d3画力导向图的边界_无边界或无导向的传输媒体

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d3画力导向图的边界

Unguided medium transport electromagnetic waves without using a physical conductor. This type of communication is often referred to as wireless communication. Signals are normally broadcast through free space and thus are available to anyone who has a device capable of receiving them.

未经引导的介质无需使用物理导体即可传输电磁波。这种类型的通信通常称为无线通信。信号通常通过自由空间进行广播，因此任何拥有能够接收信号的设备的人都可以使用。

The below figure shows the part of the electromagnetic spectrum, ranging from 3 kHz to 900 THz, used for wireless communication.

下图显示了用于无线通信的电磁频谱的一部分，范围从3 kHz到900 THz。

Unguided signals can travel from the source to the destination in several ways: Gound propagation, Sky propagation and Line-of-sight propagation as shown in below figure.

非制导信号可以通过几种方式从源传播到目的地： 地面传播 ， 天空传播和视线传播 ，如下图所示。

Propagation Modes

传播方式

Ground Propagation: In this, radio waves travel through the lowest portion of the atmosphere, hugging the Earth. These low-frequency signals emanate in all directions from the transmitting antenna and follow the curvature of the planet.
地面传播：在这种情况下，无线电波穿过大气的最低部分，并紧紧围绕着地球。这些低频信号从发射天线向各个方向发出，并跟随行星的曲率。
Sky Propagation: In this, higher-frequency radio waves radiate upward into the ionosphere where they are reflected back to Earth. This type of transmission allows for greater distances with lower output power.
天空传播：在这种情况下，较高频率的无线电波向上辐射到电离层中，并在电离层中反射回地球。这种类型的传输允许以较低的输出功率实现更大的距离。
Line-of-sight Propagation: in this type, very high-frequency signals are transmitted in straight lines directly from antenna to antenna.
视线传播：在这种类型中，非常高频率的信号直接在天线之间以直线传输。

We can divide wireless transmission into three broad groups:

我们可以将无线传输分为三大类：

Radio waves
无线电波
Micro waves
微波
Infrared waves
红外波

无线电波 (Radio Waves)

Electromagnetic waves ranging in frequencies between 3 KHz and 1 GHz are normally called radio waves.

频率范围在3 KHz和1 GHz之间的电磁波通常称为无线电波。

Radio waves are omnidirectional. When an antenna transmits radio waves, they are propagated in all directions. This means that the sending and receiving antennas do not have to be aligned. A sending antenna send waves that can be received by any receiving antenna. The omnidirectional property has disadvantage, too. The radio waves transmitted by one antenna are susceptible to interference by another antenna that may send signal suing the same frequency or band.

无线电波是全向的。当天线发射无线电波时，它们会向各个方向传播。这意味着发送和接收天线不必对齐。发送天线发送可以被任何接收天线接收的波。全向属性也有缺点。一个天线发射的无线电波容易受到另一天线的干扰，而另一天线可能会以相同的频率或频带发送信号。

Radio waves, particularly with those of low and medium frequencies, can penetrate walls. This characteristic can be both an advantage and a disadvantage. It is an advantage because, an AM radio can receive signals inside a building. It is a disadvantage because we cannot isolate a communication to just inside or outside a building.

无线电波，尤其是低频和中频的无线电波，可以穿透墙壁。此特性既可以是优点，也可以是缺点。这是一个优势，因为AM收音机可以接收建筑物内部的信号。这是一个缺点，因为我们无法将通信隔离到建筑物内部或外部。

无线电波全向天线 (Omnidirectional Antenna for Radio Waves)

Radio waves use omnidirectional antennas that send out signals in all directions.

无线电波使用全向天线，可以全方位发送信号。

无线电波的应用 (Applications of Radio Waves)

The omnidirectional characteristics of radio waves make them useful for multicasting in which there is one sender but many receivers.
无线电波的全向特性使它们对于多播有用，在多播中有一个发送者但有多个接收者。
AM and FM radio, television, maritime radio, cordless phones, and paging are examples of multicasting.
AM和FM广播，电视，水上广播，无绳电话和寻呼是多播的示例。

微波 (Micro Waves)

Electromagnetic waves having frequencies between 1 and 300 GHz are called micro waves. Micro waves are unidirectional. When an antenna transmits microwaves, they can be narrowly focused. This means that the sending and receiving antennas need to be aligned. The unidirectional property has an obvious advantage. A pair of antennas can be aligned without interfering with another pair of aligned antennas.

频率在1至300 GHz之间的电磁波称为微波。微波是单向的。当天线发射微波时，它们可以集中地聚焦。这意味着发送和接收天线需要对齐。单向属性具有明显的优势。一对天线可以对准而不会干扰另一对对准的天线。

The following describes some characteristics of microwaves propagation:

下面介绍微波传播的一些特征：

Microwave propagation is line-of-sight. Since the towers with the mounted antennas need to be in direct sight of each other, towers that are far apart need to be very tall.
微波传播是视线。由于装有天线的塔架必须彼此直视，因此相距较远的塔架必须很高。
Very high-frequency microwaves cannot penetrate walls. This characteristic can be a disadvantage if receivers are inside the buildings.
高频微波无法穿透墙壁。如果接收器在建筑物内，则此特性可能是不利的。
The microwave band is relatively wide, almost 299 GHz. Therefore, wider sub-bands can be assigned and a high date rate is possible.
微波频带相对较宽，接近299 GHz。因此，可以分配更宽的子带，并且可以实现较高的日期速率。
Use of certain portions of the band requires permission from authorities.
使用频段的某些部分需要获得主管部门的许可。

微波单向天线 (Unidirectional Antenna for Micro Waves)

Microwaves need unidirectional antennas that send out signals in one direction. Two types of antennas are used for microwave communications: Parabolic Dish and Horn.

微波需要单向天线，以单向发送信号。微波通信使用两种类型的天线： 抛物面天线和喇叭天线。

A parabolic antenna works as a funnel, catching a wide range of waves and directing them to a common point. In this way, more of the signal is recovered than would be possible with a single-point receiver.

抛物线形天线就像一个漏斗，捕获各种波并将其指向一个公共点。这样，与单点接收器相比，可以恢复更多的信号。

A horn antenna looks like a gigantic scoop. Outgoing transmissions are broadcast up a stem and deflected outward in a series of narrow parallel beams by the curved head. Received transmissions are collected by the scooped shape of the horn, in a manner similar to the parabolic dish, and are deflected down into the stem.

喇叭天线看起来像个巨大的勺子。向外传输的信号沿茎杆传播，并由弯曲的头部在一系列狭窄的平行波束中向外偏转。接收到的声音以类似于抛物线形的方式被喇叭状的喇叭形收集，并向下偏转到茎中。

微波的应用 (Applications of Micro Waves)

Microwaves, due to their unidirectional properties, are very useful when unicast(one-to-one) communication is needed between the sender and the receiver. They are used in cellular phones, satellite networks and wireless LANs.

由于微波具有单向性，因此在发送者和接收者之间需要单播(一对一)通信时非常有用。它们用于蜂窝电话，卫星网络和无线局域网。

There are 2 types of Microwave Transmission :

微波传输有2种类型：

Terrestrial Microwave
地面微波
Satellite Microwave
卫星微波

微波传输的优点 (Advantages of Microwave Transmission)

Used for long distance telephone communication
用于长途电话通讯
Carries 1000's of voice channels at the same time
同时传送1000个语音通道

微波传输的缺点 (Disadvantages of Microwave Transmission)

It is very costly
这是非常昂贵的

地面微波 (Terrestrial Microwave)

For increasing the distance served by terrestrial microwave, repeaters can be installed with each antenna .The signal received by an antenna can be converted into transmittable form and relayed to next antenna as shown in below figure. It is an example of telephone systems all over the world

为了增加地面微波的服务距离，可以在每个天线上安装中继器。天线接收到的信号可以转换成可传输的形式，并转发到下一个天线，如下图所示。它是全世界电话系统的一个例子

There are two types of antennas used for terrestrial microwave communication :

地面微波通信使用两种类型的天线 ：

1.抛物面碟形天线 (1. Parabolic Dish Antenna)

In this every line parallel to the line of symmetry reflects off the curve at angles in a way that they intersect at a common point called focus. This antenna is based on geometry of parabola.

在这种情况下，平行于对称线的每条线都以一定角度在曲线上反射，这些角度在称为焦点的公共点处相交。该天线基于抛物线的几何形状。

2.喇叭天线 (2. Horn Antenna)

It is a like gigantic scoop. The outgoing transmissions are broadcast up a stem and deflected outward in a series of narrow parallel beams by curved head.

这是一个巨大的独家新闻。传出的传输沿茎杆广播，并通过弯曲的头部在一系列狭窄的平行波束中向外偏转。

卫星微波 (Satellite Microwave)

This is a microwave relay station which is placed in outer space. The satellites are launched either by rockets or space shuttles carry them.

这是一个放置在太空中的微波中继站。卫星是由火箭发射的，还是由航天飞机运载的。

These are positioned 36000 Km above the equator with an orbit speed that exactly matches the rotation speed of the earth. As the satellite is positioned in a geo-synchronous orbit, it is stationery relative to earth and always stays over the same point on the ground. This is usually done to allow ground stations to aim antenna at a fixed point in the sky.

它们位于赤道上方36000 Km，轨道速度与地球的旋转速度完全匹配。由于卫星位于地球同步轨道上，因此它相对于地球是静止的，并且始终停留在地面上的同一点上。通常这样做是为了使地面站将天线对准天空中的固定点。

卫星微波的特点 (Features of Satellite Microwave)

Bandwidth capacity depends on the frequency used.
带宽容量取决于所使用的频率。
Satellite microwave deployment for orbiting satellite is difficult.
用于轨道卫星的卫星微波部署是困难的。

卫星微波的优点 (Advantages of Satellite Microwave)

Transmitting station can receive back its own transmission and check whether the satellite has transmitted information correctly.
发射站可以接收自己的发射，并检查卫星是否正确发射了信息。
A single microwave relay station which is visible from any point.
单个微波中继站，从任何位置都可见。

卫星微波的缺点 (Disadvantages of Satellite Microwave)

Satellite manufacturing cost is very high
卫星制造成本很高
Cost of launching satellite is very expensive
发射卫星的成本非常昂贵
Transmission highly depends on whether conditions, it can go down in bad weather
传输高度取决于条件是否会在恶劣天气下下降

红外波 (Infrared Waves)

Infrared waves, with frequencies from 300 GHz to 400 THz, can be used for short-range communication. Infrared waves, having high frequencies, cannot penetrate walls. This advantageous characteristic prevents interference between one system and another, a short-range communication system in on room cannot be affected by another system in the next room.

频率范围从300 GHz到400 THz的红外波可用于短距离通信。具有高频的红外波无法穿透墙壁。这种有利的特性防止了一个系统与另一个系统之间的干扰，机房中的短距离通信系统不会受到隔壁机房中另一系统的影响。

When we use infrared remote control, we do not interfere with the use of the remote by our neighbours. However, this same characteristic makes infrared signals useless for long-range communication. In addition, we cannot use infrared waves outside a building because the sun's rays contain infrared waves that can interfere with the communication.

当我们使用红外遥控器时，我们不会干扰邻居使用遥控器。但是，这种相同的特性使红外信号无法用于远程通信。此外，我们不能在建筑物外使用红外线，因为太阳光线中的红外线会干扰通讯。

红外波的应用 (Applications of Infrared Waves)

The infrared band, almost 400 THz, has an excellent potential for data transmission. Such a wide bandwidth can be used to transmit digital data with a very high data rate.
接近400 THz的红外波段具有出色的数据传输潜力。如此宽的带宽可用于以很高的数据速率传输数字数据。
The Infrared Data Association(IrDA), an association for sponsoring the use of infrared waves, has established standards for using these signals for communication between devices such as keyboards, mouse, PCs and printers.
红外数据协会(IrDA)是赞助使用红外波的协会，它已经建立了使用这些信号在键盘，鼠标，PC和打印机等设备之间进行通信的标准。
Infrared signals can be used for short-range communication in a closed area using line-of-sight propagation.
红外信号可用于通过视线传播在封闭区域内进行短距离通信。