Antennas and Propagation( 天线与传播 )

Chapter 5

Introduction An antenna is an electrical conductor 　（电子导线） or system of conductors （导线系统）

发送 - radiates electromagnetic energy into space 接收 - collects electromagnetic energy from space

In two-way communication, the same antenna can be used for transmission and reception

假定：两个方向上使用同一频率

Radiation Patterns （辐射模式） Radiation pattern

Graphical representation of radiation properties of an antenna Depicted as two-dimensional cross section

Beam width (or half-power beam width) （光束宽度或称半功率光束宽度）

Measure of directivity of antenna （计算天线方向性的计量值）半功率光束宽度表示一个角度，在这个角度内，天线辐射功率至少是该天线在最佳方向上辐射功率的一半

Reception pattern Receiving antenna’s equivalent to radiation pattern

Types of Antennas Isotropic antenna (idealized) （各向同性天线）

Radiates power equally in all directions 辐射模式：以天线为中心的一个球体 通常表示为三维模式的一个二维剖面

Dipole antennas （偶极天线） Half-wave dipole antenna (or Hertz antenna) （半波偶极天线或称赫兹天线） Quarter-wave vertical antenna (or Marconi antenna) （ 1/4 波垂直天线或称马可尼天线）

Parabolic Reflective Antenna （抛物反射天线）

半波偶极 ¼ 波垂直天线

Antenna Gain （天线增益） Antenna gain

Power output, in a particular direction, compared to that produced in any direction by a perfect omnidirectional antenna (isotropic antenna)

天线增益不是为了似的输出功率大于输入功率，而主要是为了方向性 Effective area

Related to physical size and shape of antenna 针对不同天线的经验计算公式见表 5.2, 如：抛物线，抛物面面积为 A ，则其有效面积为 0.56A.

Antenna Gain Relationship between antenna gain and effective

area

G = antenna gain Ae = effective area f = carrier frequency c = speed of light (» 3 ´ 108 m/s) = carrier wavelength

2

2

2

44c

AfAG ee

Propagation Modes Ground-wave propagation Sky-wave propagation Line-of-sight propagation

Ground Wave Propagation

Ground Wave Propagation Follows contour of the earth ( 沿地表传播 ) Can Propagate considerable distances Frequencies up to 2 MHz…>1km Example

AM radio ( 调幅无线电 )

Sky Wave Propagation

Sky Wave Propagation Signal reflected from ionized layer of atmosphere

back down to earth Signal can travel a number of hops, back and forth

between ionosphere and earth’s surface Reflection effect caused by refraction ( 折

射 ) 。。。尽管实际效果看起来象反射 Examples

Amateur radio CB radio （民用波段无线电） 国际广播 ( BBC, VOA)

Line-of-Sight Propagation

Line-of-Sight Propagation Transmitting and receiving antennas must be within line of

sight Satellite communication – signal above 30 MHz not reflected by

ionosphere Ground communication – antennas within effective line of sight ( 有效的直线范围 ) due to refraction 。。因为微波会被大气弯曲或折射，大多数微波弯曲程度与地球曲率相同，因而比直线广播传输的远些

Refraction – bending of microwaves by the atmosphere Velocity of electromagnetic wave is a function of the density of the

medium When wave changes medium, speed changes Wave bends at the boundary between mediums

光波地平线和无线电地平线

Line-of-Sight Equations Optical line of sight

Effective, or radio, line of sight

d = distance between antenna and horizon (km) h = antenna height (m) K = adjustment factor to account for refraction,

rule of thumb ( 经验法 ) K = 4/3

hd 57.3

hd 57.3

Line-of-Sight Equations Maximum distance between two antennas

for LOS propagation:

h1 = height of antenna one h2 = height of antenna two

2157.3 hh

天线高度计算 例 5.2: 假定天线高度 100m, 另一天线在地平线上，则两天线直线传播的最大距离是

假设接收天线 10 m ，试问为获得同样传播距离，发送天线高度几何？

h1=7.842/1.33=46.2 m

kmKhd 4113357.357.3

)3.13(57.341 1 Kh

84.73.1357.3

411 Kh

LOS Wireless Transmission Impairments ( 传输损伤 ) Attenuation and attenuation distortion Free space loss Noise Atmospheric absorption Multipath Refraction Thermal noise

Attenuation Strength of signal falls off with distance over

transmission medium Attenuation factors for unguided media:

Received signal must have sufficient strength so that circuitry in the receiver can interpret the signal

Signal must maintain a level sufficiently higher than noise to be received without error

Attenuation is greater at higher frequencies, causing distortion

跨频带的衰减均等化

Free Space Loss Free space loss, ideal isotropic antenna

Pt = signal power at transmitting antenna Pr = signal power at receiving antenna = carrier wavelength d = propagation distance between antennas c = speed of light (3 10 8 m/s)where d, , and c are in the same units (e.g., meters)

2

2

2

2 44cfdd

PP

r

t

Free Space Loss Free space loss equation can be recast:

d

PPL

r

tdB

4log20log10

dB 98.21log20log20 d

dB 56.147log20log204log20

df

cfd

Free Space Loss Free space loss accounting for gain of other

antennas

Gt = gain of transmitting antenna Gr = gain of receiving antenna At = effective area of transmitting antenna Ar = effective area of receiving antenna

trtrtrr

t

AAfcd

AAd

GGd

PP

2

22

2

224

Free Space Loss Free space loss accounting for gain of other

antennas can be recast as

rtdB AAdL log10log20log20

dB54.169log10log20log20 rt AAdf

表 5.2: 天线增益与等效面积

Categories of Noise Thermal Noise Intermodulation noise ( 互调噪声 ) Crosstalk ( 串扰 ) Impulse Noise

Thermal Noise Thermal noise due to agitation of electrons （电子的热搅动） Present in all electronic devices and transmission

media Cannot be eliminated Function of temperature Particularly significant for satellite communication

因为卫星地面站收到的卫星信号较弱

Thermal Noise Amount of thermal noise to be found in a

bandwidth of 1Hz in any device or conductor is:

N0 = noise power density in watts per 1 Hz of bandwidth

k = Boltzmann's constant = 1.3803 10-23 J/K T = temperature, in kelvins (absolute temperature)

W/Hz k0 TN

Thermal Noise Noise is assumed to be independent of frequency Thermal noise present in a bandwidth of B Hertz

(in watts):

or, in decibel-watts

TBN k

BTN log10 log 10k log10 BT log10 log 10dBW 6.228

Noise Terminology Intermodulation noise – occurs if signals with different

frequencies share the same medium Interference caused by a signal produced at a frequency that is the

sum or difference of original frequencies Crosstalk – unwanted coupling between signal paths （不希望的信号路径之间的偶合） Impulse noise – irregular pulses or noise spikes

Short duration and of relatively high amplitude Caused by external electromagnetic disturbances, or faults and

flaws in the communications system （外部电磁干扰或通信系统的故障或缺陷）脉冲噪音是数字数据误码的一个主要来源如： 0.01 秒的脉冲在 56kbps 的传输中，会破坏 560bits 数据

Expression Eb/N0

Ratio of signal energy per bit to noise power density per Hertz

R: 数据比特率 ; S 信号功率 The bit error rate for digital data is a (decreasing) function

of Eb/N0 Given a value for Eb/N0 to achieve a desired error rate, parameters of

this formula can be selected As bit rate R increases, transmitted signal power must increase to

maintain required Eb/N0

TRS

NRS

NEb

k/

00

Other Impairments Atmospheric absorption – water vapor （水蒸气） and oxygen contribute to

attenuation Multipath – obstacles reflect signals so that

multiple copies with varying delays are received

Refraction – bending of radio waves as they propagate through the atmosphere

Multipath Propagation

反射 (R), 散射 (S), 衍射 (D)

Multipath Propagation Reflection ( 反射 ) - occurs when signal

encounters a surface that is large relative to the wavelength of the signal

Diffraction( 衍射 ) - occurs at the edge of an impenetrable body that is large compared to wavelength of radio wave

Scattering – occurs when incoming signal hits an object whose size in the order of the wavelength of the signal or less

The Effects of Multipath Propagation Multiple copies of a signal may arrive at different phases

If phases add destructively, the signal level relative to noise declines, making detection more difficult

Intersymbol interference (ISI) One or more delayed copies of a pulse may arrive at the same time as the primary pulse for a subsequent bit

Types of Fading Fast fading Slow fading Flat fading ( 平面衰落 ) Selective fading Rayleigh fading ( 瑞利衰落 ) Rician fading ( 瑞森衰落 )

Error Compensation Mechanisms( 差错补偿机制 ) Forward error correction Adaptive equalization Diversity techniques

Forward Error Correction Transmitter adds error-correcting code to data

block Code is a function of the data bits

Receiver calculates error-correcting code from incoming data bits If calculated code matches incoming code, no error

occurred If error-correcting codes don’t match, receiver attempts

to determine bits in error and correct

Adaptive Equalization（自适应均衡） Can be applied to transmissions that carry analog

or digital information Analog voice or video Digital data, digitized voice or video

Used to combat intersymbol interference Involves gathering dispersed symbol energy back

into its original time interval Techniques

Lumped analog circuits （集总模拟电路） Sophisticated digital signal processing algorithms

Diversity Techniques （分集技术） Diversity is based on the fact that individual channels

experience independent fading events Space diversity – techniques involving physical

transmission path Frequency diversity – techniques where the signal is

spread out over a larger frequency bandwidth or carried on multiple frequency carriers

如：扩频 Time diversity – techniques aimed at spreading the data

out over time 交织 (interleaving) 代价是时延。。交织度越大，接收端重构原序列的时延越大

作业1. 命名并简要定义四类噪声2. 抛物反射天线的优点是什么 ?3. 假设发送器产生 50W 的功率

(1) 以 dBm 和 dBW 为单位表示传输功率 (2) 如果上述发送器的功率输入到一个增益为 1 、载波频率为

900MHz 的发送天线上 ,则在距离 100m 远的自由空间处 , 接收功率是多少 dBm? （假定接收天线增益也为 1） (3) (2) 中 ,如果距离为 10km, 距离如何 ? (4) (3) 中 ,假设接收天线有 2的增益 , 结果又如何 ?

4. 确定信号必须能够达到 80km 远的用户处的电视台天线的高度

下周六停一次课！