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

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tech/Ham.wiki
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[[Propagation]] [[Propagation]]
[[Antenna]] [[Antenna]]
[[SWR]]

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tech/SWR.wiki Normal file
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= SWR =
Standing Wave Radio (SWR)