Antennas are the core components of modern wireless communication systems, responsible for converting the energy of guided waves into electromagnetic waves radiated into free space, and performing the reverse reception process. Their fundamental operation lies in the transformation of electromagnetic energy forms and directional spatial radiation. This article systematically explains the physical principles, key parameters, and engineering considerations behind this conversion from the perspective of a broadcast communication engineer.
1. The Physical Basis of Antenna Operation: From Transmission Line to Radiator
Within communication systems, signals travel along transmission lines (such as coaxial cables, microstrip lines) in the form of guided waves. The electromagnetic energy of guided waves is confined within conductor or dielectric boundaries, with electric and magnetic fields perpendicular to each other and to the direction of propagation. However, transmission line structures are generally ineffective at electromagnetic radiation because the spacing between their conductors is much smaller than the wavelength, causing the field energy to remain largely bound nearby.
The core function of an antenna is to break this confinement. When an antenna structure is connected to a transmission line and meets specific dimensional criteria (typically comparable to the wavelength), high-frequency alternating currents are excited along the antenna conductor. The accompanying electromagnetic fields can then break free from the conductor’s constraints, forming electromagnetic waves that propagate freely through space. This process is deeply rooted in Maxwell’s equations, particularly Faraday’s Law of Induction and the Ampere-Maxwell Law:
∇×E= –∂B/∂t
∇×H= J+ ∂D/∂t
These equations show that a changing electric field generates a changing magnetic field, and vice versa, creating self-sustaining electromagnetic oscillations that propagate outward [1].
2. The Key Conversion Process: How Antennas Achieve Effective Radiation
2.1 Impedance Matching: The Threshold for Energy Transfer
Impedance matching between the antenna and the transmission line is the primary condition for efficient energy transfer. Transmission lines typically have a fixed characteristic impedance (e.g., 50Ω or 75Ω), while the Antenna Input Impedance is a function of frequency, consisting of a real part (radiation resistance R_r and loss resistance R_l) and an imaginary part (reactance X_a). Ideal matching requires the antenna impedance to be the complex conjugate of the transmission line’s characteristic impedance. In this state, the Voltage Standing Wave Ratio (VSWR) approaches 1:1, reflection is minimized, and most energy transfers from the line to the antenna. Mismatch causes energy reflection, reducing radiation efficiency and potentially damaging the transmitter. Engineers often use an Antenna Tuner or optimize the antenna structure to achieve broadband matching [2].
2.2 Current Distribution and Radiation Mechanism
The Current Distribution on the antenna directly determines its radiation characteristics. Taking the classic half-wave dipole antenna as an example, when its length is approximately half the operating wavelength, the current on the conductor follows an approximately sinusoidal distribution—maximum at the center (feed point) and zero at the ends. This time-varying current excites electromagnetic fields in the surrounding space that detach from the antenna structure. According to electromagnetic theory, accelerating charges (time-varying currents) are the source of radiation. The strength of the radiated field depends on the current magnitude, the antenna’s effective length, and the observation direction.
2.3 Formation of the Far-Field Radiation
The field region around an antenna can be divided into the reactive near-field, the radiating near-field, and the Far-Field Region (Fraunhofer Region). Only in the far-field region (at a distance
r > 2D2/λ ), where D is the antenna’s maximum dimension) do electromagnetic waves exhibit plane-wave characteristics: electric and magnetic fields are mutually perpendicular and in phase, their ratio equals the free-space wave impedance (approximately 377Ω), and they propagate radially. The power density (power per unit area) in the far field can be described by the Poynting vector:
S =1/2 E × H∗
This represents the final form of energy launched into free space by the antenna [3].
3. Core Performance Parameters: The Engineer’s Design Language
3.1 Directivity and Gain
Antenna Gain is a core parameter measuring its directional radiation capability. It is defined as the ratio of the radiation intensity in the antenna’s maximum radiation direction to the radiation intensity of an ideal isotropic (omnidirectional) radiator, given the same input power. It is usually expressed in dBi. Gain is closely related to Antenna Directivity, but gain incorporates the antenna’s own radiation efficiency. High-gain antennas concentrate energy more narrowly into specific sectors, thereby extending communication range, which is crucial for point-to-point microwave links or satellite communications.
3.2 Radiation Pattern and Beamwidth
The Antenna Radiation Pattern is a three-dimensional graphical representation describing the spatial distribution of its radiated energy. Engineers commonly use two-dimensional cross-sectional plots in two principal planes (E-plane and H-plane). The Half-Power Beamwidth (HPBW) is the angular width between points on the pattern where the radiated power drops to half its peak value. It intuitively reflects the concentration of the antenna beam. Sidelobe Level is another important metric for suppressing interference and improving system performance [4].
3.3 Bandwidth and Polarization
The Antenna Operating Bandwidth refers to the frequency range over which its key performance parameters (such as VSWR, gain, pattern) meet specifications. Bandwidth requirements vary by application; broadcast TV antennas may require over 10% relative bandwidth, while some satellite communication antennas might demand very narrow bandwidth to suppress interference.
Antenna Polarization describes the spatial orientation trajectory of the radiating electric field vector over time, common types being linear (vertical/horizontal) and circular polarization. Polarization matching between transmitting and receiving antennas is another key factor for maximizing energy transfer; Polarization Loss can be significant in case of mismatch.
4. Conclusion: The Antenna—A Bridge Connecting Closed Systems to Vast Space
An antenna is far from a simple metal conductor; it is a precise electromagnetic transducer that enables the transformation of wave forms. Its working principle begins with the excitation by guided waves, proceeds through antenna impedance matching and current excitation, and culminates in the effective radiation of electromagnetic waves into free space. The design of every antenna involves a delicate trade-off among parameters like radiation resistance, directivity, bandwidth, and polarization for a specific application scenario. For broadcast communication engineers, a deep understanding of the complete chain from guided waves to free-space waves is fundamental to antenna design, system integration, and solving complex interference problems. With the rapid development of 5G, IoT, and satellite internet, the demand for high-performance, integrated Antenna Technology will continue to drive progress in this ancient yet vibrant field.