Understanding 802.11 Antennas

An 802.11 antenna tutorial discussing antenna gain and providing and explanation of antenna propagation graphs. Buy high-gain 802.11 antennas from Connect802 when you need the right antenna system design.

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The antenna transmits and receives electromagnetic signals. The size and shape of the antenna and the way it's constructed determine the gain and directivity of the antenna. When gain increases the amount of desired signal energy that can be captured increases but the amount of environmental noise and interference that's captured increases by the same amount.

How Is an Antenna Manufactured to Give It "Gain"?

Most Wi-Fi wireless LAN access points are shipped from the factory with the common 5-inch long straight black antenna as standard equipment. This type of antenna is technically called a "1/2-wave dipole" and many 802.11b access points ship with two antennas (called "diversity antennas"). The standard antenna radiates RF energy around it as if a doughnut had been placed down over the stick of the antenna. This type of antenna is referred to as "omni-directional" because it radiates in all directions around the plane perpendicular to the antenna shaft. A high-gain or directional antenna is constructed in such a way that the RF energy is radiated in a differently shaped 3-dimensional volume. For example, "flattening" the doughnut would cause more energy to be directed to the sides and less towards the top and bottom - that's a "high-gain antenna". Adding elements of the antenna construction that reflect or direct the outwardly propagating RF energy predominantly in one direction creates a "directional antenna". For example, if all of the signal going out to the left of an antenna were reflected to the right then the effective net result would be that twice the RF energy would be sent out towards the right. Different types of antennas offer different ways of shaping the RF signal energy to focus it in a desired manner. The signal characteristics of an antenna are represented with a special graph called an Antenna Pattern Polar Coordinate Graph or, simply, an Antenna Pattern Graph.

RF energy radiates outward from a dipole antenna forming a 3-dimensional volume that resembles a doughnut

How is Signal Strength Increased When an Antenna Has "Gain"?

When an antenna has 'gain' or is 'directional' it doesn't make the transmitted power any greater. The way the metal elements of an antenna are constructed (their size, shape, orientation, and relationship to each other) changes the way the electromagnetic signal radiates away from the antenna. The change in the 3-dimensional shape of the propagating wave (and, consequently, the spatial volume of the resulting 3-dimensional area) changes the density of the signal. It's increased energy density in some particular direction that results in increased signal strength for an intended receiver and is the quality of an antenna called 'gain'.

We begin thinking about gain by considering a theoretical radiating point source called an 'isotropic radiator'. An isotropic antenna would radiate signal outwards equally in all directions, creating a spherical transmission volume. There is no perfect isotropic antenna. A dipole antenna (a straight 'stick') radiates RF in a manner that can be visualized by thinking about a fluorescent light bulb. Signal radiates outwards from the sides, but not from the top or bottom. This produces a torroidal (doughnut) shaped signal volume around a dipole antenna. Since the signal is 'squeezed' into a transmission volume shape that is 'flatter' than the theoretical isotropic pattern the RF energy is compressed into a smaller volume. This results in the electromagnetic energy being more 'dense' in any given area inside the transmission volume than it would have been in the spherical volume of the isotropic radiator. The increased 'density' of signal is referred to as antenna gain and it's measured in decibels relative to the isotropic case (dBi, which is often simply written as 'dB' on an antenna spec sheet).

Where Can I Find Detailed Technical Information About RF Signal Propagation?

If you want the most concise, in-depth technical document describing, in tremendous detail, the way electromagnetic energy propagates, We've Got You Covered. Our technical white paper, "I'm Going To Let My Chauffeur Answer That" (written by Connect802's Chief Scientist and President, Joseph Bardwell) delves into the math and physics that explain Maxwell's wave equations and RF signal propagation. The math is well explained and there's plenty of concept information. You can download this free engineering white paper from the Technical Documents page of the Literature section.

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Antenna Pattern Polar Coordinate Graphs

Understanding the Antenna Pattern Graphs
The graphs show signal field strength from a "side view" (called the Elevation view, or "E-Graph"). You're standing in a big room looking across the room at the antenna. This is as opposed to the "top view" (called an Azimuth view or Horizontal view or H-Graph). All the graphs are E-Graphs, no H-Graphs are shown. If you're an electronics engineer then you'll also appreciate that "E" and "H" have significance beyond the memory aid of Elevation and Horizontal.

The Omni-Directional Antenna
The RF energy spreads out equally in all directions, forming a doughnut shape around the antenna. This is generally consistent with what a typical 1/2-wave dipole on an 802.11b or 802.11g WLAN access point might produce.

A High-Gain Omni-Directional Antenna
The antenna has been constructed so as to radiate power in a "flatter" doughnut shape. Since the same input power is now being radiated in a "flatter" volume, the power per unit area at any given point on the plane perpendicular to the antenna is increased. This graph shows a 2dB gain relative to the omni-directional design above.

A Directional Antenna
A reflecting element has been placed to the left of the antenna's active radiating element. Signal energy that used to go out to the left now goes out to the right. The signal power per unit area on the plane perpendicular to the antenna has now been doubled to the right. The antenna is directional. It is, of course, "high gain" relative to an omni-directional antenna. The graph shows a 3dB gain relative to the original omni-directional design.

A High-Gain Directional Antenna
Now there is not only a reflecting element (as with the directional antenna) but the construction of the radiating element is such that the doughnut is "flattened". Both characteristics combine to make very high gain, directional antennas. The graph shows a 20dB gain relative to the original omni-directional design.

Definitions and descriptions of Antennas that you might find on the Web:

A conductive structure specifically designed to couple or radiate electromagnetic energy. In RFID systems, the antenna may be used to both transmit and receive electromagnetic energy.
A device for transmitting and/or receiving signals. The size and shape of antennas are determined, in large part, by the frequency of the signal they are receiving.
An aerial for receiving or transmitting radio signals. A high gain antenna is highly focused, whereas a low gain antenna receives or transmits over a wide angle
A conductive physical device designed to radiate RF energy from a transmitter, or to capture RF energy for application to a receiver.
A device for transmitting or receiving radio waves. Also known as aerial. In satellite communication systems the antenna usually consists of a parabolic reflector and a feed horn. In a receiving system the reflector focuses radio waves onto the feedhorn for detection and conversion into electrical signals. In a transmitting systems the reflector concentrates the radio waves emitted by the feedhorn into a narrow beam aimed towards the satellite.
The device that transmits and/or receives microwave and radio energy
The part of a radio system that is designed to radiate electromagnetic waves into free space (or to receive them). This does not include the transmission lines or waveguide to the radiator.
A conductive structure specifically designed to couple or radiate electromagnetic energy. In a driven mode the structure is a transmitter antenna. In receiver mode the structure is a receiver antenna. Antenna structures, often encountered in radio frequency identification systems, may be used to both transmit and receive electromagnetic energy, particularly data modulated electromagnetic energy.
Any system of wires, poles, rods, reflecting discs, or similar devices used for the transmission or reception of electromagnetic waves.
An Antenna is any device used for transmitting signals. Antennas come in all shapes and sizes, their size and shape depending on the frequency and use of the signal transmitted. Some antennas can broadcast signals in all directions, they are called omnidirectional antennas. Other antennas can also broadcast signals in a fine straight line - like a flashlight, they are called directional antennas. Electrical signals with frequencies higher on the spectrum, for example, are shorter and more directional. As they get higher on the spectrum, they behave more like light. These must be focused and thus, require antennas which are shaped like the mirror reflector of a focusing flashlight. This parabolic shape focuses the broad beam (of the bulb or the electrical signal), into a narrow, focused beam. The weaker the received signal the bigger the antenna must be.
An electrical device that sends or receives radio or television signals

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