Here you can study about the basics of antenna
Antenna gain is essential for microwave communication - since it helps both transmitting and receiving, it is doubly valuable. Practical microwave antennas provide high gain within the range of amateur fabrication skills and budgets. Three types of microwave antennas meet these criteria: horns, lenses, and dishes. Horns are simple, foolproof, and easy to build; a 10 GHz horn with 17 dB of gain fits in the palm of a hand. Metal-plate lenses are easy to build, light in weight, and non-critical to adjust. Finally, dishes can provide extremely high gain; a 2-foot dish at 10 GHz has more than 30 dB of gain, and much larger dishes are available. These high gains are only achievable if the antennas are properly implemented before we talk about specific microwave antennas, there a few common terms that must be defined and explained as follows.
Aperture
Gain
Usually we are only
interested in the maximum gain the direction in which the antenna is radiating
most of the power. An antenna with a large aperture has more gain than a
smaller one; just as it captures more energy from a passing radio wave, it also
radiates more energy in that direction. Gain may be calculated as following
equation 1.1.
If we transmit alternately with a smaller and a larger dish, is there
any reason that the relative power received at a distant antenna would be any
different than the relative power received by the two dishes? No, but a mathematical proof is surprisingly difficult.
Transmitting and receiving gains and antenna patterns are identical. However, the relative noise received by different types of
antennas may differ, even with identical antenna gains. Thus, the received signal-to-noise ratio may
be better with one type of antenna compared to another.
Side lobes
ANTENNA BASICS
Antenna gain is essential for microwave communication - since it helps both transmitting and receiving, it is doubly valuable. Practical microwave antennas provide high gain within the range of amateur fabrication skills and budgets. Three types of microwave antennas meet these criteria: horns, lenses, and dishes. Horns are simple, foolproof, and easy to build; a 10 GHz horn with 17 dB of gain fits in the palm of a hand. Metal-plate lenses are easy to build, light in weight, and non-critical to adjust. Finally, dishes can provide extremely high gain; a 2-foot dish at 10 GHz has more than 30 dB of gain, and much larger dishes are available. These high gains are only achievable if the antennas are properly implemented before we talk about specific microwave antennas, there a few common terms that must be defined and explained as follows.
The aperture of an antenna is the area that
captures energy from a passing radio wave. For a dish antenna, it is not
surprising that the aperture is the size of the reflector, and for a horn, the
aperture is the area of the mouth of the horn.
Wire antennas are not so simple a thin dipole has almost no area, but
its aperture is roughly an ellipse with an area of about 0.13λ2 and Yagi-Uda antennas have even larger
apertures.
The hypothetical isotropic
antenna is a point source that radiates equally in all directions. Any real
antenna will radiate more energy in some directions than in others. Since it cannot create energy, the total
power radiated is the same as an isotropic antenna driven from the same
transmitter; in some directions it radiates more energy than an isotropic
antenna, so in others it must radiate less energy. The gain of an antenna in a
given direction is the amount of energy radiated in that direction compared to
the energy an isotropic antenna would radiate in the same direction when driven
with the same input power.
Where η is the efficiency of the antenna.
Efficiency
Consider a dish antenna
pointed at an isotropic antenna transmitting some distance away. We know that
the isotropic antenna radiates uniformly in all directions so it is a simple matter
of spherical geometry to calculate how much of that power should be arriving at
the dish over its whole aperture. Now we
measure how much power is being received from the dish (at the electrical
connection to the feed) never greater than is arriving at the aperture. The ratio of power received to power arriving
is the aperture efficiency.
How much efficiency should we expect? For dishes, all the books say that 55% is
reasonable, and
70 to 80% is possible with very good feeds. To achieve this efficiency we need small
modification takes place on there.
Reciprocity
Reciprocity
Reciprocity principle says “both
transmitting and receiving it follows same principles”.
If we transmit alternately
with a smaller and a larger dish, is there any reason that the relative power
received at a distant antenna would be any different than the relative power
received by the two dishes? No, but a
mathematical proof is difficult. Transmitting and receiving gains and antenna
patterns are identical. However, the relative noise
received by different types of antennas may differ, even with identical antenna
gains. Thus, the received
signal-to-noise ratio may be better with one type of antenna compared to
another.
Directivity and Beam width
Suppose an antenna has 20 dB of gain in some
direction. That means it is radiating
100 times as much power in that direction compared to radiation from an
isotropic source, which is uniformly distributed over the surface of an
arbitrarily large sphere which encloses it.
If all the energy from the 20 dB gain antenna were beamed from the
center of that same sphere, then it would pass through an area 100 times
smaller than the total surface of the sphere.
Since
there are 41,253 solid degrees in a sphere, the radiation must be concentrated
in 1/100th of that, or roughly 20 degrees beam width. The larger the
gain, the smaller the beam width. The directivity of an antenna is the maximum
gain of the antenna compared with its gain averaged in all directions. It is calculated by calculating the gain,
using the previous formula, with 100% efficiency.
Side lobes
No antenna is able to
radiate all the energy in one preferred direction. Some is inevitably radiated in other
directions. Often there are small peaks
and valleys in the radiated energy as we look in different directions (Figure
1.1). The peaks are referred to as sidelobes, commonly specified in dB down from the main lobe, or
preferred direction. Are sidelobes important?
Let's suppose that we could make an antenna with a 1 degree beamwidth,
and in all other directions the average radiation was 40 dB down from the main
lobe.
Fig no1: Typical
antenna pattern with side lobes
E-plane and H-plane
An antenna is a transducer
which converts voltage and current on a transmission line into an
electromagnetic field in space, consisting of an electric field and a magnetic
field travelling at right angles to each other.
An ordinary dipole creates electric field, creating a pattern with
larger amplitude in planes which include the dipole. The electric field travels in the E-plane;
the H-plane, perpendicular to it, is the field in which the magnetic field travels. When we refer to polarization of an antenna,
we are referring to the E-plane. However, for three-dimensional antennas like
horns, dishes, and lenses, it is important to consider both the E-plane and the
H-plane, in order to fully utilize the antenna and achieve maximum gain.
Where the
path length d is the distance
between the two antennas.
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