天线用于通过周围电磁场的变化来传输和接收信息。本文是天线理论的启示。
An Abridged History of Electromagnetism
Over 2600 years ago (and likely well before that) the ancient Greeks discovered that a piece of amber rubbed on a piece of fur would attract lightweight objects like feathers. Around the same time, the ancients discovered lodestone, which are pieces of magnetised rock.
It took a few hundred years more to determine that there are two different properties of attraction and repulsion (magnetic and electric): likes repel and opposites attract. Another 2000 years passed before scientists first discovered that these two entirely different novelties of nature were inextricably linked.
In the early nineteenth-century,汉斯·克里斯滕·奥斯特（Hans Christen Oersted）placed a wire perpendicular to a compass needle and saw nothing. But when he rotated the wire parallel to the compass needle and passed a current through the wire, it deflected in one direction. When he passed the current through the wire in the opposite direction, the compass needle deflected in the opposite direction.
垂直于指南针的电流携带电线不会引起任何运动。
平行于电流携带线的指南针将旋转。当电流方向逆转时，旋转方向会逆转。
该线是第一个天线发射器，指南针是第一个接收器。当时的科学家只是不知道。
While not terribly elegant, it provided a clue about the way the universe worked—that charges moving through a wire create a magnetic field that is perpendicular to the wire. (Scientists soon learned the field surrounding a wire is circular, not perpendicular.)
With this information, scientists were able to describe the ways in which electric fields and magnetic fields interact with electric charges and formed a basis of an understanding of electromagnetism.
The video above shows an alternating-current lamp filament being flexed between support points in the presence of a strong magnetic field.
Shortly after, Nikola Tesla wirelessly lit lamps in his workshop, demonstrated thefirst remote-control toy boat, and established the alternating-current system we use to transfer electricity throughout the world today.
在Orstead的实验后不到整整一个世纪，Guglielmo Marconidevised a way to send the first wireless telegraph signals across the Atlantic.
And here we stand, a full two centuries after that first compass experiment, able to capture images from distant planets and send them through the vastness of space to a device we can hold in the palm of our hand—all with antennas.
Image of Pluto, courtesy ofNASA.
建筑模块
Our universe came to us with certain rules. We discovered this thousands of years ago when we graduated from noticing the merely attractive force of gravity and first separated objects based on their ability to attract or repel other objects. Then we discovered another set of rules of attraction and repulsion that were completely separate from the first.
人类对物体进行了分类，并通过强烈的实验确定正面和负面是一个称为“电荷”的属性的相反表现，就像北极和南极是一种称为磁性的东西的相反的表现，就像左右与右手一样。
Image showing mirror-symmetry between electric charges, magnetic poles, and hands.
Something was happening in Orstead's wire whether he had a compass needle oriented beneath it or not. This leads to an idea of imperceivable electromagnetic fields that permeate the universe—through the densest matter and nature's best vacuums. Every one of our categorized objects (+/-/N/S) influences the space around it and is influenced if a field changes.
Magnetic field surrounding a current carrying solenoid. AMathematica-generated image based on code byPaul Nylander.
通过以可预测的方式移动电荷，我们可以以可预测的方式更改电磁场，并使用这些更改通过电磁波传输信息 - 电磁场中的调节振荡。
波叠加
波浪将能量从一个位置转移到另一个位置。
Left alone over a long period of time, a pool of water will appear flat and still. Disturb the water in one location and the water molecules will disturb neighboring water molecules, which will disturb neighboring water molecules and so on until the disturbance makes it to the edge of the pool.
The molecules that started the chain of events remain close to their initial locations, but the disturbance will reach the pool's edge in seconds. Waves transfer energy without transferring matter.
单个干扰in a pool
Waves are how we describe the movement of a disturbance through a medium. Whether through one initial disturbance or one million, the chain-reaction of molecular collisions in the pool is what drives the disturbance to propagate outwards.
Graphic of two waves in a pool
当两个波打扰了同一空间区域时，它们的幅度会增加或减去以产生建设性或破坏性干扰。这种瞬态添加剂或减法实践称为叠加。
Graphic of wave pulse constructive interference, courtesy of thePenn State College of Engineering.
在波浪中干扰特定位置后，它们沿相同的方向和相同的速度继续前进，只要它们保持在同一介质中。当波进入新介质时，速度和方向可能会改变。声波在空气中传播，水波穿过水 - 海浪传播的物质被称为“中等”。
Electromagnetic waves can travel through mediums such as air and water, or through the emptiness of space—they do not require a medium to propagate energy from one location to another.
波反射
When waves transition from one medium to another, part of their energy is transmitted, part of their energy is reflected, and part of their energy is dissipated into the environment.
The material properties of the two mediums determine the ratios of transmission to reflection and dissipation. And material properties also determine whether a wave will invert when it reflects or remain upright.
A singlewave-pulse传输和反射能量。图形提供Wikimedia.
连续的入射波（橙色）撞击了一个能量反映的界面（浅橙色），并传播一些能量（深橙色）
Reflection and Inversion
When waves travel from one medium to another, some of the incident energy is reflected. Depending on the material properties of the mediums, waves may invert when they reflect.
想象一个长长的春天绑在杆子上。如果您要将弹簧轻拂到中心的左侧，干扰将传播弹簧的长度，直到撞到杆子，此时它将反向方向，然后开始向另一侧向您移动，右 -中心。这是一个反转。
.Gif courtesy of thePenn State College of Engineering.
Take that same spring and tie it to a rope. If you were to flick the spring to the left-of-center, the disturbance would travel the length of the spring until it hit the rope, at which point it would reverse direction and start travelling back towards you on the same side, left-of-center.
.Gif courtesy of thePenn State College of Engineering.
Understanding rope reflection helps us to understand what is happening inside an antenna.
以下是四种有助于说明反思和反转概念的情况：
Whether or not a wave inverts when it reflects is determined by the properties of the mediums on either side of the interface.
If the wave does invert when it reflects, and we want constructive interference in the rope, we must have a length of rope that is half of a wavelength long, a full wavelength long, one and a half wavelengths long and so on: $$L=n\times \frac{\lambda}{2}$$, where n is a positive integer.
Antenna resonance is based on the same basic principles of reflection and interference:
Choose a length of wire that allows reflected energy to constructively interfere to create a larger signal rather than a smaller one.
站立波s
When two waves of the same wavelength travel in opposite directions in the same medium (depicted in Blue and Orange in the examples below), they can interact to form a standing wave (depicted in Green in the examples below). Standing waves are so named because while the blue waves travel to the left and the orange waves travel to the right, the green standing-waves have no apparent side-to-side motion.
An incident wave (Orange) and a reflected wave (Blue) combine to form a standing wave (Green)
站立波仅在介质中以特定的长度出现，这些长度由反射行为和入射波的波长确定。
站立波Ratio (SWR)
Standing waves of maximum amplitude occur at very precise combinations of frequency (or wavelength) and antenna length.
Unfortunately, it is impractical and actually impossible to have antennas that are the precise length to form a perfect standing wave for the desired range of operating frequencies. Fortunately, this is not necessary. A single-length antenna can function for a small range of frequencies with a small, acceptable level of imbalance.
Standing waves with net voltage shown over a period of oscillation.Image干涉师（自己的工作）[CC BY-SA 4.0], via Wikimedia Commons
Antenna length must be adjusted to create as close to a perfect standing wave as possible at the center of the operating frequency range.
SWR (Standing Wave Ratio) metersmeasure the ratio of transmitted to reflected energy, with the idea to have the ratio be as close to 1:1 as possible.
可以通过在最终电路放大阶段和天线之间引入被动电路组件来进行小型调整。天线调整中的小瑕疵将导致最终放大电路中存在电位差，从而加热传输电路的最后部分。大型失衡会使高电位差异回到发射机电路中，从而导致介电击穿和弧形。
Transmitting Information
The two types of information transfer that you are likely most familiar with are FM (Frequency Modulation) and AM (Amplitude Modulation).
Frequency Modulation
With Frequency Modulation, information is transmitted by modifying the frequency of a carrier wave.
Amplitude Modulation
With Amplitude Modulation, the frequency of the carrier wave is constant. Information is transmitted by modifying the amplitude of the carrier.
偶极天线
A simple antenna that uses two identical elements is called a dipole. The shortest dipole antennas operate at one-half wavelength and establish standing waves along their length.
Standing waves in a dipole antenna, courtesy ofwikimedia.org
天线长度的变化电场会产生向外传播的无线电波。
天线辐射能量，由wikimedia.org
Antennas allow us to transmit and receive information through influencing and being influenced by the electromagnetic fields that permeate the universe. The next article will explain different types of antennas and how they allow information to travel over large distances.
Further Reading
Antenna Basics: Radiation Patterns, Permittivity, Directivity, and Gain
This is a great intro article.
I ask the author to consider more information on two points mentioned in the sequel:
1. Whether or not a wave inverts when it reflects is determined by the properties of the mediums on either side of the interface.
在这个地区详细说明！
2. Small adjustments can be made by introducing passive circuit components between the final circuit amplification stage and the antenna.
这里需要更多。
If these two areas were covered, I think we’d have almost everything we’d need to design our own dipoles.
Beyond that, I’d think the other topics needed to round out the basics of antennas would be impedance matching and reflectors.
Good morning.
As a Brazilian, and, too, as a researcher, I could not not to mention, already you have made, regarding Marconi, to speak aboutFather Landell de Moura(Access这里, for a Portuguese reader), contemporary of Gugliemo Marconi and his companion, or competitor, of course, each one in his country, in electromagnetical researches.Landell de Moura由于缺乏对巴西政府和教会强烈的反对，丢失了无线电专利（从科学上讲，从科学上讲）是合理地说明了这一点，从而威胁了他异端，因此将他的研究视为“恶魔[Al]的事物”。帕德里（父亲）兰德尔（Padre）是广播的共同发明者，也是他时代的其他重要发现。