Comparison of Different Communication Channels | 不同通信信道比较

📚 Comparison of Different Communication Channels | 不同通信信道比较

In a communication system, the information signal must travel from a transmitter to a receiver through a physical or wireless path called a communication channel. CIE A-Level Physics asks you to compare different channels using a set of engineering criteria: bandwidth, attenuation, noise, security, cost and convenience.

在通信系统中，信息信号必须从发射机通过一条物理或无线路径到达接收机，这条路径称为通信信道。CIE A-Level 物理要求你使用一组工程标准来比较不同信道：带宽、衰减、噪声、安全性、成本和便利性。

1. What Is a Communication Channel? | 什么是通信信道？

A communication channel is the medium that carries a modulated carrier wave from transmitter to receiver. It can be guided, such as a copper wire or an optical fibre, or unguided, such as free space for radio waves and microwaves.

通信信道是将调制载波从发射机传送到接收机的介质。它可以是有导引的，例如铜线或光纤；也可以是非导引的，例如无线电波和微波在自由空间中传播。

In the CIE syllabus, the main channels are wire pairs, coaxial cables, radio waves, microwaves and optic fibres. Each has a different carrier frequency range and a different set of advantages and limitations.

在 CIE 大纲中，主要信道包括双绞线、同轴电缆、无线电波、微波和光纤。每种信道都有不同的载波频率范围以及不同的优点和局限。

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2. Key Performance Criteria | 关键性能指标

To compare channels fairly, we use several measurable quantities. Bandwidth is the range of frequencies a channel can carry; it determines how much information can be sent per second. Attenuation is the loss of signal power per unit length or over a path. Noise is unwanted energy added to the signal. Security describes how difficult it is for an outsider to tap the channel.

为了公平比较信道，我们使用几个可测量的物理量。带宽是信道能够承载的频率范围，它决定每秒可以发送多少信息。衰减是信号功率在单位长度或路径上的损失。噪声是叠加到信号上的多余能量。安全性描述外部人员窃听信道的难易程度。

Cost and convenience are also important. A cable may be cheap but difficult to install over mountains; a microwave link may be expensive but avoids digging. In the exam, you should always link a channel property to one of these criteria.

成本和便利性也很重要。电缆可能便宜，但在山区难以铺设；微波链路可能昂贵，但可避免开挖。在考试中，你应始终将信道特性与这些标准之一联系起来。

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3. Wire Pairs (Twisted Pair) | 双绞线

A wire pair consists of two insulated copper wires twisted together. It is used mainly for short-distance telephone connections and local area networks. The twisting reduces electromagnetic interference by making induced currents cancel over adjacent twists.

双绞线由两根相互绞合的绝缘铜线组成，主要用于短距离电话连接和局域网。绞合通过使相邻绞距上的感应电流相互抵消来减少电磁干扰。

Its main drawbacks are low bandwidth and rapid attenuation at high frequencies. Coaxial cable or fibre is preferred when signals must travel more than a few kilometres or carry many channels.

其主要缺点是带宽低，高频时衰减快。当信号需要传输数公里以上或承载许多信道时，同轴电缆或光纤更受欢迎。

Typical bandwidth is up to about 100 MHz for short runs, but attenuation rises quickly because the copper pair acts as a low-pass filter.

短距离使用时，典型带宽可达约 100 MHz，但衰减随频率迅速增大，因为双绞线铜线就像一个低通滤波器。

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4. Coaxial Cable | 同轴电缆

A coaxial cable has an inner copper conductor surrounded by a dielectric insulator, a braided outer conductor and a plastic sheath. The outer conductor acts as a shield, so the signal is less affected by external noise than in a wire pair.

同轴电缆由内铜导体、绝缘介质、编织外导体和塑料护套组成。外导体起屏蔽作用，因此信号受外部噪声的影响小于双绞线。

Coaxial cable can carry higher frequencies than a wire pair, typically up to about 1 GHz, with moderate attenuation. It is widely used for cable television, broadband Internet and some radio-frequency links.

同轴电缆可承载比双绞线更高的频率，通常可达约 1 GHz，衰减中等。它广泛用于有线电视、宽带互联网和一些射频链路。

However, repeaters are needed every few kilometres to restore signal strength, and the copper conductor still loses power through resistance and dielectric loss.

然而，每隔几公里就需要中继器来恢复信号强度，而且铜导体仍会因电阻和介质损耗损失功率。

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5. Radio Waves | 无线电波

Radio waves are electromagnetic waves with frequencies from about 30 kHz to 300 MHz. They can travel long distances by ground wave, space wave or sky wave, depending on frequency and ionospheric conditions.

无线电波是频率约 30 kHz 至 300 MHz 的电磁波。根据频率和电离层条件，它们可以通过地波、空间波或天波传播很长的距离。

Radio channels are unguided, so no physical cable is needed. This makes broadcasting cheap and mobile, but the bandwidth is limited and the signal can suffer from interference, fading and noise.

无线电信道是非导引的，因此不需要物理电缆。这使广播成本低且可移动，但带宽有限，信号可能受到干扰、衰落和噪声的影响。

In the CIE syllabus, radio waves are used for sound broadcasting and emergency communication, but they are not suitable for high-data-rate digital services compared with microwaves or fibres.

在 CIE 大纲中，无线电波用于声音广播和应急通信，但与微波或光纤相比，它们不适合高数据速率的数字业务。

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6. Microwaves | 微波

Microwaves occupy frequencies roughly from 1 GHz to 300 GHz. They are used for point-to-point links, satellite communication, mobile phone backhaul and radar. Their short wavelength allows high-gain dish antennas to focus the beam.

微波的频率大致在 1 GHz 到 300 GHz 之间。它们用于点对点链路、卫星通信、移动电话回传和雷达。波长短，可用高增益碟形天线聚焦波束。

Microwave links require a clear line of sight because the waves travel in straight lines; tall towers on hills or geostationary satellites are used to overcome the Earth’s curvature.

微波链路需要清晰的视距，因为微波沿直线传播；通常使用高山上的高塔或地球静止卫星来克服地球曲率。

They offer much larger bandwidth than radio waves and moderate attenuation in clear air, but rain, fog and atmospheric absorption can cause fading. The beam is also directional, which improves security but requires precise alignment.

微波提供的带宽远大于无线电波，在晴朗空气中衰减中等，但雨、雾和大气吸收会引起衰落。波束具有方向性，这提高了安全性，但需要精确对准。

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7. Optic Fibres | 光纤

An optic fibre is a thin strand of glass or plastic that carries infrared or visible light by total internal reflection. A core of higher refractive index is surrounded by a cladding of lower refractive index, so light rays above the critical angle stay inside the core.

光纤是一种细玻璃或塑料丝，通过全内反射传输红外光或可见光。纤芯折射率较高，包层折射率较低，因此大于临界角的光线会留在纤芯内。

Fibres have enormous bandwidth, typically in the terahertz range, and very low attenuation compared with copper cables. They are immune to electromagnetic interference and are difficult to tap without detection.

光纤具有极大的带宽，通常在太赫兹范围，衰减远低于铜缆。它不受电磁干扰，且难以在不被察觉的情况下窃听。Published by TutorHao | A-Level Physics Revision Series | aleveler.com

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