Antennas are fundamental components operating at the Physical Layer (Layer 1) of the OSI model in Wireless Local Area Networks (WLANs). They function as transducers, converting electrical signals into radio frequency (RF) waves for transmission and vice versa for reception. The choice of antenna significantly impacts a network's coverage area, signal strength, and overall performance, making it a crucial aspect for WLAN professionals involved in design, analysis, and troubleshooting. Various antenna types, including omnidirectional, patch, Yagi, sector, and parabolic dish antennas, possess distinct characteristics tailored for different deployment scenarios.
A key characteristic of an antenna is its gain, typically measured in dBi (decibels isotropic) or dBd (decibels dipole). Gain represents the increase in power achieved by concentrating signal energy in a particular direction compared to a theoretical isotropic radiator (for dBi) or a dipole antenna (for dBd). This concentration is referred to as passive gain and directs existing energy more effectively rather than generating more power. The Equivalent Isotropically Radiated Power (EIRP), a vital metric for compliance and link budget calculations, is the transmitter's output power combined with any gains or losses up to the antenna connection point, plus the antenna's directional gain. The dBi or dBd value must be calculated against the input power supplied to the antenna to determine the actual output power in the direction of propagation.
Another important characteristic is beamwidth, which describes the angular spread of the concentrated RF energy from the antenna's primary lobe. Beamwidth is measured in both horizontal and vertical planes at the points where the signal power is 3 dB lower (half power) than the maximum power point. Azimuth and Elevation diagrams visually depict an antenna's radiation pattern. Azimuth diagrams show the top-down (horizontal) pattern, while Elevation diagrams show the side (vertical) pattern. These diagrams are essential for predicting coverage, understanding propagation, and minimizing interference. The use of detachable or external antennas offers flexibility to position the antenna separately from the Access Point (AP) and select different antenna types or gain levels to optimize coverage (Basic Service Area - BSA) or address specific propagation issues. Antenna selection should consider manufacturer support, gain needs, and connector types.
Antenna selection is a critical component in designing Wireless LANs (WLANs), influencing signal propagation, coverage, capacity, and overall network performance. Different antenna types focus and radiate Radio Frequency (RF) energy in distinct patterns, making certain types more suitable for specific environments and use cases. Antenna gain, referenced in dBi, and directionality, often visualized through Azimuth and Elevation charts, are key characteristics that define how an antenna will perform. Beamwidth, measured horizontally and vertically at the -3 dB (half power) points, provides a quantitative measure of how broad or narrow the antenna's focus is. Understanding these characteristics and the different antenna types—Omnidirectional, Directional, Semi-directional, Patch/Panel, Sector, Yagi, and Parabolic Dish—is essential for effective WLAN design.
Omnidirectional antennas are a primary type of antenna used in WLANs. Characterized by a horizontal beamwidth of 360 degrees, they radiate RF energy outward in all directions within a horizontal plane. While they provide uniform coverage horizontally, their vertical beamwidth can vary significantly, typically ranging from 7 to 80 degrees. In general indoor implementations, internal antennas with omni-optimized patterns are often sufficient to provide coverage across an area. Standard omnidirectional antennas usually have a gain around 2.2 dBi, although higher-gain omni antennas (like 7 or 12 dBi) can be beneficial. Outdoor Access Points (APs) commonly use external omni antennas, often high-gain variants. When performing site surveys, using omni-directional antennas similar to what will be deployed is important for accurate measurements. Mismatched links can occur in bridge configurations if one side uses a high-output power omnidirectional antenna while the other uses a lower-power, high-gain antenna.
Directional and semi-directional antennas are designed to focus RF energy in a specific direction or within a specific area, unlike omnidirectional antennas which spread energy in all horizontal directions. This focus provides higher gain in the desired direction and can be used to minimize Co-channel interference (CCI) by limiting signal propagation into unwanted areas. Beamwidth and Azimuth (top-down view) and Elevation (side view) charts are used to visualize and understand the specific propagation pattern of directional antennas. Semi-directional antennas are often used in environments like manufacturing or warehousing. Several specific types fall under the directional or semi-directional categories, each with distinct characteristics and applications:
Patch/Panel Antennas: These are listed as a type of antenna. Patch/panel antennas typically have a horizontal beamwidth ranging from 30 to 180 degrees and a vertical beamwidth from 6 to 90 degrees. They are often used in manufacturing or warehousing environments more frequently than standard offices. Patch or panel antennas can be mounted on a wall and aimed inward or down a hallway to provide focused coverage. Testing with patch or panel antennas during a site survey allows evaluation of their performance compared to omni antennas.
Sector Antennas: Sector antennas are characterized by a horizontal beamwidth between 60 and 180 degrees and a narrow vertical beamwidth ranging from 7 to 17 degrees. These are particularly useful in high-density (HD) and very high-density (VHD) deployments, such as Large Public Venues (LPV). They can also be advantageous for outdoor networks. A sector antenna is intended to propagate its energy in one direction but across a fairly wide path, as illustrated by its Azimuth chart. Like patch/panel antennas, sector antennas are more common in manufacturing or warehousing than in standard offices.
Yagi Antennas: Yagi antennas have a horizontal beamwidth of 30 to 78 degrees and a vertical beamwidth of 14 to 64 degrees. They are another type of antenna used in environments like manufacturing or warehousing.
Parabolic Dish Antennas: Parabolic dish antennas are a type of highly directional antenna, more likely to be needed for outdoor site surveys. They offer the narrowest beamwidth among the listed types, ranging from 4 to 25 degrees horizontally and 4 to 21 degrees vertically. This focused energy makes them particularly suitable for long-distance outdoor links and bridge links, including Point-to-Point (PtP) and Point-to-Multipoint (PtMP) configurations. When designing bridge links, antenna selection is based on the required link distance, data-rate requirements, and regulatory constraints, with antenna gain impacting the Signal-to-Noise Ratio (SNR) at the receiver.
The choice of antenna type has a profound effect on WLAN design and implementation. The propagation pattern of the selected antenna dictates the coverage area of an AP. For instance, omnidirectional antennas are used for broad area coverage in standard office environments, while directional antennas are employed to focus signal in specific areas or directions, which is crucial in environments like warehouses or stadiums.
Antenna gain contributes to achieving the necessary signal strength and SNR for reliable connections and required data rates, especially over distance. High-gain antennas should be used cautiously during site surveys on the survey client, as they might falsely represent sufficient coverage that standard client devices will not experience. Proper antenna selection is vital for mitigating interference. Using directional antennas can help prevent or minimize CCI in specific areas by containing the RF signal.
During site surveys, it is essential to use APs equipped with external antenna connectors to test different power levels and antenna types. Accurate predictive modeling relies on defining the correct antenna types and their properties. Post-installation validation should ensure that APs, including their antennas, are mounted in the correct location and orientation as specified in the design recommendation, because deviation can significantly alter the RF propagation pattern and coverage. Antenna selection is also a key consideration when designing bridge links, impacting link budgets and signal reach. For specific use cases like high-density designs, Large Public Venues, and outdoor deployments, the appropriate selection and placement of antennas, including sector or directional types, are critical for success.
In conclusion, selecting the appropriate antenna type based on its characteristics, such as gain and beamwidth, and understanding its propagation pattern is fundamental to successful WLAN design. The choice of omnidirectional or various directional antennas like Patch/Panel, Sector, Yagi, and Parabolic Dish directly impacts coverage, capacity, interference management, and the accuracy of site surveys and installations, ultimately determining whether the WLAN meets the required performance objectives.