I. Basic Characteristics of Radio Waves

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Estimated reading time: 15 minutes

1.1 Definition of Radio Waves

Radio waves serve as the carrier of signals and energy, generated by the mutual coupling of oscillating electric and magnetic fields, adhering to the alternating coupling law of "electricity generates magnetism and magnetism generates electricity". During propagation, the electric and magnetic fields are always perpendicular to each other and both perpendicular to the propagation direction of the wave, making them **Transverse Electromagnetic Waves (TEM waves)**.

Their generation originates from high-frequency oscillating circuits: when the current in a circuit changes rapidly over time, an alternating electromagnetic field is excited in the surrounding space. Once this electromagnetic field detaches from the wave source, it propagates through space in the form of radio waves, without relying on any medium—they can even transmit in a vacuum.

1.2 Relationship between Wavelength, Frequency and Propagation Speed

The core formula governing the relationship between the wavelength (λ), frequency (f) of radio waves and their propagation speed (speed of light \( C \) in a vacuum, approximately \( 3×10^8 \, \text{m/s} \)) is:

\[ \lambda = \frac{C}{f} \]

**Key Conclusion**: In the same medium, frequency and wavelength are strictly inversely proportional—the higher the frequency, the shorter the wavelength. This relationship directly dictates the design dimensions of antennas: for example, the wavelength of a 2.4GHz WiFi signal is approximately 12.5 cm, corresponding to a half-wave dipole antenna length of about 6.25 cm; for a 700MHz low-frequency communication signal, the wavelength is approximately 42.8 cm, requiring a half-wave dipole length of 21.4 cm. Additionally, the electrical performance of an antenna (such as radiation efficiency, gain, and impedance) is directly related to its **electrical length** (the ratio of physical length to wavelength). In practical engineering, the required electrical length must be converted to the specific physical length to ensure the antenna operates properly.

1.3 Polarization of Radio Waves

Polarization refers to the variation law of the electric field direction as a radio wave propagates, determined by the spatial motion trajectory of the electric field vector, forming a complete spectrum: **Circular Polarization ← Elliptical Polarization → Linear Polarization**. The core characteristics and application scenarios of the three are as follows:

- **Linear Polarization**: The electric field direction remains fixed, the most commonly used polarization form. A wave with an electric field perpendicular to the ground is a **vertically polarized wave**, which has strong resistance to ground reflection interference and is suitable for terrestrial mobile communications (e.g., traditional 2G/3G base stations); a wave with an electric field parallel to the ground is a **horizontally polarized wave**, commonly used in radio and television transmission, microwave relay communications, and other scenarios.

- **Circular Polarization**: The trajectory of the electric field vector is circular, divided into **left-hand circular polarization** and **right-hand circular polarization**, which are mutually exclusive (a left-hand antenna can only receive left-hand circularly polarized waves, and vice versa). Its core advantage is strong resistance to multipath interference and polarization torsion, making it widely used in satellite communications (e.g., Beidou, GPS satellites), unmanned aerial vehicle (UAV) remote control, and other scenarios.

- **Elliptical Polarization**: The trajectory of the electric field vector is elliptical, the general form of polarization—circular polarization occurs when the major and minor axes of the ellipse are equal, and linear polarization when the minor axis approaches zero. In actual communication environments, due to multipath reflections, obstacle occlusion, and other factors, pure linear or circular polarized waves are often converted into elliptically polarized waves.

1.4 Multipath Propagation

When radio waves propagate, in addition to direct waves, they undergo reflection, diffraction, and transmission when encountering obstacles such as hills, forests, and buildings, resulting in the receiving terminal simultaneously receiving multi-path radio waves—a phenomenon known as **multipath propagation**. Its core impacts include: (1) Complicating the signal strength distribution, causing "shadow fading" and "fast fading" and leading to severe fluctuations in signal strength at the receiving end; (2) Altering the polarization direction of the radio wave, resulting in polarization mismatch and reducing the received signal strength; (3) Generating delay spread (the time difference between signals arriving via different paths), causing intersymbol interference; (4) Causing local signal superposition (enhancement) or cancellation (weakening, depending on the relationship between path difference and wavelength). For instance, in dense urban areas, building reflections generate a large number of multi-path signals, leading to frequent fluctuations in the signal strength received by mobile phones.

The core solution to this issue is **diversity reception technology**, which receives and combines multi-path signals to mitigate interference. It is divided into two categories:

1. **Spatial Diversity**: Utilizes multiple single-polarized antennas with a reasonable spatial layout (spacing greater than 10 times the wavelength) to receive signals via different paths. Suitable for scenarios with low polarization requirements.

2. **Polarization Diversity**: Leverages the orthogonal characteristics of dual-polarized antennas to simultaneously receive two vertically polarized signals (e.g., +45°/-45°). Due to the low correlation of signals, the combined output significantly improves reception reliability, making it the mainstream solution for current 5G base stations.

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