In electronics, an antenna is a device for receiving or
transmitting of electromagnetic radiation. The simplest form of antenna
would be a rod of a electrically leading material. The basic physical
principles of antennas are
- Transmitting antenna: An alternating current in the antenna couples
with an external electromagnetic field.
Receiving antenna: An external electromagnetic field induces
an alternating current in the antenna.
There is no difference in principle between a transmitting and a receiving
antenna. For instance, cellular phones use the same antenna for both. However,
there are great differences in what you want to optimize for either type, so if
you want high performance you would use separate antennas. For transmitting
antennas you want to maximize gain, and the antenna must be able to handle
high effects and large currents. On the receiving antenna, on the other
hand, it is paramount to maximize the signal to noise ratio. The reason
that we know how to construct antennas at all, is because we know about electromagnetism.
More specifically, we know about Maxwell's equations, which describe
time-variant electrical and magnetic fields, and how electricity, magnetism
and light interact. They give us the current distributions, and with these
we can calculate the radiating fields for any antenna, in theory.
Microwave antennas
When the feeding current of the antenna reaches microwave frequencies (> 1
GHz), several possibilities open. First, the antenna itself becomes several wavelengths
in size, which makes the analysis of it similar to optics, which in turn makes
things easier. Also, it is a lot easier because of less diffraction and
refraction, to make a highly directional beam. Examples of these are
satellite dishes where a 1 meter antenna is about 30-40 wavelengths in size,
giving excellent sensitivity in a very specific direction, preferably towards
the satellite. Another important feature of microwave antennas is that you can
build advanced array antennas, where each radiating element is fed at a
certain phase, which gives the possibility of directing the beam without
moving the antenna. Commonly used for radars.
Important concepts
Radiation direction:
There is no antenna that transmits equally in all directions. This is because
the electromagnetic field have perpendicular components of the electric field E
and the magnetic field H, and the emitted transverse electromagnetic
wave moves perpendicular to both of them. Trying to construct an antenna that transmits
uniformly in all directions, called an isotropic radiator, is sometimes
referred to as the hairy ball problem.
- Imagine a ball covered with hair. Try to comb the hair so that there is
no parting anywhere on the ball.
This is as you realize impossible, and the same applies to antennas. All
antennas will have discontinuity somewhere, because of how the E- and H-field
interact in creating the emitted wave. Therefore the emitted signal will differ
depending on direction, and radiation direction is always specified with the horizontal
angle φ (0 - 360 degrees) and the vertical angle θ
(-90 to +90 degrees).
Antenna diagram
The antenna diagram is a representation of the radiation pattern, due to the
above mentioned directivity of antennas. It shows in what direction there's most
and least power radiated. Ideally, this would be a 3-dimensional representation,
but generally these are drawn 2-dimensional. The further from the axis the
diagram goes, the more power in that direction. Examples:
. <--- main lobe
. .
. . . . . . . . <--- side lobe
. . | . . . . . . . .
. . | . . . . . . . . . .
. .|. . . .. .. .. .. .
----------------|--------------- -------------------------------
. .|. .
. . | . . HHHHHHHHHHHHHHHHHHHHHHHHHHH
. . | . . (Slotted waveguide array)
. . . .
Rod antenna Slot antenna
Typical for microwave antennas is that they're highly directional, as the
aperture antenna above. It usually has a main lobe, which points in the
desired direction. The side lobes however are desired to be as low as
possible, since this signal, received or transmitted, will interfere with the
main lobe signal.
Antenna gain, directivity:
The term gain of an antenna is a measurement of the power density in a certain
direction (φ,θ) at a certain distance (R),
compared to an ideal isotropic radiator. The term directivity is similar, as it
is the power density in a certain direction (φ,θ) compared to the
average power density of all directions for that antenna. The power density is
measured in watts per square meter, W/m2. The antenna gain
includes internal effects such as power loss in circuits as well as external
losses due to interfering objects in the near field. An example of the latter
would be a bird sitting on the antenna. The "near field" is usually
defined as R < 2 ยท d2 / λ , where d is the
maximum aperture, and λ is the antenna wavelength.
An ideal isotropic antenna has the antenna gain of 1. A simple half wave
dipole antenna, just a rod of certain length, has the antenna gain of 1.65,
which means that it has 1.65 times higher intensity in its most favorable
direction, at the same feeding effect. Usually, antenna gain is expressed in
dB, which gives 2.15 dB for the half wave dipole. Antennas for radio astronomy
can have a gain of 60-80 dB in certain directions, more than a million times the
gain of the dipole.
Types of antennas
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Ok, that's a brief introduction without digging too deep in the mathematics.
Reference: ne.se, Antenna Theory and Design (Stutzman, Thiele)