I. Basic Characteristics of Radio Waves WWW.WHWIRELESS.COM 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 pol...

Classification of array antennas. WWW.WHWIRELESS.COM Estimated reading time: 15 minutes Array antennas are typically categorized based on the arrangement of their individual units. Linear array: An array of antenna elements arranged along a straight line, with unit spacing that can be equal or unequal. It can be further divided into edge-illuminated arrays and end-illuminated arrays based on the direction of concentrated radiation energy. Planar array: An array of antenna elements arranged at the centers of a single plane. If all the elements in a planar array are arranged in a rectangular grid, it is called a rectangular array; if all the element centers are located on concentric circles or elliptical rings, it is called a circular array. Planar arrays can also have arrays with equal or unequal spacing. Conformal arrays: arrays of antennas that are attached and conform to the shape of the carrier. Cylindrical-surface arrays, spherical-surface arrays, and conical-surface arrays are all examples of conformal arrays. Array antenna unit configuration. Linear antenna array elements: dipole types, monopole types, ring-shaped elements (such as slot antennas), and spiral elements. Diaphragm-type elements: horn antenna elements, open-slot waveguide elements, microstrip patch elements. Hybrid and specialized elements: Yagi-Uda units, logarithmic-periodic dipole array units, medium-resonance antenna units, metasurface/metamaterial units. The theoretical basis of array antennas. ① Principle of Interference and Superposition of Electromagnetic Waves: Array antennas can create radiation characteristics that differ from those of conventional individual antenna units. One of the primary reasons for this is that the electromagnetic waves emitted by multiple coherent radiation units interfere and superimpose on each other in space, with some areas experiencing increased radiation and others experiencing decreased radiation. This results in a redistribution of the constant total radiation energy across different spatial regions. ② The Directional Diagram Product Theorem: Under far-field conditions, the overall normalized directional function of an antenna array composed of multiple identical elements, excited with fixed amplitude and phase, and arranged in fixed geometric positions, can be decomposed as follows: Primary factor F(θ, φ): The directionality of a single unit in free space (including the unit’s polarization and orientation). Array factor AF(θ, φ): This is determined solely by the geometric layout, spacing, excitation amplitude, and phase of the array, and is independent of the specific shape of the elements. That is, the composite overall direction diagram D(θ,φ) = F(θ,φ) · AF(θ,φ). Analysis of array antennas. The analysis of an array antenna involves determining its radiation characteristics under the assumption that four parameters are known (the total number of elements, the spatial distribution of elements, the distribution of excitation amplitudes...

What Is an Antenna? WWW.WHWIRELESS.COM Estimated reading time: 10 minutes An antenna is a device used to transmit and receive radio waves. It is a key component in wireless communication systems, capable of converting high-frequency electrical currents (which flow in transmission lines) into electromagnetic waves (which propagate through free space), and vice versa. Antennas are widely used in radio broadcasting, television, mobile communication, satellite communication, radar systems, and many other fields. Specifically, the functions of an antenna include: Radiating Electromagnetic Waves: On the transmitting side, the antenna converts high-frequency electrical energy generated by electronic equipment into radio waves and radiates them into surrounding space for long-distance transmission. Receiving Electromagnetic Waves: On the receiving side, the antenna captures radio waves from space and converts them into high-frequency electrical currents. These signals can then be processed—such as demodulation, amplification, and decoding—to recover the original information or data. Energy Conversion: The antenna acts as a medium for energy conversion, efficiently transferring energy between guided waves (in transmission lines) and free-space waves (radio waves). Directivity and Polarization: Many antennas have specific directivity and polarization characteristics. Directivity refers to the antenna’s ability to radiate or receive energy more effectively in certain directions than others. Polarization describes the orientation of the electric field of the radio wave emitted or received by the antenna. These properties help optimize communication performance, reduce interference, and extend communication distance. Impedance Matching: To ensure minimal signal reflection and energy loss during transmission, the antenna must be impedance-matched with the transmission line (feed line). This means the antenna’s input impedance should match the characteristic impedance of the line to allow efficient power transfer. Signal Enhancement and Coverage: In some systems, antennas are used to enhance signal strength or extend coverage. For example: In mobile base stations, high-gain antennas can expand signal coverage areas. In satellite communications, directional and high-gain antennas improve signal reception quality and reliability. WWW.WHWIRELESS.COM

Why Impedance Matching Is Necessary WWW.WHWIRELESS.COM Estimated reading time: 15 minutes The biggest difference between radio frequency (RF) and hardware lies in impedance matching, and the reason for impedance matching is the transmission of electromagnetic fields. As we all know, an electromagnetic field is the interaction between an electric field and a magnetic field. The loss in the transmission medium occurs because the electric field causes oscillations in its effect on electrons. The higher the frequency, the more cycles of electromagnetic waves there are in a transmission line of the same length, and the higher the frequency of current changes. As a result, the heat loss generated by oscillations increases, leading to greater losses in the transmission line. At low frequencies, since the wavelength is much longer than the transmission line, the voltage and current on the transmission line in the circuit remain almost unchanged, so the transmission line loss is very small. Meanwhile, if reflection occurs during wave output, the superposition of the reflected wave with the original input wave may lead to a decline in signal quality and also reduce the efficiency of signal transmission. Whether working on hardware or RF systems, the goal is to achieve better signal transmission, and no one wants energy to be lost in the circuit. When the load resistance is equal to the internal resistance of the signal source, the load can obtain the maximum output power. This is what we often refer to as impedance matching.  It is important to note that conjugate matching is for maximum power transmission.    According to the voltage reflection coefficient formula \( \Gamma = \frac{Z_L - Z_0}{Z_L + Z_0} \), \( \Gamma \) is not equal to 0 at this time, meaning there is voltage reflection.    For distortionless matching, the impedances are completely equal, so there is no voltage reflection. However, the load power is not maximized in this case. Return Loss (RL) = \( -20\log|\Gamma| \) Voltage Standing Wave Ratio (VSWR) = \( \frac{1 + |\Gamma|}{1 - |\Gamma|} \) The relationship between standing wave ratio and transmission efficiency is shown in the table below:    Impedance matching involves a rather tedious calculation process. Fortunately, we have the Smith Chart, an essential tool for impedance matching. The Smith Chart is a diagram composed of many intersecting circles. When used correctly, it allows us to obtain the matching impedance of a seemingly complex system without any calculations. The only thing we need to do is read and track data along the circular lines.    ## Smith Chart Method  1. After connecting a series capacitor component, the impedance point moves counterclockwise along the constant-resistance circle it is on.  2. After connecting a shunt capacitor component, the impedance point moves clockwise along the constant-conductance circle it is on.  3. After connecting a series ind...