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By Marlon Schafer
[Author's Note: Many thanks to Bill Drach, Radio Frequency Systems Antenna Product Manager, Seth Hanson and Bud Bayer at Gabriel Electronics Inc., Linda D'Evelyn Applications Engineering Manager for Ball Commercial Products & Technologies, and Daniel Warren. Without these folks I could never have gotten so much highly technical information translated into layman's terms.] So, what's an antenna?
Now, the Federal Communications Commission often calls antennas, radiators—go figure? For the purpose of my discussion, we'll think of antennas as lenses and reflectors, even though I know that sounds strange, please bear with me. But you can always review a more accurate list of fixed wireless definitions, then you'll find from in Webster's Dictionary or from the FCC. All antennas are shown as dBi—with the dB designation an abbreviation for decibels and the lowercase-i short for interference. It is a mathematical expression showing the relationship between two values. All radio frequencies (RF), in relation to antenna gain, start out as the power that would be put out by a perfect point in space that would radiate the signal in all directions and create a perfect sphere of energy—this is called an isotropic radiator. This theoretical device—you can't really build one because the energy has to get into it some how so you loose the perfect shape of your isotropic antenna, has a gain of 0 dB. An antenna has gain when you start to focus its energy toward the direction of your choice. Don't forget that antennas have gain in two directions—both sending and receiving. Antennas work like a lens for outgoing energy and a magnifying glass for incoming energy, which is why I like to think of antennas as lenses and reflectors When choosing an antenna, it's important to first determine the area of coverage needed and the distance of the link. You'll have to use an antenna with enough gain to establish a stable link—but not so much that you'll overpower the other end or cause excessive reflections. This phenomenon, in turn could make the RF signal fade. Known as multipathing the transmitted signal arrives at the receiver from different directions, with different path lengths, attenuation and delays. The summed signal at the receiver may result in an attenuated signal—thin and weak. Antennas are also used in different polarizations—whereby the action or process of affecting radiation and especially light so that the vibrations of the wave assume a definite form. Antenna polarizations could be:
It's easy if you think about RF energy as a two dimensional image and picture it that way in your mind's eye. Think of RF as Long, as it travels to it's destination and Tall, like wave height. When you orient the wave vertically, the antenna is vertically polarized—horizontal would be when you rotate the antenna sideways. Think of it like a ruler—antennas don't care about 180 degree changes.
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