Update for 12-11-21 13:00
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= Antenna =
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See [[#Antenna Types]]
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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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@ -69,4 +71,31 @@ 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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== Antenna Types ==
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=== Dipole ===
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A straight conductor that is half of the wavelength. Transmits in the same
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orientation as the dipole (usually horizontally). The feed line is usually
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along the antenna, usually the center. The radiation pattern looks like a
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circular donut.
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=== Ground Planes ===
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A ground plane is a single conductor with the feed point at the bottom of the
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antenna. The antenna is usually a fourth of the wavelength. A metal base below
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the antenna provides an electrical mirror (exactly like how mirrors work for
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light). This creates the other half of the signal, allowing for the full
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wavelength to be created.
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Another common length is 5/8 a wavelength, which focuses more to the horizion.
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When operating at HF bands, especially 24Mhz (12.5m), an inductor is usually
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introduced into the antenna, making the antenna longer electically then it is
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physically.
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*Ground plane antennas are omnidirectional when mounted parallel to the ground*
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== Yagis ==
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Yagi antennas are the common house mounted antenna you see for TV.
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@ -11,4 +11,16 @@ signal.
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SWR is the same as the ratio of antenna-to-feed or feedline-to-antenna
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impedances, whichever is greater than 1. No reflected power is a ratio of 1,
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and means that there is no interference pattern. Because [[Impedance]] changes with
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frequency, the SWR also changes with frequency.
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frequency, the SWR also changes with frequency.
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Becuase of higher SWR means more reflected energy, a lower SWR means more of
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the transmitters power actually makes it out through the antenna as a radio
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wave. The power feeding back to the transmitter also creates interference with
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the signal itself, which can cause voltages higher than what the transmitter is
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designed for. Due to this *most equipment can only handle SWR of 2:1*.
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If you expereince SWR, check
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* Antenna length vs frequency
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* Faulty feed lines/feed line connectors
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* Erratic SWR means loose connections in feed line
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