce the desired effect of communication, you must use the appropriate antenna it! on what structure of conductors to achieve efficient, transmit and receive, also formed the antenna to this subject.
High-frequency electromagnetic waves propagate in the air, in case of a conductor, the induced effect occurs, the conductor to generate high frequency current so that we can use the wire to receive radio signals from far away.
2. antenna
In the wireless communication system, you need to import from the transmitter wave energy into radio waves, or radio waves to guided wave energy, radiation and receive radio waves device known as the antenna. The transmitter has a high-frequency current energy (or guided wave energy) through the feeder to the transmitting antenna, antenna will be converted to a polarisation of electromagnetic energy, and go out to the desired direction. Reaches the receiving point, receiving antenna in a specific direction from the space of a polarization of the electromagnetic energy is converted to have high-frequency current energy modulated by the feeder to the receiver input.
In sum, the antenna should have the following features:
1) antenna should be guided wave energy as much as possible into the electromagnetic energy. This first requires the antenna is a good electromagnetic open systems, followed by the required antenna and transmitter or receiver match.
2) as far as possible, the antenna should be concentrated on electromagnetic wave direction, or to determine the direction of the wave maximum acceptable, i.e. direction directional.
3) antenna should be able to transmit or receive electromagnetic waves, the provisions of the antenna are appropriate polarization.
4) antennas should have sufficient working band.
This point is the most basic function of the antenna, as far as this can define several parameters as a design and evaluation of antenna.
The antenna and transmitter or receiver connected system known as a feeder system. Feeder in the form of a different frequency and conductor transmission line, into a coaxial cable transmission lines, waveguides or microstrip line, etc. Therefore, the so-called feeder, is actually a transmission line.
2.1. antenna power parameters
The basic functionality of the antenna is energy conversion and directional radiation, the so-called electrical parameters of the antenna, is able to quantitative characterization of its energy conversion and directional radiation capacity.
2.1.1. the antenna direction
Measure the antenna will be energy to the desired direction of the radiation. the main lobe width:
Main lobe width is the maximum measuring antenna radiation area of degree of physical quantities. Wider and better. the sidelobe level:
Sidelobe level is off the main flap recently and the level of the highest level of the first side flap. In fact, sidelobe zone is the zone without radiation, so its level, the lower the better.
(Antenna radiation of the main lobe sidelobe similar square wave signal spectrum) the front and rear ratio:
Before and after more than a maximum radiation direction (forward) level and its opposite directions (back) level. The larger the antenna around than backward radiation (or receive). Front and rear than the F/b calculates is simple---F/B = 10 Lg {(forward power density)/(back to power density)} direction of coefficients:
At a distance from the antenna, antenna in the direction of maximum radiation power density of radiation with the same radiation power ideal non-directional antenna in the same distance of radiated power flux density ratio. This is the direction of the most important indicators, accurately compare different antenna directivity, represents the antenna beam energy of electric parameters.
2.1.2. the antenna efficiency
Antenna efficiency is defined as the antenna radiated power ratio of the input power.
Common antenna radiation resistance R to question the ability of the antenna radiated power. Antenna radiation resistance is a virtual volume, defined as follows: there is a resistance R, through its current equal to the antenna on the maximum current, the loss of power is equal to its radiation power. Obviously, radiation resistance level is a measure of the antenna radiation capacity is an important indicator, i.e. radiation resistance, the larger the description more antenna radiation.
2.1.3. gain coefficientR > gain coefficient is a comprehensive measure of antenna energy conversion and direction properties of parameters, it is defined as: direction coefficient and antenna efficiency of product, write to:

D-oriented coefficient, for antenna efficiency.
Visible, the higher the antenna factor and, the higher the gain factor.
Physical significance: antenna gain coefficient describes the antenna and the ideal of non-directional antenna with a maximum radiation than a multiple of the output power amplification. Also can this popular understanding, ideal for directional and omnidirectional antenna (its radiation in the direction of parity) within a certain distance of a point on a certain size ratio of the signal.
Example: If an ideal non-directional point source as the transmitting antenna, you need to input power 100W, and gain g = 13 dB = 20 a directional antenna as the transmitting antenna, the input power just 100/20 = 5W. In other words, the gain of an antenna, its maximum radiation effects of radiation in the direction, and without direction compared to the ideal point source, the input power amplification of multiples.
2.1.4. the polarization direction
Polarization characteristics of refers to the antenna in the direction of maximum radiation electric field vector direction change over time.
Polarization direction antenna in the direction of the electric field. Antenna polarization wired polarization cable polarity (horizontal polarization and vertical polarization) and circular polarization (l-polarization and right-hand polarized) etc.
How to understand linear polarization? first imagine the classic electromagnetic wave propagation figure, electric field in a plane by the sine wave propagation, the magnetic field in the field of orthogonal planes also by the sine wave, we spread from the starting point along the way to go to the electric field, see is a short line, this polarization is linear polarization. Then the linear polarization directions how to determine? when high-frequency electric current through an antenna, antenna on the high-frequency voltage, the formation of high-frequency electric field, the field direction of General and antenna towards consistent, i.e. linear polarization of polarization direction is consistent with the trend of the antenna. If the antenna is horizontally mounted wire, electric field is generated in the horizontal direction in called it "the level of polarization antenna"; If the antenna is mounted perpendicular to the ground conductors, the electric field is vertical, called it "vertical polarization" antenna. (Usually a straight wire structure of linear polarization antenna for)
How to understand the circular polarization? is the classic electromagnetic wave propagation figure, but this time the farm size is always the same, but the direction around the x axis constant rotation changes, but in any one of the plane of the projection is a sine wave, is somewhat similar to our signal processing arm of the same, but a phase in a constantly changing. At this point, from the point of origin to the propagation direction to see the electric field, see is a circle, this polarization is circularly polarized. Of course, to the left is the left-hand polarization, to the right is the right-hand polarization. (Usually the helical structure of antenna for circular polarization)
Only the reader antenna polarization direction and receives electromagnetic waves polarization orientation to induction maximum signal. According to this principle, we can infer the following conclusions.
For linear polarization, when the reader antenna polarization direction consistent with linear polarization direction (electric field orientation), induction of signal maximum (electromagnetic wave on the polarization direction projection maximum); with the recipient antenna polarization direction and linear polarization direction off more and more, induction, the smaller the signal (projection constantly decrease); when the reader antenna polarization direction and line orthogonal polarization direction (the direction of magnetic field), induction of signal is zero (projection of zero). Line polarization antenna direction request high. Of course the actual conditions, electromagnetic wave propagation in catadioptric encountered en route, will cause the polarization direction deflection, sometimes a signal can be received, the level of the antenna can also be vertical antenna receiving, but in any case, the antenna polarization direction often is an important issue to consider.
For circular polarization, regardless of the recipient of the direction of polarized antenna, the induction of the signal are the same, there won't be any difference (electromagnetic wave in any direction projection are the same). Therefore, using circular polarization mode, allows the system to the antenna azimuth (bearing here is the orientation of the antenna, and the previously mentioned direction system of azimuth is different) to lower the sensitivity. Thus, the majority of occasions are circular polarization.
Make a figuratively, linear polarization similar bent in the ground crawling snakes, similar to the snake around the circular polarization in a stick on bypass. Then make a metaphor, you get a rope, rope swing up and down, passing wave is linear polarization form; continually draw a circle, the delivery of the wave is circularly polarized.
2.1.5. band width
Antenna power parameters and frequencies for, that is to say, the electrical parameters are for a frequency, when the operating frequency deviations from the frequency, were often designed to arouse the antenna parameters change. When the operating frequency of the changes, the electrical parameters of the antenna should not exceed the scope provided for in this frequency range is known as the band width, hereinafter referred to as the antenna of the bandwidth.
2.1.6. the input impedance
For transmitter, antenna is a load, how to make the antenna can most often get energy, we must solve a match always. Only that line its impedance and transmitter of impedance equal is to get the maximum transmitter power!
For high frequency signal, the antenna is very long wires. High-frequency signal from the antenna feed point to flow to and from the endpoint endpoint reflected the time, enough to cause antenna parts of voltage and current of amplitude and phase produce considerably, resulting in the antenna length, structure and location of the feed point, rendering of impedance. If the Center feed dipole, when the length of each arm as a quarter of a wavelength, rendering some 50 to 75 of pure resistance, easy and feeder cables and transmitter directly match.
When the conditions cannot be an antenna length trimming to the appropriate value, should generally be in antenna circuit additional inductance capacitor reactance components offset antenna itself render the reactor, and sometimes required impedance transformer the antenna impedance converter to letter circuit requirements values, these additional components of the device called "antenna tuner" or "antenna matching device".
2.1.7. the effective length
The effective length is the ability to measure the antenna radiation and an important means ofLabel.
Antenna effective length is defined as follows: keep the actual antenna maximum radiated field strength value of unchanged, assuming that the antenna current distribution for uniform distribution of equivalent length when antenna. The longer the effective length, indicating that the antenna radiation capacity.
The book has an example of the strengthening of education: length 2h, current is not uniform distribution of short oscillator in maximum radiation electric field intensity and direction of the length h, current for the uniform distribution of the oscillator at maximum intensity of the radiation. That is, the short oscillator effective length h.
2.2. receiving antenna theory
High-frequency electromagnetic waves propagate in the air, in case of a conductor, the induced effect occurs, the conductor to generate high frequency current so that we can use the wire to receive radio signals from far away. Receive the conductor of electromagnetic waves, usually called "antenna".
2.2.1. effective reception area
Effective reception area is a measure of an antenna to receive important indicators of radio waves. It is defined as: the day in line with the maximum receive direction alignment to wave direction for receiving, receiving antenna to match the load average power for PLmax, assuming that this power was a piece and DOA-which intercepted a vertical area, this area is called a receiving antenna effective reception area.
The larger the area of effective receiving antenna to receive the ability of radio waves.
2.2.2. the equivalent noise temperature
Receiving antenna of equivalent noise temperature is reflected an antenna to receive weak signals performance of important electrical parameters.
Receiving antenna to receive from the space surrounding the noise power delivered to the receiver of the process is similar to the noise power noise resistance to connected resistor network. Therefore the receiving antenna is equivalent to a temperature resistance for the Ta. The higher the antenna Ta sent to the receiver noise increases, the smaller the contrary.
3. the transmission line
The transmission line is used to transmit microwave information and energy in various forms of transport system collectively, its role is to guide the electromagnetic wave propagation along a certain direction and, therefore, also known as guided wave system. The boot of electromagnetic waves is called guided wave.
Transmission lines is also a kind of conductor, but with the antenna, you do not want the electromagnetic wave propagation in a radiation here. Therefore, the metallic structure of the transmission line is to not radiation energy.
To the coaxial cable of most often as an example, in the middle of a wire, the outside there is a ring ring wire, electromagnetic waves in a space of communication, not radiation.
The most common is the TEM (transverse wave) wave transmission lines, mainly including: double rows equal line, coaxial cable, ribbon cable, micro-strip lines, etc.

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