EM waves travel in straight lines, unless acted upon by
some outside force. They travel faster through a vacuum
than through any other medium.
As EM waves spread out from the point of origin, they
decrease in strength in what is described as an inverse
For example: a signal 2 km from its starting point will be
only 1/4 as strong as that 1 km from the source. A signal 3
km from the source will be only 1/9 that at the 1 km point.
Electromagnetic radiation comprises both an Electric and a Magnetic Field.
The two fields are at right-angles to each other and the direction of propagation is at right-angles to both
The Plane of the Electric Field defines the Polarisation of the wave.
The polarization of an antenna is the orientation of
the electric field with respect to the Earth's surface
and is determined by the physical structure of the
antenna and by its orientation.
Radio waves from a vertical antenna will usually
be vertically polarized.
Radio waves from a horizontal antenna are
usually horizontally polarized.
Direction of Propagation
ECM 717 - BY DR MOHD
Direction of Propagation
Horizontally polarized directional
yagi antennaDR MOHD
ECM 717 - BY
hat is an ‘antenna’?
An antenna is a device that is made to efficiently
radiate and receive radiated electromagnetic waves.
An antenna is an electrical conductor or system of
Transmission - radiates electromagnetic
energy into space
Reception - collects electromagnetic energy
In two-way communication, the same antenna can be
used for transmission and reception.
Horn antennas are very popular at UHF (300
MHz-3 GHz) and higher frequencies.
Horn antennas often have a directional radiation
pattern with a high antenna gain, which can range
up to 25 dB in some cases, with 10-20 dB being
Horn antennas have a wide impedance
bandwidth, implying that the input impedance is
slowly varying over a wide frequency range.
The gain of horn antennas often increases as the
frequency of operation is increased.
This is because the size of the horn aperture is always
measured in wavelengths, a higher frequency has a
Since the horn antenna has a fixed physical size, the
aperture is more wavelengths across at higher
Horn antennas have very little loss, so the directivity of
a horn is roughly equal to its gain.
The most well-known reflector antenna is the parabolic
reflector antenna, commonly known as a satellite dish
Examples of this dish antenna are shown in the following
Figure 1. The "big dish" antenna of
Figure 2. A random direcTV dish
antenna on a roof.
Parabolic reflectors typically have a very high gain (30-40 dB
is common) and low cross polarization.
They also have a reasonable bandwidth, with the fractional
bandwidth being at least 5% on commercially available
models, and can be very wideband in the case of huge dishes.
The smaller dish antennas typically operate somewhere
between 2 and 28 GHz.
The large dishes can operate in the VHF region (30-300 MHz),
but typically need to be extremely large at this operating band.
A slot antenna consists of a metal surface, usually
a flat plate, with a hole or slot cut out.
When the plate is driven as an antenna by a
driving frequency, the slot
radiates electromagnetic waves in similar way to
a dipole antenna.
Often the radio waves are provided by
a waveguide, and the antenna consists of slots in
Slot antennas are used typically at frequencies
Slot antennas are often used
at UHF and microwave frequencies instead of line
antennas when greater control of the radiation
pattern is required.
Widely used in radar antennas, for the sector
used for cell phone base stations.
Often found in standard
desktop microwave sources used for research
Dipole antenna with a very thin radius is considered.
For very small dipole antennas, the input impedance is
capacitive, which means the impedance is dominated by a
negative reactance value.
As the dipole gets larger, the input resistance increases, along
with the reactance.
At slightly less than 0.5 the antenna has zero imaginary
component to the impedance, and the antenna is said to be
If the dipole antenna's length becomes close to one
wavelength, the input impedance becomes infinite.
Dipole antenna basics
Dipole antenna consists of two terminals or "poles" into which
radio frequency current flows.
This current and the associated voltage causes and
electromagnetic or radio signal to be radiated.
Dipole is generally taken to be an antenna that consists of a
resonant length of conductor cut to enable it to be connected
to the feeder.
The basic half wave dipole antenna
The current distribution along a dipole is roughly
falls to zero at the end and is at a maximum in the
voltage is low at the middle and rises to a maximum at
It is generally fed at the centre, at the point where the
current is at a maximum and the voltage a minimum.
This provides a low impedance feed point which is
convenient to handle.
High voltage feed points are far less convenient and
more difficult to use.
When multiple half wavelength dipoles are used,
they are similarly normally fed in the centre.
Here again the voltage is at a minimum and the
current at a maximum.
Theoretically any of the current maximum nodes
could be used.
Dipole polar diagram
The polar diagram of a half wave dipole antenna that the
direction of maximum sensitivity or radiation is at right
angles to the axis of the RF antenna.
The radiation falls to zero along the axis of the RF
antenna as might be expected.
Polar diagram of a half wave dipole in free space
If the length of the dipole antenna is changed
then the radiation pattern is altered.
As the length of the antenna is extended it can
be seen that the familiar figure of eight pattern
changes to give main lobes and a few side
The main lobes move progressively towards
the axis of the antenna as the length
The dipole antenna is a particularly important
form of RF antenna which is very widely used
for radio transmitting and receiving
The dipole is often used on its own as an RF
antenna, but it also forms the essential
element in many other types of RF antenna.
As such it is the possibly the most important
form of RF antenna.
Radiation pattern and gain
Dipoles have a radiation pattern, shaped like a toroid (doughnut)
symmetrical about the axis of the dipole.
The radiation is maximum at right angles to the dipole, dropping off
to zero on the antenna's axis.
The theoretical maximum gain of a Hertzian dipole is 10 log 1.5 or
The maximum theoretical gain of a λ/2-dipole is 10 log 1.64 or 2.15
A short dipole is a physically
feasible dipole formed by two
conductors with a total length L
very small compared with the
The two conducting wires are fed
at the center of the dipole.
The current is assumed to be
maximal at the center (where the
dipole is fed) and to decrease
linearly to zero at the ends of the
Typically a dipole antenna is
formed by two quarterwavelength conductors or
elements placed back to back
for a total length of L = λ/2.
The larger the differential
voltage, the greater the current
between the elements.
The quarter-wave monopole
antenna is a single-element
antenna fed at one end, that
behaves as a dipole antenna.
The quarter-wave conductor
and its image together form a
half-wave dipole that radiates
only in the upper half of space.
A folded dipole is a half-wave
dipole with an additional wire
connecting its two ends.
the antenna are folded back until
they almost meet at the feed point,
such that the antenna comprises
one entire wavelength.
This arrangement has a greater
bandwidth than a standard halfwave dipole.
An antenna in the form of a section of dielectric rod
excited by a radio wave guide or the post of a coaxial
A surface wave is generated in the rod of the antenna
and propagates along the axis of the rod.
Dielectric antennas are essentially traveling-wave
antennas, consisting of elementary electric and magnetic
The radiation maximum coincides with the axis of the
rod, as does the maximum of any traveling-wave
The type of radiation of a dielectric antenna depends on the
phase velocity of propagation of the surface wave, which
decreases with an increase in the diameter of the dielectric
rod and in the dielectric constant of its material.
The lower the phase velocity, the greater the length of the rod.
As the phase velocity decreases, or as it approaches the
speed of light in the surrounding medium (air), the dielectric
rod begins to lose its wave-guide properties.
This leads to an abrupt decrease in the field intensity near the
end of the rod, an increase of radiation into the medium
surrounding the antenna (directly from the open end of the
wave guide), and a decrease in the antenna’s efficiency.
The rods of dielectric antennas are made from
dielectric materials with low attenuation of
Dielectric antennas are used mainly in aircraft
radio equipment, which operates on centimeter
or decimeter wavelengths.
low cost alternative to free space high gain
antenna designs such as Yagi-Uda and horn
antennas, which are often more difficult to
manufacture at these frequencies
Printed antenna technology used for wireless
Printed antenna application are:
- Arrays for low or medium directivity
- Efficient radiators
- Planar antenna
Originated from the use of planar microwave technologies.
The begin antenna printed in the mid 1970.
The layered structure with 2 parallel conductors separated
by a thin dielectric substrate and the lower conductor acting
as a ground plane.
Printed belongs to the class or resonant antennas. Printed
antennas have found use in most classical microwave
Operates typically from 1- 100 GHz.
Phase Array Antenna
A phased array is an array of antennas in which the
relative phases of the respective signals feeding the
antennas are varied in such a way that the effective
radiation pattern of the array is reinforced in a desired
direction and suppressed in undesired directions.
An antenna array is a group of multiple active
antennas coupled to a common source or load to
produce a directive radiation pattern.
Usually, the spatial relationship of the individual
antennas also contributes to the directivity of the
Phase Array Antenna
Use of the term "active antennas" is intended
to describe elements whose energy output is
modified due to the presence of a source of
energy in the element or an element in which
the energy output from a source of energy is
controlled by the signal input.
One common application of this is with a
standard multiband television antenna, which
has multiple elements coupled together.
Antenna feed refers to the components of an antenna which
feed the radio waves to the rest of the antenna structure, or in
receiving antennas collect the incoming radio waves, convert
them to electric currents and transmit them to the receiver.
Antennas typically consist of a feed and additional reflecting or
directive structures whose function is to form the radio waves
from the feed into a beam or other desired radiation pattern.
Feed consists of a dipole driven element, which converts the
radio waves to an electric current, and a coaxial cable or twin
lead transmission line which conducts the received signal from
the antenna into the house to the television receiver.
It is a type of parabolic antenna used at earth
Its design is similar to Cassegrain antenna
(secondary reflector is convex), only
Gregorian antenna’s secondary reflector is
In another word, Gregorian antenna is a
double reflector antenna with the second
reflector located at a distance greater than the
In modern world, Gregorian is less compact
than Cassegrain in term of design but with a
smaller second reflector thus the advantages
of beam blocking result in more efficient
of s is
an sseg mp
ten ra le
Cassegrain antennas are a subcategory of reflector antennas. Reflector
antennas have been used from discovery of electromagnetic wave propagation
Cassegrain antenna consists of two reflectors (primary and secondary) and a
feeder. The main characteristics of Cassegrain antennas is their high
directivity.The bigger diameter of antenna reflector is used, the better
gain is achieved.
Application Cassegrain Antenna.
Operation cassegrain antenna
In telecommunications and radar, a Cassegrain antenna is a parabolic antenna in
which the feed radiator is mounted at or behind the surface of the concave main
parabolic reflector dish and is aimed at a smaller convex secondary reflector
suspended in front of the primary reflector. The beam of radio waves from the
feed illuminates the secondary reflector, which reflects it back to the main
reflector dish, which reflects it forward again to form the desired beam.
Closeup of the convex secondary
reflector in a large satellite
communications antenna in
communication antenna in
Sweden. The convex secondary
reflector can be seen suspended
above the dish, and the feed horn
is visible projecting from the
center of the dish.
Another important aspect of the antenna is the front-to-back ratio. It
measures the directivity of the antenna. It is a ratio of energy which
antenna is directing in a particular direction, which depends on its
radiation pattern to the energy which is left behind the antenna or
wasted. The higher the gain of the antenna, the higher the front-toback ratio is. A good antenna front-to-back ratio is normally 20 dB.
Advantage of cassegrain antenna
The feed antennas and associated waveguides and "front end" electronics can be
located on or behind the dish, rather than suspended in front where they block
part of the outgoing beam.
the feed antenna is directed forward, rather than backward toward the dish as in a
front-fed antenna, the spillover side lobes caused by portions of the beam that
miss the secondary reflector are directed upwards toward the sky rather than
downwards towards the warm earth.
Dual reflector shaping: The presence of a second reflector in the signal path allows
additional opportunities for tailoring the radiation pattern for maximum
increase the focal length of the antenna, to improve the field of view Parabolic
reflectors used in dish antennas have a large curvature and short focal length, to
locate the focal point near the mouth of the dish, to reduce the length of the
supports required to hold the feed structure or secondary reflector.
Disadvantage of cassegrain
A disadvantage of the Cassegrain is that the feed horn(s) must have a
narrower beamwidth (higher gain) to focus its radiation on the smaller
secondary reflector, instead of the wider primary reflector as in front-fed
dishes. The angular width the secondary reflector subtends at the feed
horn is typically 10° - 15°, as opposed to 120° - 180° the main reflector
subtends in a front-fed dish. Therefore the feed horn must be longer for a
Also known as microwave horn, it is an
antenna which is the waveguide is hornshaped
This is to direct radio waves into a beam
Usually used for UHF and microwave
frequency, which is above 300MHz
They used as a feeder (called feed horn) for
larger structure such as parabolic antenna
It is a small horn antenna used to convey radio
waves between transmitter and receiver, and
also the reflector
Generally used in parabolic antenna
Usually used in SHF
In transmitting antenna, it convert the radio
frequency alternating current from transmitter
to radio waves, which is then feed the radio
waves to the antenna to be focused into beam
In receiving antenna, the received radio waves
is gathered and focused by the antenna’s
reflector to the feed horn, which is then
converted into radio frequency amplified by
om is e
an irec ple
nn nal f
An omnidirectional antenna is an antenna that has a non-directional pattern (circular pattern) in a given
plane with a directional pattern in any orthogonal plane. Examples of omnidirectional antennas are dipoles
and collinear antennas. In radio communication, an omnidirectional antenna is an antenna which radiates
radio wave power uniformly in all directions in one plane, with the radiated power decreasing with elevation
angle above or below the plane, dropping to zero on the antenna's axis. This radiation pattern is often
described as "doughnut shaped".
Application of omnidirectional
Wireless computer network.
Operation of omnidirectional
The omnidirectional antenna radiates or receives equally well in all directions. It is
also called the "non-directional" antenna because it does
not favor any particular direction. The pattern for an omnidirectional antenna, with
the four cardinal signals. This type of pattern is commonly associated with verticals,
ground planes and other antenna types in which the radiator element is vertical with
to the Earth's surface.
Radiation pattern omnidirectional antenna
Omnidirectional antennas have a similar radiation pattern. These antennas provide
a 360 degree horizontal radiation pattern. These are used when coverage is
required in all directions (horizontally) from the antenna with varying degrees of
vertical coverage. Polarization is the physical orientation of the element on the
antenna that actually emits the RF energy. An omnidirectional antenna, for
example, is usually a vertical polarized antenna.
Advantage of omnidirectional
An omni directional antenne receives or transmits signals from all directions as in
Signal strength is uniform with omnidirectional antennas.
If you use an omnidirectional antenna all signals are noise sources look into the
same antenna gain. There is neither increase of the desired signal, nor
suppression of undesired signals. On a crowded band, the omnidirectional
antenna confers no SNR
HORN & PARABOLIC
Antenna Radiation Patterns
An antenna radiation pattern or antenna pattern is defined
as a mathematical function or a graphical representation
of the radiation properties of the antenna as a function of
In most cases, the radiation pattern is determined in the
far field region and is represented as a function of the
Types of Radiation Patterns
Main beam / side lobe region
The peak of the main beam represents the
highest level of field strength and
approximately 70% of the radiated energy is
The side lobe region represents a potential
source of interference into the communication
link, and for this reason is generally required
to be of low level.
Antenna Side lobes
While most of the power radiated by an antenna
is contained in the ”main lobe", a certain amount
of power can be transmitted, (or received), in offaxis directions.
Side lobes are an intrinsic property of antenna
radiation and cannot be completely eliminated.
However, side lobes are also due to antenna
defects that can be minimized with proper
Horn Antenna Radiation Pattern
This antenna is
simulated using a
The radiation pattern at
2 GHz is shown in Figure
The gain of the horn is
18.1 dB in the +z-
Parabolic Antenna Radiation
This example will be for a parabolic dish reflector with
the diameter of the dish D equal to 11 wavelengths.
A circular horn antenna will be used as the feed.
The maximum gain from the physical aperture is the
actual gain is 29.3 dB = 851, so we can conclude that
the overall efficiency is 77%.
Gain, directivity and efficiency
Gain and directivity are quantities which define
the ability to concentrate energy in a particular
direction and are directly related to the antenna
It includes all ohmic and dissipative losses arising
from conductivity of metal and dielectric loss.
Antenna efficiency is a coefficient that accounts
for all the different losses present in an antenna
• The fields across the aperture of the parabolic
reflector is responsible for this antenna's
• Directivity or gain is the ratio of the power
radiated by an antenna in its direction of
maximum radiation to the power radiated by a
reference antenna in the same direction.
• Is measured in dBi (dB referenced to an
isotropic antenna) or dBd (dB referenced to a
half wavelength dipole).
Parabolic Antenna Gain,
G = 6D2/λ2
where D = diameter
Horn Antenna Gain
G = 10A/λ2
A =flange area
Antenna efficiency is affected by:
1.The sub reflector and supporting structure
2.The main reflector rms surface deviation.
3.Illumination efficiency, which accounts for the non
uniformity of the illumination, phase distribution
across the antenna surface, and power radiated in
the side lobes.
4.The power that is radiated in the side lobes.
The radiation efficiency is the usual efficiency
that deals with ohmic losses.
Horn antennas are often used as feeds, and
these have very little loss.
Parabolic reflector is typically metallic with a
very high conductivity, this efficiency is
typically close to 1 and can be neglected.
The effective antenna aperture is the ratio of the
available power at the terminals of the antenna to
the power flux density of a plane wave incident upon
the antenna, which is polarization matched to the
If there is no specific direction chosen, the direction
of maximum radiation intensity is implied.