Rf antenna basics

CONTENTS
1. Preliminaries
2. Introduction
3. Parameters
4. Radiation Pattern
5. Polarization
6. Types of Antenna
7. Antenna Designs
8. Radio Wave Propagation
9. Conclusion

Preliminaries
Waves:
A wave is a vibratory disturbance in a medium which carries energy from one
point to another without any actual movement of the medium.Types of waves:
1. Mechanical waves:
The waves which require a medium for propagation are called mechanical
waves e.g sound waves,water waves etc.
2. Electromagnetic Waves:
The waves which do not require any medium for propagation are called
electromagnetic waves e.g light waves,radio waves etc.
Nature of waves:
1. Transverse Waves:
In which the particles vibrates at right angles to the direction of propagation of
wave e.g. light waves
2. Longitudinal Waves:
In which particles vibrates in the direction of propagation of wave e.g sound
waves.
Electromagnetic theory:
 An electric current inside a wire creates a corresponding magnetic field
outside the wire. Its direction (clockwise or counter-clockwise) depends on
the direction of the current in the wire.
 A current is induced in a loop of wire when it is moved toward or away from
a magnetic field, or a magnet is moved towards or away from it; the
direction of current depends on that of the movement
Note: due to Maxwell's Equations, changing electric fields give rise to changing
magnetic fields, and hence we have electromagnetic radiation.

Electromagnetic Waves:
 This is an electric field that travels away from some source (an antenna, the
sun, a radio tower etc.). A traveling electric field has an associated magnetic
field with it, and the two make up an electromagnetic wave.
 These are transverse waves hence the two fields are perpendicular to each
other and perpendicular to the direction of wve propagation.
 All electromagnetic waves propagate at the same speed in air or in space
 Speed of electromagnetic waves in a medium depends upon properties of
that medium.
Electromagnetic Spectrum:
 The electromagnetic spectrum is the range of frequencies of electromagnetic
radiation and their respective wavelengths.
 This frequency range is divided into separate bands, and the electromagnetic
waves within each frequency band are called by different names; radio
waves, microwaves, infrared, visible light, ultraviolet, X-rays, and gamma
rays at the high-frequency (short wavelength) end.
 The electromagnetic waves in each of these bands have different
characteristics, such as how they are produced, how they interact with
matter, and their practical applications.
 Typically, lower-energy radiation, such as radio waves, is expressed as
frequency; microwaves, infrared, visible and UV light are usually expressed
as wavelength; and higher-energy radiation, such as X-rays and gamma rays,
is expressed in terms of energy per photon.

Radio Frequency:
 It is a frequency range from around 20kHz to around 300GHz.
 This is roughly upper limit of audio frequency and lower limit of infrared
frequencies.
 Radio waves are easy to generate and are widely used for both indoor and
outdoor communications because of their ability to pass through buildings
and travel long distances.
Frequency Band
Name
Frequency Range
Wavelength
(Meters)
Application
Extremely Low
Frequency (ELF)
3-30 Hz 10,000-100,000 km Underwater Communication
Super Low Frequency
(SLF)
30-300 Hz 1,000-10,000 km
AC Power (though not a
transmitted wave)
Ultra Low Frequency
(ULF)
300-3000 Hz 100-1,000 km
Very Low Frequency
(VLF)
3-30 kHz 10-100 km Navigational Beacons
Low Frequency (LF) 30-300 kHz 1-10 km AM Radio
Medium Frequency
(MF)
300-3000 kHz 100-1,000 m Aviation and AM Radio
High Frequency (HF) 3-30 MHz 10-100 m Shortwave Radio
Very High Frequency
(VHF)
30-300 MHz 1-10 m FM Radio
Ultra High Frequency
(UHF)
300-3000 MHz 10-100 cm
Television, Mobile Phones,
GPS
Super High Frequency
(SHF)
3-30 GHz 1-10 cm
Satellite Links, Wireless
Communication
Extremely High
Frequency (EHF)
30-300 GHz 1-10 mm Astronomy, Remote Sensing
Visible Spectrum
400-790 THz
(4*10^14-7.9*10^14)
380-750 nm
(nanometers)
Human Eye
 At low frequencies, the waves can pass through obstacles easily. However,
their power falls with respect to the distance.

 The higher frequency waves are more prone to absorption by rain drops and
they get reflected by obstacles.
Frequency:
 The rate of repetition of a wave over a particular period of time, is called as
frequency.Simply, frequency refers to the process of how often an event
occurs.
 Mathematically this is written as:
frequency is the number of cycles per second (Hertz)
 The speed that the waves travel depends on the wavelength multiplied by
the frequency. The equation that relates frequency, wavelength and the
speed of light is as:
 Basically, the frequency is just a measure of how fast the wave is oscillating.
Wavelength:
 According to the standard definition, “The distance between two
consecutive maximum points (crests) or between two consecutive minimum
points (troughs) is known as the wavelength.”
 And since all EM waves travel at the same speed, the higher the frequency,
the shorter will be the wavelength and vice versa.

Bandwidth:
 The signal when transmitted or received, is done over a range of frequencies
called bandwidth.
 This particular range of frequencies are allotted to a particular signal, so that
other signals may not interfere in its transmission.
 The bandwidth once allotted, cannot be used by others.
 The whole spectrum is divided into bandwidths to allot to different
transmitters.
Impedance:
 impedance is the measure of the opposition that a circuit presents to a
current when a voltage is applied.
 Impedance possesses both magnitude and phase, unlike resistance, which
has only magnitude.
 When a circuit is driven with direct current (DC), there is no distinction
between impedance and resistance or we can sat impedance is at zero
degree phase.
Impedance Matching:
 The approximate value of impedance of a transmitter, when equals the
approximate value of the impedance of a receiver, or vice versa, it is termed
as Impedance matching.
 To ensure maximum power transmission it is important to match source
impedance with load impedance.
VSWR & Reflected Power:
 The term, which indicates the impedance mismatch is VSWR.
 VSWR stands for Voltage Standing Wave Ratio. It is also called as SWR.
 The ratio of the maximum voltage to the minimum voltage in a standing
wave is known as Voltage Standing Wave Ratio.
 If the impedance of the antenna, the transmission line and the circuitry do
not match with each other, then the power will not be radiated effectively.
Instead, some of the power is reflected back.
 The higher the impedance mismatch, the higher will be the value of VSWR.
 The ideal value of VSWR should be 1:1 for effective radiation.
 Reflected power is the power wasted out of the forward power. Both
reflected power and VSWR indicate the same thing.

Introduction
Antenna:
Electrical Symbol
 An Antenna is a transducer, which converts electrical power into
electromagnetic waves and vice-versa
 An Antenna can be used either as a transmitting antenna or a receiving
antenna.
 A transmitting antenna is one, which converts electrical signals into
electromagnetic waves and radiates them.
 A receiving antenna is one, which converts electromagnetic waves from the
received beam into electrical signals.
 In two-way communication, the same antenna can be used for both
transmission and reception.
 Antenna can also be termed as an Aerial
Need of Antenna:
whenever the need for wireless communication arises, there occurs the
necessity of an antenna. Antenna has the capability of sending or receiving the
electromagnetic waves for the sake of communication, where you cannot
expect to lay down a wiring system.
Working Principle:
 The working principle of an antenna is that it converts electrical
current (carried along by metallic conductors) into EM radiation in free space
or vice versa.
 The sole functionality of an antenna is power radiation or reception. Antenna
can be connected to the circuitry at the station through a transmission line

 A transmitter sends a high frequency wave into a co-axial cable and a pulsing
electric field is created between the wires,which cannot free itself from the
cable.
 The end of the cable is bent open.The field lines becomes longer and
orthogonal to the wires.
 The cable is bent open at right angles.The filed lines now reached a length
which allows the wave to free itself from the cable.Thus radiates an
electromagnetic wave, whereby the length of two bent pieces of wire
corresponds to the half of the wavelength
 The functioning of an antenna depends upon the radiation mechanism of a
transmission line.A straight transmission line conducting current with
uniform velocity, with infinite extent, radiates no power.
 If the power has to be radiated, though the current conduction is with
uniform velocity, the wire or transmission line should be bent, truncated or
terminated.
 If this transmission line has current, which accelerates or decelerates with a
time varying constant, then it radiates the power even though the wire is
straight.

Parameters
1. Directivity:
 Radiation emitted from an antenna which is more intense in a particular
direction, indicates the maximum intensity of that antenna. The emission of
radiation to a maximum possible extent is the radiation intensity.
 The radiation intensity of an antenna is focused in a particular direction,
while it is transmitting or receiving. Hence, the antenna is said to have its
directivity in that particular direction.
 It is a measure of how 'directional' an antenna's radiation pattern is.
 So the ratio of maximum radiation intensity of an subject antenna to the
radiation intensity of an isotropic or reference antenna, radiating the same
total power is called the directivity.
 An antenna that radiates equally in all directions would have effectively zero
directionality, and the directivity of this type of antenna would be 1 (or 0
dB).
 Small antennas have broad radiation patterns (low directivity), and antennas
with large uniform voltage or current distributions have very directional
patterns (and thus, a high directivity).
 Antennas for cell phones should have a low directivity because the signal can
come from any direction, and the antenna should pick it up. In contrast,
satellite dish antennas have a very high directivity, because they are to
receive signals from a fixed direction.
2. Antenna Gain:
 The term antenna gain describes how much power is transmitted in the
direction of peak radiation to that of an isotropic source.Gain is usually
measured in dB.
 According to the standard definition, “Gain of an antenna is the ratio of the
radiation intensity in a given direction to the radiation intensity that would
be obtained if the power accepted by the antenna were radiated
isotropically.

 Unlike directivity, antenna gain takes the losses that occur also into account
and hence focuses on the efficiency.
 The equation of gain, G is as shown below.
 A transmitting antenna with a gain of 3 dB means that the power received
far from the antenna will be 3 dB higher (twice as much) than what would be
received from a lossless isotropic antenna with the same input power.
Is a High Gain Antenna Advantageous?
The answer is: it depends. If you know exactly where your desired signal is
coming from, you would like to have maximum gain (towards the desired)
direction. However, if you don't know where the desired signal will be coming
from, it is better to have a low gain antenna.
3. Antenna Efficiency:
 According to the standard definition, “Antenna Efficiency is the ratio of the
radiated power of the antenna to the input power accepted by the antenna.”
 Simply, an Antenna is meant to radiate power given at its input, with
minimum losses. The efficiency of an antenna explains how much an
antenna is able to deliver its output effectively with minimum losses in the
transmission line.
 Antenna Efficiency is same for antenna during transmission and reception.
 The mathematical expression for antenna efficiency is given below −
Where,
ηe is the antenna efficiency.
Prad is the power radiated.
Pinput is the input power for the antenna.
What causes an antenna to not have an efficiency of 100% (or 0 dB)? :
 conduction losses (due to finite conductivity of the metal that forms the
antenna)
 dielectric losses (due to conductivity of a dielectric material near an antenna)
 impedance mismatch loss

4. Aperture Efficiency:
 The effective aperture describes how much power is captured by antenna
aperture from a given plane wave and delivered by antenna.
 According to the standard definition, “Aperture efficiency of an antenna, is
the ratio of the effective radiating area (or effective area) to the physical
area of the aperture.”
The mathematical expression for aperture efficiency is as follows:
Where,
εA is Aperture Efficiency
Aeff is effective area
Ap is physical area

Radiation Pattern
The energy radiated by an antenna is represented by the Radiation pattern of
the antenna.One can simply understand the function and directivity of an
antenna by having a look at its radiation pattern.Types of Radiation patterns
are:
1. Isotropic:
A pattern is "isotropic" if the radiation pattern is the same in all directions.
Antennas with isotropic radiation patterns don't exist in practice, but are
sometimes discussed as a means of comparison with real antennas.
2. Omni-directional:
Some antennas may also be described as "omnidirectional", which for an actual
antenna means that the radiation pattern is isotropic in a single
plane.Omni-directional pattern (also called non-directional pattern): The
pattern usually has a doughnut shape in three-dimensional view. However, in
two-dimensional view, it forms a figure-of-eight pattern.

3. Directional:
 The third category of antennas are "directional", which do not have a
symmetry in the radiation pattern. These antennas typically have a single
peak direction in the radiation pattern; this is the direction where the bulk of
the radiated power travels.
 To have a better understanding, consider the following figure, which
represents the radiation pattern of a dipole antenna.
 The major part of the radiated field, which covers a larger area, is the main
lobe or major lobe. This is the portion where maximum radiated energy
exists. The direction of this lobe indicates the directivity of the antenna.
 The other parts of the pattern where the radiation is distributed side wards
are known as side lobes or minor lobes. These are the areas where the
power is wasted.
 There is other lobe, which is exactly opposite to the direction of main lobe. It
is known as back lobe, which is also a minor lobe. A considerable amount
of energy is wasted even here.
 Elimination of these side lobes is must, in order to improve the
performance and save the energy.
 If these minor lobes are eliminated and this energy is diverted into one
direction (that is towards the major lobe), then the directivity of the
antenna gets increased which leads to antenna’s better performance

Beam Width:
 Another important factor in the radiation pattern of an antenna, known
as beam width. It is the measure of directivity of an antenna.
 The antenna beam is an angular width and measured on main lobe of
antenna including Half Power Beam Width (HPBW) and First Null Beam
Width (FNBW).
1. Half-Power Beam Width
It is angular width measured on the main beam of antenna radiation pattern.
The angular width on major lobe between two points where power is half of
maximum power radiated is known as HPBW.
2. First Null Beam Width
It is n angular width which is measured between first nulls or first side lobes on
antenna radiation pattern.
It defines ability of the system to separate two adjacent targets.

Antenna Polarization
A radio wave is made up of both electric and magnetic fields. In free space, the
electric and magnetic fields are mutually perpendicular and are also
perpendicular to the direction of propagation. The direction of oscillation of the
electric field component, when a radio wave is propagating in a medium, is
called the polarization of the radio wave.
Antennas are usually developed to receive and transmit radio waves that are
polarized in a specific way. The different types of polarization are discussed
below:
1. Linear polarization:
When the electric field is oscillating in the horizontal or vertical direction, the
radio wave is said to be linearly polarized.
The linear polarization of the antenna helps in maintaining the wave in a
particular direction, avoiding all the other directions. Hence, we use this linear
polarization to improve the directivity of the antenna.
2. Circular polarization:
When the polarization of a radio wave rotates while the signal propagates. A
Circularly Polarized signal consists of two perpendicular electromagnetic plane
waves of equal amplitude, which are 90 degree out of phase.

Based on the direction the signal is rotating, circular polarization can be
classified as two types: Right Hand Circular Polarization (RHCP) and Left Hand
Circular Polarization (LHCP)
By using circular polarization, the effect of multi-path gets reduced and hence it
is used in satellite communications such as GPS.

Types Of Antennas
Type of antenna Examples Applications
Wire Antennas
Dipole antenna, Mono
pole antenna, Helix
antenna, Loop antenna
Personal applications,
buildings, ships,
automobiles, space crafts
Aperture
Antennas
Waveguide (opening),
Horn antenna
Flush-mounted
applications, air-craft,
space craft
Reflector
Antennas
Parabolic reflectors,
Corner reflectors
Microwave
communication, satellite
tracking, radio astronomy
Lens Antennas
Convex-plane,
Concave-plane,
Convex-convex, concave
lenses
Used for very high
frequency applications
Micro strip
Antennas
Planar Inverted-F
Antennas (PIFA)
Rectangular Micro strip
(Patch) Antennas
Air-craft, space-craft,
satellites, missiles, cars,
mobile phones etc.
Array Antennas
Yagi-Uda antenna, Micro
strip patch array, Aperture
array, Slotted wave guide
array
Used for very high gain
applications, mostly
when needs to control
the radiation pattern

Planar Inverted-F Antenna:
 PIFA is refereed as short-circuited microstrip antenna
 A Planar Inverted-F Antenna can be considered as a type of antenna in which
the wire radiating element is replaced by a plate to increase the bandwidth.
 The advantage of these antennas is that they can be hidden into the housing
of the mobile when compared to different types of antennas like a whip, rod
or helical antennas, etc.
 The other advantage is that they can reduce the backward radiation towards
the top of the antenna by absorbing power, which enhances the efficiency.
They provides high gain in both horizontal and vertical states. This feature is
most important for any kind of antennas used in wireless communications.
 The PIFA structure consists of a rectangular planar which is installed on the
ground plane, it is always parallel to reduce the height.
 By varying the size of the ground plate the bandwidth of PIFA can be
adjusted and optimized.
Advantages:
 It has omni-directional radiation pattern and provides high gain in vertical
and horizontal, both directions
 Low SAR values
 Simple and small structure

Antenna Designs
Mobile phone Antenna Designs:
Nowadays internal antenna has been used in cell phones instead of external
antenna and the main reason of that is internal antenna has good relation with
SAR rate and also size of the phone remains smaller.
There are many types of internal antennas like PIFA,fractal antenna or
mono-pole antenna. These types of antenna can cover single band, dual band
and wide band based on design of antenna.There are multiple antennas on each
phone:
1. Primary Cellular Antenna Design: (Transmit and Receive)
 The primary cellular antenna is the primary communication antenna on the
smart phone, and hence is extremely important. This antenna typically is the
only cellular antenna that transmits, so it has many specifications and
requirements to meet.
 The primary cellular antenna will typically have both a low-band (somewhere
between 700 and 960 MHz), and a high-band (somewhere between 1710
and 2700 MHz)
 The location of the primary cellular antenna will almost always be at the
lower end of the device. The reason for this is because of the SAR
requirements.
2. Diversity cellular antenna:(Receive Only)
 This antenna typically does receive only. This means it only needs to cover
the Rx frequencies, and not the Tx. frequencies for the cellular frequency
bands the phone covers.
 The mobile phone's receiver then will typically do switched diversity (i.e.
choose the receive signal with the most energy) or combined diversity (in
order to sum the powers from the two receiving cellular antennas).
3. GPS Antenna:(Receive Only)
 GPS antennas are somewhat unique, in that their bandwidth is fairly small.
The GPS frequency is 1.575 GHz, with virtually no bandwidth.
 GPS antennas on smart phones are receive only antennas, and therefore you
don't have to worry about any transmitting issues (SAR)
 The GPS antenna is most often used in portrait mode, which means the
mobile phone is held vertically in the hand. As a result, it is advantageous for
the antenna to have a radiation pattern that is directed upwards, instead of
downwards.

 Because the GPS antenna is used when the user is holding the phone
vertically, they will typically have their hands on the lower end of the device.
We therefore prefer to have the GPS antenna towards the top of the device.
4. Wi-Fi Antenna:(Transmit and Receive)
The WIFI frequencies are the highest of all frequency on the device (relative to
cell frequencies, gps, nfc, etc). The WIFI frequency is divided into two bands:
2400-2484 MHz (which also includes Bluetooth) and 5150-5850 MHz.
typically the WIFI antenna is connected to a chip that does both WIFI and
Bluetooth. This means designing antennas for WIFI and Bluetooth is basically
the same thing.
Because WIFI is the highest frequency on the mobile device, the WIFI antenna
will be the smallest antenna. A half-wavelength at 2.4 GHz is 6.25 cm (2.5"), and
a half-wavelength at 5 GHz is 3 cm or just over an inch.

Radio Waves Propagation
1. Line of Sight (LOS) Propagation
In the line-of-sight communication the wave travels a minimum distance of
sight. Which means it travels to the distance up to which a naked eye can see.
This is better understood with the help of the following diagram.
The line-of-sight propagation will not be smooth if there occurs any obstacle in
its transmission path. As the signal can travel only to lesser distances in this
mode, this transmission is used for infrared or microwave transmissions.
2. Ground Wave Propagation
The wave when propagates through the Earth’s atmosphere is known
as ground wave. The direct wave and reflected wave together contribute the
signal at the receiver station. When the wave finally reaches the receiver, the
lags are canceled out. In addition, the signal is filtered to avoid distortion and
amplified for clear output.

Sky Wave Propagation
Sky wave propagation is preferred when the wave has to travel a longer
distance. Here the wave is projected onto the sky and it is again reflected back
onto the earth.
The sky wave propagation is well depicted in the above picture. Here the
waves are shown to be transmitted from one place and where it is received by
many receivers. Hence, it is an example of broadcasting.
The waves, which are transmitted from the transmitter antenna, are reflected
from the ionosphere. It consists of several layers of charged particles ranging in
altitude from 30- 250 miles above the surface of the earth. Such a travel of the
wave from transmitter to the ionosphere and from there to the receiver on
Earth is known as Sky Wave Propagation. Ionosphere is the ionized layer
around the Earth’s atmosphere, which is suitable for sky wave propagation.

Conclusion
An average person now carries one or more antennas on them wherever they
go.the strong growth in RFID devices suggests that the number of antennas in
use may increase to one antenna per object in the world.
The huge development of the mobile phones have grown up rapidly in the last
years, and the market is asking for smaller mobile phones with more services.
On top of that it is important to reduce the risks affecting in the human body
because of the antenna radiation.
Hence wireless performance is completely dependent on high performance
antennas and implementations.

Rf antenna basics

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