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= Antenna =
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An antenna is some type of conductor, used to propagate radio signals. This is
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done because an electric current in metal induces an electromagnetic field.
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This can be transmitted and received by an antenna. When the antenna receives
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the signal, an electric current is induced in the antenna due to the
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intersection of the electromagnetic radiation in a conductor.
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*Polarization* is what orientation the electric field is from an antenna. This
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is the same orientation as the conductor of the antenna. To get the best
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signal, the sending and receiving signals must be of the same polarization.
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Due to how signals propagate through the atmosphere, their polarization
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changes dramatically, and therefore it often doesn't matter what orientation an
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antenna has when working with signals that propagate often (such as HF, see
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[[frequency_range]]). This phenomenon is called elliptical polarization.
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== Feed line ==
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A feed line delivers a signal to the antenna. The ratio of voltage to current
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is [[Impedance]]. This is due to the ratio of the physical dimensions of the
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feed point, and the frequency of the signal. They are said to be in _resonance_
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when the feed point impedance is all resistance with no reactance. All feed
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lines have a characteristic impedance. This is different then the resistance of
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the feed line. Things like length, conductors, spacing, etc determine the
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impedance.
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The feed points impedance is heavily affected by nearby conductors and its
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height above ground.
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A common feed line in Coax, due to its ease of use. Most for radio
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coax has an characteristic impedance of *50 ohms*. TV coax has an impedance of
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75 ohms.
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Another common feed line is made of two leads, seperated by plastic. It is
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called open wire, ladder line, window line, or twin lead. Becuase there is
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little to no shielding, there is less loss than coax. However, it is very
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sensitive to other nearby conductors.
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== Gain ==
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Gain is concentrating an antenna's radiated signals in a specific direction.
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Gain aids in concentrating the antenna in an intended direction. This is done
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by creating waves that constructively add in certain directions, and destructively
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subtract in others.
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*Omnidirectional antenna* radiate in all directions, and therefore have an
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equal gain in all directions.
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*Beam* or *directed antenna* have gain in a single direction, and are great for
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communicating in a specific direction.
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An *isotropic antenna* is a theoretical antenna that has no gain, because it
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radiates equally in all directions. No such antenna exists, and is used for
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reference.
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Gain is measured in decibels (dB) and is in reference to something, most often
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an isotropic antenna (abbr. dBi) or with respect to a dipoles peak gain (abbr.
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dBd). The gain is often shown using an azimuthal and elevation diagram,
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showing views from the top and side respectively.
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Decibels are logarithmic in nature and therefore are calculated as shown
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dB = 10 log (power ratio)
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db = 20 log (voltage ratio)
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Example: what is the power change from 5W to 10W, in dB?
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10 log (10 / 5) = 10 (0.3010) = 3dB
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