2021-11-11 21:45:01 +00:00
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= Antenna =
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An antenna is some type of conductor, used to propogate 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 recieved by an antenna. When the antenna recieves
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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 recieving signals must be of the same polarization.
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Due to how signals propagate through the atmostphere, their polarization
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changes dramaticly, and therefore it often doesnt matter what oerientation an
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antenna has when working with signals that propogate often (such as HF, see
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[[frequency_range]]). This phenomenon is called eliptical polarization.
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2021-11-11 22:00:01 +00:00
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== Feed line ==
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2021-11-11 21:45:01 +00:00
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2021-11-11 22:00:01 +00:00
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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 dimesnions 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.
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The feed points impredance is heavily affected by nearby conductors and its
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height above ground.
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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 constructivly add in certain directions, and destruvily
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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 showns using an azimuthal and elevation diagram,
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showing views from the top and side respecivly.
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