Antenna

An antenna gives the wireless system three fundamental properties: gain, direction and polarization. Gain is a measure of increase in power. Gain is the amount of increase in energy that an antenna adds to a radio frequency (RF) signal. Direction is the shape of the transmission pattern. As the gain of a directional antenna increases, the angle of radiation usually decreases. This provides a greater coverage distance, but with a reduced coverage angle. The coverage area or radiation pattern is measured in degrees. These angles are measured in degrees and are called beamwidths.

An antenna is a passive device which does not offer any added power to the signal. Instead, an antenna simply redirects the energy it receives from the transmitter. The redirection of this energy has the effect of providing more energy in one direction, and less energy in all other directions.

Beamwidths are defined in both horizontal and vertical plains. Beamwidth is the angular separation between the half power points (3dB points) in the radiation pattern of the antenna in any plane. Therefore, for an antenna you have horizontal beamwidth and vertical beamwidth.

Antennas are rated in comparison to isotropic or dipole antennas. An isotropic antenna is a theoretical antenna with a uniform three-dimensional radiation pattern (similar to a light bulb with no reflector). In other words, a theoretical isotropic antenna has a perfect 360 degree vertical and horizontal beamwidth or a spherical radiation pattern. It is an ideal antenna which radiates in all directions and has a gain of 1 (0 dB), i.e. zero gain and zero loss. It is used to compare the power level of a given antenna to the theoretical isotropic antenna.

Radiation Pattern of an Isotropic Antenna

Antennas can be broadly classified as omnidirectional and directional antennas, which depends on the directionality.

Unlike isotropic antennas, dipole antennas are real antennas. The dipole radiation pattern is 360 degrees in the horizontal plane and approximately 75 degrees in the vertical plane (this assumes the dipole antenna is standing vertically) and resembles a donut in shape. Because the beam is slightly concentrated, dipole antennas have a gain over isotropic antennas of 2.14 dB in the horizontal plane. Dipole antennas are said to have a gain of 2.14 dBi, which is in comparison to an isotropic antenna. The higher the gain of the antennas, the smaller the vertical beamwidth is.

Imagine the radiation pattern of an isotropic antenna as a balloon, which extends from the antenna equally in all directions. Now imagine that you press in on the top and bottom of the balloon. This causes the balloon to expand in an outward direction, which covers more area in the horizontal pattern, but reduces the coverage area above and below the antenna. This yields a higher gain, as the antenna appears to extend to a larger coverage area.

v types of antenna 

1.  There are three major types of antenna systems in use today.
   1. Omnidirectional
   2.  Semidirectional
   3.  Highly directional
2. Sectorized and Phase-Array Antennas
3. MIMO (Multiple-Input, Multiple-Output) Antenna systems

v omnidirectional antenna  

An omnidirectional antenna is an antenna system which radiates power uniformly in one plane with a directive pattern shape in a perpendicular plane. This pattern is often described as "donut shaped". Omnidirectional antenna can be used to link multiple directional antennas in outdoor point-to-multipoint communication systems including cellular phone connections and TV broadcasts

Common low gain omnidirectional antennas are the whip antenna, a vertically oriented dipole antenna, the discone antenna, and the horizontal loop antenna (or halo antenna) (Sometimes known colloquially as a 'circular aerial' because of the shape).

Higher gain omnidirectional antennas are the Coaxial Colinear (COCO) antenna and Omnidirectional Microstrip Antenna (OMA). 

Omni Antenna Pros and Cons 

Omni antennas are very easy to install. Due to the 360 degrees horizontal pattern, it can even be mounted upside down from a ceiling in the indoor environment. Also, because of its shape it is very convenient to attach these antennas to the product. For example, you might see Rubber Duck antennas attached to the wireless APs. In order to obtain an omnidirectional gain from an isotropic antenna, energy lobes are pushed in from the top and the bottom, and forced out in a doughnut type pattern. If you continue to push in on the ends of the balloon (isotropic antenna pattern), a pancake effect with very narrow vertical beamwidth results, but with a large horizontal coverage. This type of antenna design can deliver very long communications distances, but has one drawback which is poor coverage below the antenna.

Omni Antenna with No Coverage below the Antenna 

If you try to cover an area from a high point, you see a big hole below the antenna with no coverage.

This problem can be partially solved with the design of something called downtilt. With downtilt, the beamwidths are manipulated to provide more coverage below the antenna than above the antenna. This solution of downtilt is not possible in an omni antenna because of the nature of its radiation pattern.

The omni antenna is usually a vertically polarized antenna, so you cannot have advantages of using cross polarization here to fight interference.

A low gain omni antenna provides a perfect coverage for an indoor environment. It covers more area near the AP or a wireless device in order to increase the probability of receiving the signal in a multipath environment.

v Semi Directional Antenna

 There are three types of semi-directional antennas: flat patch and panel antennas are designed for wall-mounting, and cylinder shaped Yagi antennas, according to the Wireless Computing and Communications website. As the name would suggest, semi-directional antennas are more focused in directing a signal than omnidirectional antennas, while having a broader range of coverage than highly directed antennas, such as satellite dish antennas. However, semi-directional antennas are not the ideal solution for all reception related circumstances, and have a number of disadvantages as well.

Semi-directional antennas radiate RF in a fashion similar to the way a wall sconce is designed to radiate light away from the wall or the way a street lamp is designed to shine light down on a street or a parking lot, providing a directional light across a large area. 

Small Coverage Area

 Depending on its size, a semi-directional antenna can have a very long range of signal transmission. This makes semi-directional antennas ideal for use in libraries, where rows of bookshelves create long corridors, as well as in hospitals with their long hallways. However, semi-directional antennas have a fairly small coverage area with a limited width when compared with omnidirectional antennas, according to the Cisco website. Omni-directional antennas are designed to have a coverage area of 360 degrees. Semi-directional antennas, on the other hand, have coverage areas ranging from 30 to 180 degrees for patch and panel antennas and 30 to 78 degrees for Yagi antennas. Satellite dish antennas average four to 25 degrees coverage range, according to the Wireless Computing and Communications website.

Signal Interference

 Another disadvantage with semi-directional antennas is the potential for signal interference. Trees, walls or any objects located within the line of sight of the antenna signal can be a potential source of interference. This is especially true when semi-directional antennas are mounted outdoors, according to. In the case of interference from trees, the problem often becomes worse as trees grow taller and block more of the line of sight between the antenna and the signal source, according to the Wireless Computing and Communications website.

Limited Mounting Options

 A third disadvantage for semi-directional antennas is that they must face the signal source in order to be effective. This can severely limit the mounting options for the antenna, according to the Cisco website. This is especially true when the antenna must be mounted in congested urban areas or indoors, and even more so when the antenna itself is large. Yagi antennas, which protrude from the mounting location, can be a particular challenge to mount in a constricted space.

v Highly directional antenna

Highly-directional antennas are strictly used for point-to-point communications, typically to provide network bridging between two buildings. They provide the most focused, narrow beamwidth of any of the antenna types.

There are two types of highly-directional antennas: parabolic dish and grid antennas. The parabolic dish antenna is similar in appearance to the small digital satellite TV antennas that can be seen on the roofs of many houses.

The grid antenna resembles the rectangular grill of a barbecue, with the edges slightly curved inward. The spacing of the wires on a grid antenna is determined by the wavelength of the frequencies that the antenna is designed for.

Because of the high gain of highly-directional antennas, they are ideal for long-distance communications as far as 35 miles (58 km). Due to the long distances and narrow beamwidth, highly-directional antennas are affected more by antenna wind loading, which is antenna movement or shifting caused by wind.

Even slight movement of a highly-directional antenna can cause the RF beam to be aimed away from the receiving antenna, interrupting the communications. In high-wind environments, grid antennas, due to the spacing between the wires, are less susceptible to wind load and may be a better choice.

Another option in high-wind environments is to choose an antenna with a wider beamwidth. In this situation, if the antenna were to shift slightly, due to its wider coverage area, the signal would still be received. No matter which type of antenna is installed, the quality of the mount and antenna will have a huge effect in reducing wind load.

Highly directional antennas kinds 

Highly directional antennas are of two kinds, the grid style seen on top of television sets and the dish-shaped ones commonly used to receive satellite TV stations. A highly directional antenna offers a number of advantages over the other two types, which are omni-directional and semi-directional antennas. As its name suggests, the design of the highly directional antenna allows it to hone in on one direction more efficiently.

Types of Highly Directional Antennas

Grid antennas are flat with slightly curved edges. They look rather like barbecue grills with a device in a central point to collect the signals as they are relayed off a mesh of bars which make up the grid. Dish antennas are more properly known as parabolic antennas, from their inverted-bowl shape. They work by concentrating all the signals into a center point, usually a raised receiver or transmitter located a distance above the surface of the bowl, which is the point where the signal is at its strongest.

How Highly Directional Antennas Work

To explain how this kind of antenna offers better reception or transmission, imagine one hand cupped around one of your ears and pointed in one direction, while the other ear is covered by the other hand. You have in effect transformed yourself into something similar to a highly directional antenna. Sounds from the direction at the open end of the cupped hand reach the ear more loudly and more clearly, while noises from the direction of the blocked ear are muted or inaudible. This is how a highly directional antenna works.

Advantages of Highly Directional Antennas 

v A highly directional antenna improves the signal reception because it is pointed at the origin of the signal. Multi- and omni-directional antennas pick up all signals from all directions, resulting in too many incoming signals and a weaker signal from the direction of choice. Highly directional antennas also "ignore" signals coming from places other than the one they are directed to, which cuts down the interference with a chosen signal. Another advantage these antennas offer is the ability to change the focus of the receiver to another direction. For example, when switching channels on a TV set, the grid aerial, or antenna, is moved around to get the best reception or signal. To get the best signal from any highly directional antenna, it is recommended to mount it as high as possible on a given building. This cuts down the chances of signals being blocked by tall buildings and landscape features such as trees, hills and mountains 

v Sectorized and Phase-Array Antennas

v Phase-Array Antennas

A phased array antenna is actually an antenna system and is made up of multiple antennas that are connected to a signal processor. The processor feeds the individual antennas with signals of different relative phases, creating a directed beam of RF signal aimed at the client device.

Because it is capable of creating narrow beams, it is also able to transmit multiple beams to multiple users simultaneously. Phased array antennas do not behave like other antennas since they can transmit multiple signals at the same time.

Because of this unique capability, they are often regulated differently by the local RF regulatory agency. Phased array antennas are extremely specialized, expensive, and not commonly used in the 802.11 market.

In fact, the leading manufacturer of 802.11 phase array antenna systems recently went out of business. It is an interesting and very capable technology; however, time will tell whether it has a future in the 802.11 market. 

v Sectorized antenna

Sector antennas are a special type of high-gain, semi-directional antennas that provide a pie shaped coverage pattern. These antennas are typically installed in the middle of the area where RF coverage is desired and placed back to back with other sector antennas.

Individually, each antenna services its own piece of the pie, but as a group, all of the pie pieces fit together and provide omni-directional coverage for the entire area.

Unlike other semi-directional antennas, a sector antenna generates very little RF signal behind the antenna (back lobe) and therefore does not interfere with the other sector antennas that it is working with.

The horizontal beamwidth of a sector antenna is from 60 to 180 degrees, with a narrow vertical beamwidth of from 7 to 17 degrees. Sector antennas typically have a gain of at least 10 dBi.

Installing a group of sector antennas to provide omni-directional coverage for an area provides many benefits over installing a single omni-directional antenna.

To begin with, sector antennas can be mounted high over the terrain and tilted slightly downward, with the tilt of each antenna at an angle appropriate for the terrain it is covering.

Omni-directional antennas can also be mounted high over the terrain; however, if an omni-directional antenna is tilted downward on one side, the other side will be tilted upward.

Since each antenna covers a separate area, each antenna can be connected to a separate transceiver and can transmit and receive independently of the other antennas.

This would provide the capability for all of the antennas to be transmitting at the same time, providing much greater throughput. A single omni-directional antenna would be capable of transmitting to only one device at a time.

The last benefit of the sector antennas over a single omni-directional antenna is that the gain of the sector antennas is much greater than the gain of the omni-directional antenna, providing a much larger coverage area. Sector antennas are used extensively for cellular telephone communications and are starting to be used for 802.11 networking.

v MIMO (Multiple-Input, Multiple-Output) Antenna systems

Multiple input multiple outputs (MIMO, pronounced “MY-moh”) are another, more sophisticated form of antenna diversity. Unlike conventional antenna systems, where multipath propagation is impairment, MIMO systems take advantage of multipath.

There is much research and development currently happening with this technology and thus much disagreement about MIMO. There currently are no official or de facto standards for the technology.

MIMO can safely be described as any RF communications system that has multiple antennas at both ends of the communications link being used concurrently. How the antennas are to be used has not yet been standardized.

There are multiple vendors providing different current and proposed solutions. Complex signal processing techniques known as Space Time Coding (STC) are often associated with MIMO.

These techniques send data using multiple simultaneous RF signals and the receiver then reconstructs the data from those signals. The proposed 802.11n standard will include MIMO technology.

Author Profile: Jaber Swati

This author has published 24 articles so far. More info about the author is coming soon.