vimwiki/tech/Antenna.wiki

= Antenna =

See [[#Antenna Types]]

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.
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
intersection of the electromagnetic radiation in a conductor.

*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
signal, the sending and receiving signals must be of the same polarization.

Due to how signals propagate through the atmosphere, their polarization
changes dramatically, and therefore it often doesn't matter what orientation an
antenna has when working with signals that propagate often (such as HF, see
[[frequency_range]]). This phenomenon is called elliptical polarization.

== Feed line ==

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 dimensions of the
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. 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 impedance is heavily affected by nearby conductors and its
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. In general, larger diamater coax has less loss compared to thinner
cable. Performance of Coax is dependent on the integrity of the outer jacket.

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 is concentrating an antenna's radiated signals in a specific direction.
Gain aids in concentrating the antenna in an intended direction. This is done
by creating waves that constructively add in certain directions, and destructively
subtract in others.

*Omnidirectional antenna* radiate in all directions, and therefore have an
equal gain in all directions.

*Beam* or *directed antenna* have gain in a single direction, and are great for
communicating in a specific direction.

An *isotropic antenna* is a theoretical antenna that has no gain, because it
radiates equally in all directions. No such antenna exists, and is used for
reference.

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.
dBd). The gain is often shown using an azimuthal and elevation diagram,
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

*dBi* means gain with respect to an isotropic antenna.

*dBd* means gain with respect to a dipole antenna

== Antenna Types ==

=== Dipole ===

A straight conductor that is half of the wavelength. Transmits in the same
orientation as the dipole (usually horizontally). The feed line is usually
along the antenna, usually the center. The radiation pattern looks like a
circular donut.

=== Ground Planes ===

A ground plane is a single conductor with the feed point at the bottom of the
antenna. The antenna is usually a fourth of the wavelength. A metal base below
the antenna provides an electrical mirror (exactly like how mirrors work for
light). This creates the other half of the signal, allowing for the full
wavelength to be created.

Another common length is 5/8 a wavelength, which focuses more to the horizion.

When operating at HF bands, especially 24Mhz (12.5m), an inductor is usually
introduced into the antenna, making the antenna longer electically then it is
physically.

*Ground plane antennas are omnidirectional when mounted parallel to the ground*

=== Yagis ===

Yagi antennas are the common house mounted antenna you see for TV. They are
very directional, focusing half of their power 60 degrees, along the axis of
the Yagi. They also can ignore the interference caused by other signals coming
from the sides of the antenna.

=== Dish ===

A Dish in a very focused type of antenna, often used for the 1Ghz+ range of
frequencies. They are directional in the dirction of the dish, and can send and
recieve very focused signals.