Radio Waves: Generation, Propagation and Communication | 无线电波：产生、传播与通信

📚 Radio Waves: Generation, Propagation and Communication | 无线电波：产生、传播与通信

Radio waves are the longest-wavelength and lowest-frequency members of the electromagnetic spectrum. In CIE A Level Physics, they provide a key test of your understanding of electromagnetic wave production, the wave equation, modulation, and how waves propagate, reflect and diffract around the Earth.

无线电波是电磁波谱中波长最长、频率最低的成员。在 CIE A Level 物理中，它们是检验你对电磁波产生、波动方程、调制以及波在地球周围传播、反射和衍射等知识的重点内容。

1. Radio Waves in the Electromagnetic Spectrum | 电磁波谱中的无线电波

Radio waves occupy the region of the electromagnetic spectrum with frequencies from about 3 kHz to 300 GHz, although the boundary with microwaves is sometimes taken at 300 MHz or 1 GHz depending on the convention. Their wavelengths in free space range from about 100 km down to 1 mm.

无线电波在电磁波谱中的频率范围约为 3 kHz 到 300 GHz，不过与微波的分界有时按惯例取在 300 MHz 或 1 GHz。它们在自由空间中的波长范围约为 100 km 到 1 mm。

All electromagnetic waves, including radio waves, are transverse waves consisting of oscillating electric and magnetic fields at right angles to each other and to the direction of energy propagation. They can travel through a vacuum and do not require a medium.

所有电磁波（包括无线电波）都是横波，由相互垂直且与能量传播方向垂直的振荡电场和磁场组成。它们可以在真空中传播，不需要介质。

2. Wave Equation and Units | 波动方程与单位

For any electromagnetic wave in a vacuum, the relationship between speed c, frequency f and wavelength λ is given by:

对于真空中的任何电磁波，波速 c、频率 f 和波长 λ 之间的关系为：

c = fλ

where c = 3.00 × 10⁸ m s⁻¹. Since c is constant in a vacuum, radio waves with lower frequencies must have longer wavelengths, and higher-frequency radio waves must have shorter wavelengths.

其中 c = 3.00 × 10⁸ m s⁻¹。由于真空中 c 恒定，频率较低的无线电波必然波长较长，频率较高的无线电波必然波长较短。

For example, a radio station broadcasting at f = 100 MHz has a wavelength of λ = c / f = (3.00 × 10⁸) / (100 × 10⁶) = 3.00 m. A long-wave transmitter at 150 kHz has λ = 2000 m.

例如，一个以 f = 100 MHz 广播的电台，其波长为 λ = c / f = (3.00 × 10⁸) / (100 × 10⁶) = 3.00 m。一个 150 kHz 的长波发射机，其波长为 2000 m。

3. Generation by Oscillating Charges | 振荡电荷产生无线电波

Radio waves are produced when charged particles undergo acceleration. In practice, an alternating current in a transmitting antenna forces electrons to oscillate up and down the metal conductor, producing a changing electric field and an associated changing magnetic field that propagate outwards as an electromagnetic wave.

无线电波是由带电粒子加速运动产生的。实际上，发射天线中的交变电流迫使电子沿金属导体上下振荡，产生变化的电场和伴随的变化磁场，以电磁波的形式向外传播。

The frequency of the emitted radio wave is equal to the frequency of the alternating current driving the antenna. If the oscillator frequency is 1 MHz, the radio wave frequency is also 1 MHz.

发射的无线电波频率等于驱动天线的交流电频率。如果振荡器频率为 1 MHz，那么无线电波频率也是 1 MHz。

The electric field vector is parallel to the transmitting antenna, so a vertical transmitting dipole produces vertically polarised radio waves. This is why adjusting the orientation of a receiving aerial can improve signal strength.

电场矢量与发射天线平行，因此垂直发射偶极天线产生垂直偏振的无线电波。这就是为什么调整接收天线的方向可以改善信号强度。

4. Transmitting and Receiving Antennas | 发射天线与接收天线

A basic transmitting antenna is a conducting rod connected to a high-frequency alternating voltage source. The alternating voltage accelerates free electrons along the rod, and maximum radiation occurs when the antenna length is comparable to λ/2 or λ/4, creating a standing wave of current.

基本发射天线是一根连接到高频交变电压源的导体棒。交变电压沿棒加速自由电子，当天线长度约为 λ/2 或 λ/4 时，会形成电流驻波，辐射最强。

A receiving antenna placed in the path of a radio wave experiences an oscillating electric field. This field exerts forces on free electrons in the antenna, inducing an alternating potential difference at the same frequency as the incoming wave. The induced p.d. can then be amplified and decoded.

位于无线电波传播路径上的接收天线会受到振荡电场的作用。该电场对天线中的自由电子施加力，感应出与入射波频率相同的交变电势差。感应电动势随后可以被放大和解码。

For efficient reception, the receiving antenna should be parallel to the electric field of the incoming wave, or circularly polarised waves may require different arrangements. This explains why FM radio reception in a car can vary with aerial orientation.

为了有效接收，接收天线应与入射波的电场平行；对于圆偏振波则可能需要不同的布置。这就解释了为什么汽车中 FM 收音机的接收效果会随天线方向而变化。

5. Modulation: AM and FM | 调制：调幅与调频

Audio signals have frequencies from about 20 Hz to 20 kHz, which are too low to be efficiently radiated over long distances. Modulation combines the audio signal with a high-frequency carrier wave so that the information can be transmitted effectively.

音频信号的频率约为 20 Hz 到 20 kHz，频率太低

Published by TutorHao | A-Level Physics Revision Series | aleveler.com

更多咨询请联系16621398022（同微信）