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.

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?

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Antenna Pattern Polar Coordinate Graphs