116 lines
4.7 KiB
Plaintext
116 lines
4.7 KiB
Plaintext
= 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.
|