Antennas
Antennas
Antenna
It is a metallic conductor system capable of radiating and capturing
electromagnetic energy.
The antenna couples energy received from a transmission line to the
atmosphere an energy received from the atmosphere to the transmission line.
Antennas
Radio Waves and Wavefront
These are electrical energy traveling at approximately the speed of light that
has escaped into free space in the form of transverse electromagnetic wave.
A wavefront is the set of all locations in a
medium where the wave is at the same phase.
Antennas
Basic Antenna Operation
All electrical circuits that carry alternating current radiate a certain amount
of electromagnetic energy. However, the amount of energy is negligible unless the
physical dimensions of the circuit approach the dimensions of a wavelength of the
wave.
Antennas
Antenna Reciprocity
An antenna is a passive reciprocal device in that the transmit and receive
characteristics and performance are identical (gain, directivity, frequency of
operation, BW, etc.).
Transmit antennas must be capable of handling high powers, therefore,
must be constructed with materials that can withstand high voltages and currents.
Receive antennas, however produce very small voltages and currents and can be
constructed from a small-diameter wire.
In many radio communications systems, the same antenna is used for
transmitting and receiving . If this is the case, the antenna must be constructed
from heavy duty materials.
Antennas
Antenna Coordinate System and Radiation Pattern
Spherical Coordinate System
Antenna Radiation Pattern
using a polar diagram
Antennas
Near Field and Far Field
The term near field (induction field) refers to the field pattern that is close
to the antenna, and the term far (radiation field) feild refers to the field pattern that
is at great distance.
Antennas
Radiation Resistance and Antenna Efficiency
Radiation resistance is an ac antenna resistance that if it replaced the
antenna, it would dissipate exactly the same amount of power that the antenna
radiates. Mathematically, it is
Antenna efficiency is the ratio of the radiated power of the antenna to the
input power accepted by the antenna.
Antennas
Directive Gain
Is the ratio of the power density radiated in a particular direction to the
power density radiated to the same point by a reference antenna.
Power density at some point 𝑅:
where:
• 𝒫 = power density at some point with a given antenna (W/m )
• 𝒫
= power density at the same point with a reference antenna (W/m )
Maximum directive gain of an antenna is called directivity.
Antennas
Antenna Gain
It is the same as the directive gain except that the total power fed to the
antenna is used. It is assumed that the given antenna and the reference antenna
have the same input power and that the reference antenna is lossless.
Mathematically, it is
Antennas
Effective Isotropic Radiated Power (EIRP)
is
Is defined as an equivalent transmit power and is expressed mathematically
Where:
•
total radiated power (W)
•
transmit antenna directive gain (unitless)
•
total antenna input power
•
transmit antenna power gain
Antennas
Example: EIRP
For a transmit antenna with a
determine
and an input power
a)
in watts, dBm and dBW
b) Power density at a point 10 km from the transmit antenna
c) Power density had an isotropic antenna been used with the same input power and efficiency
Ans: a)
b)
c)
,
Antennas
Captured Power Density
Since antenna are reciprocal devices the captured power density can be
expressed as:
Where:
• C = captured power density
• P = transmit antenna input power
• A = transmit antenna power gain
• A = receive antenna power gain
• R = distance between transmit and receive antennas
Antennas
Captured Area and Captured Power
There is an obvious relationship between an antenna’s size and its ability to
capture electromagnetic energy. Mathematically, it is:
Because antennas are reciprocal devices, the power received or captured by
an antenna is the product of the power density and the antenna’s effective area.
Antennas
Example: Captured Power
For a receive power density of
and a receive antenna with a
capture area of
, determine the captured power in watts and in dBm.
Ans:
and
Antennas
Antenna Polarization
Polarization of an antenna refers simply to the orientation of the electric
field radiated from it.
Antennas
Antenna Beamwidth and Bandwidth
Is the angular span of the main lobe of the antenna radiation pattern, which
is the region of the pattern where most of the power is radiated.
The bandwidth of an antenna refers to the
range of frequencies over which the antenna
satisfies a particular parameter specification. It is
normally taken as the difference between the half
power frequencies.
Antennas
Antenna Input Impedance
Radiation from an antenna is a direct result of the flow of RF current and is
simply the ratio of the antenna’s input voltage to input current.
Where:
• Z
= antenna input impedance
• E = antenna input voltage
• I = antenna input current
Antennas
Types of Basic Antenna
1. Elementary Doublet
2. Half-wave Dipole
3. Monopole Antenna
4. Antenna Arrays
1. Broadside Array
2. End-Fire Array
3. Rhombic Antenna
5. Special Purpose Antenna
•
•
•
•
•
•
•
Folded Dipole
Yagi-Uda Antenna
Turnstile Antenna
Log-Periodic Antenna
Loop Antenna
Phased Array Antenna
Helical Antenna
6. UHF and Microwave Antenna
•
•
Parabolic Reflector Antenna
Conical Horn Antenna
Antennas
Elementary Doublet
Considered as the simplest type of antenna. It is often referred to as short
dipole, elementary dipole, or Hertzian dipole. Electrically short means short
compared with one-half wavelength but not necessarily one with a uniform current
(
).
Antennas
Half-wave Dipole
Is one of the most widely used antennas at frequencies above 2 MHz and is
generally referred to as a Hertz antenna.
It is a resonant antenna. That is, it is a multiple of quarter-wavelengths long
and open circuited at the far end.
Ground Effects on a half-wave dipole
Antennas
Monopole Antenna (Marconi Antenna)
It is an antenna one-quarter wavelength long, mounted vertically with the
lower end either connected directly to ground or grounded through the antenna
coupling network.
To improve the conductivity of the ground area, a
counterpoise may be implemented. A counterpoise is a form of
capacitive ground system formed between the counterpoise and
Earth’s surface
Antennas
Antenna Loading
It is possible to increase the electrical length of an antenna by a technique
called loading. When an antenna is loaded, its physical length remains unchanged,
although its effective electrical length is increased.
Effects:
1. Increased radiation resistance
2. Increased power loss
3. Possibility of corona
4. Reduced radiation efficiency
Loading Coil
Effects:
1. Increased radiation resistance
2. Increased radiation efficiency
3. Reduced voltage SWR
Top Loading
Antennas
Antenna Arrays
It is formed when two or more antenna elements are combined to form a
single antenna. The elements are physically placed in such a way that their
radiation fields interact with each other.
Parts:
1. Driven elements – elements that is directly connected to the TL and
receive power from or are driven by the source.
2. Parasitic elements – elements that are not connected to the TL.
They only receive energy through mutual induction with a driven
element or another parasitic element.
• Reflector – element that is longer than the driven element.
• Director – element that is shorter than the driven element
Antennas
Broadside Array
Is one of the simplest types of antenna arrays. It is made by simply placing
several resonant dipoles of equal size (both length and diameter) in parallel with
each other and in a straight line (collinear).
Radiation Pattern
Antennas
End-fire Array
Is essentially the same element configuration as the broadside array except
that the transmission line is not crisscrossed between elements.
Radiation Pattern
Antennas
Nonresonant Array: Rhombic Antenna
It is an antenna capable of operating satisfactorily over a relatively wide
bandwidth, making it ideally suited for HF transmission (3 MHz – 30 MHz).
It is made up of 4 non-resonant elements each several wavelengths long.
Radiation Pattern
Antennas
Special Purpose Antenna: Folded Dipole
A single antenna made up of two elements. One element is fed directly,
whereas the other is conductively coupled at the ends. Each element is one-half
wavelength long.
The input impedance of a folded dipole is equal to the half-wave impedance
times the number of folded wires squared.
The bandwidth can also be increased by making
the dipole elements larger in diameter (fat dipole).
Antennas
Special Purpose Antenna: Yagi-Uda Antenna
An antenna that uses a
folded dipole as the driven element. Also, it is a
linear array consisting of a dipole and two or more parasitic elements: one
reflector and one or more directors.
Its bandwidth can be increased by using
more than one folded dipole, each cut to a
slightly different length.
The Yagi antenna is commonly used for
VHF television reception (54 MHz to 216
MHz) because of its wide bandwidth.
Antennas
Special Purpose Antenna: Yagi-Uda Antenna
Antennas
Special Purpose Antenna: Turnstile Antenna
Is formed by placing two dipoles at right angles
to each other, 90 degrees out of phase. The turnstile
antenna produces a nearly omnidirectional radiation
pattern. The typical gain of the turnstile antenna is
around 10 or more dB.
Antennas
Special Purpose Antenna: Log-Periodic Antenna
Is a multi-element, directional antenna designed to
operate over a wide band of frequencies. Its primary advantage
is the independency of its radiation resistance and radiation
pattern to frequency.
Log-Periodic antennas also have bandwidth ratios of 10:1
or greater. The bandwidth ratio is the ratio of the highest to the
lowest frequency over which an antenna will satisfactorily
operate.
The physical structure of this antenna is repetitive, which
results in a repetitive behavior in its electrical characteristics.
Antennas
Special Purpose Antenna: Log-Periodic Antenna
Dipole lengths and spacing are related by the
formula:
1
R
L
=
=
τ R
L
Where:
• R = dipole spacing
• L = dipole length
• τ = design ratio (< 1)
Antennas
Special Purpose Antenna: Loop Antenna
A single-turn coil of wire that is significantly shorter than one wavelength
and carriers RF current. If the radius is small compared with a wavelength, current
is essentially in phase throughout the loop.
The radiation pattern for a loop antenna is essentially the same as that of a
short horizontal dipole.
Radiation resistance:
where
is the area of the loop
Antennas
Special Purpose Antenna: Phased Array Antenna
Is a group of antennas or a group of
antenna arrays that, when connected together,
function as a single antenna whose beamwidth
and direction can be changed electronically
without having to physically move any of the
individual antenna or antenna elements within
the array.
Its primary application is in radar when
radiation patterns must be capable of being
rapidly changed to follow a moving object.
Antennas
Special Purpose Antenna: Helical Antenna
Is a broadband VHF or UHF antenna that is ideally suited for applications
for which radiating circular rather than linear polarized electromagnetic waves are
required.
There are two modes of propagation for helical antennas: normal and axial.
In normal mode, the wave is in a direction at right angles to the axis of the helix.
In axial mode, the wave is in axial direction and produces a broadband, relatively
directional pattern.
Antennas
Special Purpose Antenna: Helical Antenna
The gain and beamwidth of the helical antenna is:
Where:
•
helix diameter
•
number of turns
•
pitch
•
wavelength
Antennas
UHF and Microwave Antennas: Parabolic Reflector Antenna
An antenna that provides extremely high gain and directivity and are very
popular for microwave radio and satellite communication links. It consists of two
main parts: a parabolic reflector and a feed mechanism.
Antennas
Special Purpose Antenna: Parabolic Reflector Antenna
Parabolic antenna beamwidth and power gain:
Where:
•
antenna mouth diameter
•
wavelength
•
antenna efficiency (typically 0.55)
•
beamwidth b/w nulls
Receiver Power Gain:
where
• k = aperture efficiency
• A = actual area
when η = 0.55
Antennas
Example: Parabolic Reflector Antenna
For a 2 m diameter parabolic reflector with 10 W of power radiated by the
feed mechanism operating at 6 GHz with a transmit antenna efficiency of 55% and
an aperture efficiency of 55%, determine:
a. Beamwidth
b. Transmit power gain
c. Receiver power gain
d. EIRP
Ans: a)
b)
c)
d)
Antennas
Special Purpose Antenna: Parabolic Reflector Antenna
Types of Feed Mechanisms
Antennas
Special Purpose Antenna: Conical Horn Antenna
Consists of a cone that is truncated in a piece of circular waveguide. The
waveguide in turn connects the antenna to either the transmitter or receiver.
If the horn itself is used as the
antenna, the cone angle (flare angle) is
made approximately 50°. In this case, the
length of the truncated cone determines the
antenna gain.