- Introduction to Antenna Concepts
- Antenna Types
- Understanding Antenna Radiation
- Beamwidth and Polarisation
Introduction to Antenna Concepts
In the realm of wireless networking, antennas serve as the physical interface between the radio frequency (RF) circuitry and the airwaves. They are the gatekeepers of wireless communication, responsible for transmitting and receiving RF signals between devices within a network. An antenna’s primary functions can be distilled into two core activities:
- Transmission: When an antenna functions in the transmission mode, it takes an electrical signal from the connected device (like an Access Point (AP) or smartphone) and converts it into an electromagnetic wave. This wave is then propagated through the air to reach other devices within or outside the network.
- Reception: Conversely, when operating in reception mode, the antenna captures incoming electromagnetic waves and converts them back into electrical signals, which are then processed by the device it’s connected to.
Antenna Types
Antennas come in a variety of types, each with unique characteristics that make them suitable for specific applications, environments, and performance requirements. Understanding these different types and their properties can significantly optimize a wireless network’s performance, range, and reliability.
Understanding antennas is the foundation upon which advanced topics like Multiple Input Multiple Output (MIMO), antenna diversity, and beamforming are built. As you delve into these advanced topics, you’ll appreciate the pivotal role antennas play in enabling high-speed, reliable wireless communication.
Omnidirectional antennas
Not all antennas are created equal. Different types of antennas are optimised for specific roles and environments, from broadcasting Wi-Fi in homes to facilitating high-speed data transfer in industrial settings. This section will delve into the diverse world of antenna types, discussing their unique characteristics, advantages, and applications.
Omni-directional antennas are often used in Wi-Fi routers, walkie-talkies, mobile communications, and other wireless systems where the orientation of transmitting and receiving can’t be controlled. They are designed to cover a wide area and are generally less powerful over a distance compared to directional antennas, which focus the signal in a specific direction. The trade-off is that omnidirectional antennas don’t need to be pointed toward a specific signal source, making them more versatile for certain applications.
The image below shows a side view of the radiation patterns of an omnidirectional antenna with different gains.
The same antenna from a top-down view:
Semi-directional Antennas
Unlike omnidirectional antennas, which radiate an RF signal in all directions, semi-directional antennas are designed to direct a signal in a specific direction. Semidirectional antennas are used for short to medium-distance communications, with long-distance communications served by highly directional antennas. It is common to use semi-directional antennas to provide a network bridge between two buildings in a campus environment or down the street from each other.
The image below displays a top-down view of a generic semi-directional antenna.
Planar Antenna
Derived from the Latin word ‘planus’ meaning ‘flat’, planar antennas are aptly named. They typically offer a horizontal (H-Plane) beamwidth ranging from 40° to 120°. These antennas can be further divided into two categories: Patch and Panel.
Patch antennas consist of a flat rectangular or square-shaped metal patch mounted above a ground plane. A dielectric substrate separates the patch and the ground plane.
On the other hand, panel antennas are array antennas that consist of multiple individual radiating elements arranged in a flat or slightly curved panel. These elements produce a slightly more specific radiation pattern than a patch antenna.
Planar antennas can facilitate outdoor communications over distances up to a mile. Yet, they’re more frequently found indoors, serving as central devices. They provide unidirectional signals from APs to client devices. Imagine a library, warehouse, or retail store with long aisles lined with shelves. In such places, omnidirectional antennas might struggle to offer consistent RF coverage due to obstructions like tall shelves.
Here’s where planar antennas shine. You can mount them high on a building’s sidewalls, directing signals through the aisles. You ensure a uniform signal distribution by alternating their placement between rows and positioning them on opposite walls. Given their horizontal beamwidth of 180 degrees or less, there’s little signal spillage outside the building. This strategic placement ensures signals travel down the aisles, offering the required coverage.
Before the introduction of 802.11 MIMO radios, indoor environments often used patch and panel antennas with older 802.11/a/b/g radios. The goal was to minimize reflections and counteract the adverse effects of multipath. Places with many metal racks or shelves, like warehouses or retail stores, were especially challenging due to high multipath interference.
However, with the rise of MIMO technology, which uses multipath to its advantage, the primary purpose of planar antennas shifted. While they’re still used indoors, the reasons are different. Consider a high-density setting, such as a packed school gym or a conference hall brimming with Wi-Fi users. In such scenarios, a standard omnidirectional antenna might not suffice. When mounted on ceilings or walls and directed downwards, MIMO patch and panel antennas create focused coverage areas or “sectors.” Hence, in indoor settings, the primary role of MIMO patch antennas now revolves around catering to high-density areas.
Sector Antenna
Sector antennas are high-gain, semi-directional antennas known for their pie-shaped coverage pattern. They’re designed to cover specific segments of an area, and when combined, they can provide comprehensive 360-degree coverage (a complete pie). This combination is often referred to as a sectorized array.
In cell phone communications, sector antennas play a pivotal role. Cell towers use these antennas to divide their coverage area into distinct sectors, allowing them to efficiently distribute cellular signals. Each sector antenna handles its designated slice of the coverage pie, ensuring that users within that segment receive a strong and consistent signal.
But let’s pivot to Wi-Fi, the main focus of our discussion.
Wi-Fi deployments can benefit from sector antennas, especially in large outdoor areas. Here’s why:
- Height and Tilt: Unlike omnidirectional antennas, sector antennas can be mounted high and tilted slightly downward to cater to specific terrains. This positioning ensures the signal is directed where it’s most needed, optimising coverage and signal strength.
- Simultaneous Transmission: Each sector antenna can connect to its own transceiver. This setup allows multiple antennas to transmit and receive data simultaneously, boosting the overall network throughput. In contrast, a single omnidirectional antenna can only communicate with one device at a time.
- Enhanced Gain: Sector antennas generally have a higher gain compared to omnidirectional antennas. This translates to a larger coverage area, making them ideal for vast outdoor spaces.
While sector antennas are a staple in cell phone communications, their use in Wi-Fi is more niche. However, they’re becoming more popular, especially for Internet service providers offering last-mile connectivity in areas where laying traditional cables might be challenging or impractical. By leveraging the focused coverage of sector antennas, providers can ensure robust and reliable Wi-Fi connections even over extended distances.
Highly Directional Antennas
Highly directional antennas excel in focusing signals in a specific direction, ensuring strong connections and minimal interference. They’re the go-to choice for point-to-point, point-to-multipoint, and backhaul links. Their focused beamwidth is crucial in scenarios requiring bridging distant locations or bypassing obstacles, guaranteeing consistent and reliable communication.
The image below illustrates a top-down view of a generic highly-directional antenna..
Yagi Antenna
With its driven element, reflectors, and multiple directors on a straight rod, the Yagi antenna radiates mainly in one direction, reducing interference. While commonly used for TV reception in areas with weak signals, in Wi-Fi, they shine in long-distance point-to-point links, like connecting distant buildings, and in boosting reception in interference-prone areas.
Parabolic Dish Antenna
With its curved dish design, the parabolic dish antenna concentrates radio waves to a focal point, enhancing signal strength and directionality. In 802.11 Wi-Fi, they’re the top pick for long-distance links, such as connecting buildings across vast terrains.
Grid Antenna
Grid antennas merge the principles of parabolic designs with an open lattice structure, offering resilience against wind. In Wi-Fi, they’re favoured for outdoor point-to-point links in windy areas, ensuring stability against gust-induced misalignments or damage.
Antenna Comparison
The table below provides a comparison of different Wi-Fi antenna types, highlighting their key characteristics and typical applications. This summary helps in understanding which antenna type is best suited for specific scenarios and requirements.
| Antenna Type | Characteristics | Typical Applications |
|---|---|---|
| Omnidirectional | Radiates equally in all directions; typical beamwidth: 360° horizontal, 25° vertical | Used in most Wi-Fi routers and access points, client devices |
| Planar | Horizontal beamwidth of 40°-120°, flat design; includes Patch and Panel antennas | Indoor coverage in aisles of warehouses, retail stores, high-density areas like conference halls and school gyms |
| Sector | High gain, pie-shaped coverage; typical beamwidth: 60°-120° horizontal | Large outdoor areas, cellular networks, campus-wide Wi-Fi coverage, outdoor event venues |
| Yagi | Highly directional, multiple directors; typical beamwidth: 15°-40° horizontal | Long-distance point-to-point links, connecting distant buildings on a campus, extending Wi-Fi to remote areas |
| Parabolic Dish | High gain, focused signal; typical beamwidth: 3°-10° horizontal | Long-distance Wi-Fi links, bridging gaps across large open areas such as fields or between hills |
| Grid | Resilient to wind, lattice structure; typical beamwidth: 5°-12° horizontal | Outdoor links in windy areas, providing stable point-to-point connections in rural or industrial settings |
Understanding Antenna Radiation
Azimuth and Elevation Charts
To aid potential buyers in making informed decisions, antenna manufacturers provide azimuth and elevation charts for their antennas, commonly referred to as radiation patterns.
These radiation patterns are precisely created in controlled environments where external influences cannot distort the results. They represent the signal pattern emitted by a specific antenna model, often referred to as polar charts or antenna radiation envelopes.
The image below shows an azimuth and elevation chart of a typical enterprise access point with an omnidirectional antenna.
The azimuth chart displays the antenna’s radiation pattern from a top-down perspective. Given its omnidirectional nature, the radiation pattern is nearly circular, as the azimuth chart shows. On the other hand, the elevation chart offers a side view of the antenna’s radiation pattern. Unfortunately, there’s no standard dictating that antenna manufacturers align the degree marks on the chart with the antenna’s orientation, placing the onus on the chart’s reader to decipher its meaning.
To assist in interpreting these radiation charts, consider the following pointers:
- In both charts, the access point or antenna is centred.
- The azimuth chart, or H-plane, provides a top-down view.
- The elevation chart, or E-plane, offers a side view.
- The outer ring on the chart signifies the antenna’s strongest signal.
Interpreting Polar Charts
Antenna azimuth (H-plane) and elevation (E-plane) charts, often referred to as polar charts, are essential tools in understanding the directional characteristics of antennas. These charts commonly use decibel (dB) values to map the antenna’s radiation pattern. This logarithmic approach, rather than a linear one, is crucial because it allows the charts to cover a broad range of signal strengths effectively, despite the exponential nature of decibel measurements.
The Logarithmic Nature of dB Scales
The use of decibel scales can be misleading without proper interpretation. For instance, a decrease from 0 dB to -10 dB doesn’t represent a halving of the signal strength but rather a tenfold decrease. This exponential relationship means that even small numerical changes on a dB scale represent significant changes in signal strength, which is vital for precise antenna configuration.
Consider the image below, where boxes appear to be of equal size but actually represent values that double with each subsequent box. This analogy helps explain how dB scales work on polar charts. Even though areas on the chart may look similar in size, they can represent vastly different signal strengths.
Practical Application
When configuring or choosing antennas, it’s important to understand how to read polar charts correctly. By knowing where the signal strength drops off – indicated by the points moving towards the center of the chart – engineers can better predict how the antenna will perform in a given environment. This is crucial for applications requiring precision, such as point-to-point communications, where a focused beam is necessary.
To further illustrate the point, the image below compares a typical logarithmic chart measured in dBi (left), to a linear chart which has been normalised to maximum gain (right). This example serves to highlight that while linear charts can sometimes offer a clearer basic understanding, logarithmic charts are essential and standard for detailing antenna radiation patterns, ensuring all nuances are accurately represented for technical assessments.
Understanding polar charts and the data they represent allows network technicians and engineers to optimise antenna placement and configuration, ensuring efficient and reliable communication networks. By mastering the interpretation of polar charts, professionals can enhance both the performance and reliability of wireless communications systems.
Beamwidth and Polarisation
Beamwidth
Beamwidth is akin to the adjustable lens on a flashlight, allowing the user to focus or spread the light. However, unlike flashlights, antenna beamwidth is fixed, and one must choose the desired focus at the time of purchase. Beamwidth refers to the angle from which the strength of the signal radiates from the antenna, defining how narrow or broad the signal spread is. It’s measured both horizontally and vertically from the center or peak of the signal, extending to the points along the axes where the signal strength falls to half its power (-3 dB), also known as the half-power points.
This angular measurement is critical when selecting an antenna for your specific communication needs. For instance, a narrower beamwidth focuses the signal more tightly, enhancing the reach and reducing interference, which is ideal for long-distance links. Conversely, a broader beamwidth offers wider coverage, suitable for indoor areas where devices are spread out. Understanding these specifications, provided on manufacturer datasheets, helps in making informed decisions about the appropriate antenna for various scenarios.
Antenna Polarisation
Antenna polarisation refers to the orientation of the electromagnetic waves radiating from an antenna, which can be either horizontal or vertical. This characteristic is crucial for the alignment of transmitting and receiving antennas to ensure optimal signal reception. In simple terms, if one antenna sends out waves horizontally, the receiving antenna should also be oriented horizontally to capture the most signal. The same applies for vertical orientation.
While polarisation may not significantly impact indoor communications – where signals often reflect off surfaces and change direction – it plays a vital role in outdoor settings, particularly in point-to-point or point-to-multipoint communications. Incorrect polarisation alignment can lead to cross-polarisation, where the mismatch between transmitting and receiving antennas results in a significant signal degradation. Thus, ensuring the polarisation matches between antennas can markedly enhance communication effectiveness.
By understanding both beamwidth and antenna polarisation, network designers and technicians can optimise antenna placement and configuration, enhancing both the range and quality of wireless communications. This knowledge is essential for efficient network planning and implementation, especially in environments that require precise directional control and signal integrity.