Definition: The bandwidth of an antenna refers to the frequency range within which the antenna’s performance parameters (such as input impedance, gain, VSWR, radiation pattern, etc.) meet certain specified requirements. From a straightforward perspective, it is the frequency range in which the antenna can effectively operate. For example, for a specific antenna, if the VSWR is below a certain threshold in a particular frequency range, that range is considered the bandwidth of the antenna in terms of VSWR. For a TV antenna, for instance, the traditional VHF antenna bandwidth might cover a frequency range of 47-230 MHz, which is used to receive different TV channels. Within this bandwidth, the antenna can effectively convert received electromagnetic waves into electrical signals for the television.
Bandwidth Measurement Indicators:
VSWR Bandwidth: VSWR (Voltage Standing Wave Ratio) is an important metric for measuring the impedance matching in the antenna system. The frequency range where VSWR is below a certain threshold (e.g., 1.5 or 2) is defined as the VSWR bandwidth. Generally, within this bandwidth range, the antenna is well matched to the feed line and source impedance, with minimal signal reflection, thus ensuring effective power transmission. For instance, if an antenna has a VSWR less than 2 in the frequency range of 2 GHz to 6 GHz, this range is considered its VSWR bandwidth.
Gain Bandwidth: The gain of an antenna is the ratio of the signal power density produced by the actual antenna to the signal power density produced by an ideal radiation unit at the same point in space under equal input power conditions. Gain bandwidth refers to the frequency range where the antenna’s gain does not drop by more than a specified value (e.g., 3 dB). For example, if an antenna has a gain of 10 dB at its center frequency and the gain decreases by no more than 3 dB in the frequency range of 3 GHz to 5 GHz, this 3-5 GHz range would be its gain bandwidth.
Efficiency Bandwidth: The efficiency of an antenna is the ratio of the power radiated by the antenna to the power input into the antenna. Efficiency bandwidth refers to the frequency range where the antenna maintains an efficiency above a certain level. For instance, if the antenna radiation efficiency is required to be greater than 50% within a certain frequency range, this range is considered its efficiency bandwidth.
Factors Affecting Antenna Bandwidth:
Antenna Design Factors:
Physical Size and Shape: Larger antennas typically radiate over a broader frequency range, resulting in a wider bandwidth. For example, large parabolic antennas tend to have a wider bandwidth. Different geometric shapes like helical, horn, and patch antennas can also affect the antenna’s operating frequency range. A helical antenna can operate over a wider frequency range, while smaller patch antennas might have a narrower bandwidth.
Load Technologies: Techniques like loading loops can extend the effective electrical length of the antenna, thereby affecting its impedance characteristics and possibly increasing its bandwidth.
Material Properties: The conductivity and permeability of the materials used to construct the antenna can affect current distribution and losses, which in turn influences the antenna's bandwidth. For example, materials like copper and aluminum, which have good conductivity, reduce resistive losses and can help broaden the bandwidth. The dielectric constant and loss tangent of the substrate material used for microstrip antennas also significantly impact the resonant frequency and bandwidth. Thicker substrates, lower dielectric constants, and higher loss tangents can increase bandwidth but may lead to increased losses, reducing efficiency.
Impedance Matching: Proper impedance matching between the antenna and the transmission line is critical for maximizing power transfer and minimizing reflection losses. An antenna with good impedance matching will perform better within its specified bandwidth. If impedance mismatching occurs, signal reflection increases, leading to reduced antenna efficiency and narrowed bandwidth.
Multi-resonant Modes and Parasitic Structures: Exciting multiple resonant modes in an antenna can increase bandwidth. For instance, adding U-shaped slots, pins, or using triangular, circular, or elliptical patches in microstrip antennas, or using serpentine or bent dipole arms can achieve multiple resonances. Additional parasitic structures can form extra resonant circuits, expanding bandwidth when the resonant frequencies overlap, but this might affect the antenna's radiation pattern.
Environmental Factors:
Nearby Objects: The presence of nearby metallic objects, buildings, and other structures can affect antenna performance, changing its effective operating frequency and bandwidth. Metal objects, in particular, may cause signal reflection and scattering, altering the radiation pattern and influencing the bandwidth.
Ground Conditions: The electrical properties of the ground, such as conductivity and dielectric constant, affect the antenna’s grounding and electromagnetic field distribution, which in turn influences bandwidth. For example, antennas installed on different types of ground (e.g., grass, concrete, or sand) may exhibit different bandwidth characteristics.
Atmospheric Conditions: Climate conditions, such as temperature, humidity, and air pressure, may indirectly affect the electrical properties of antenna materials, thereby impacting the bandwidth.