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Antenna basics
By: Masoud Baghelani
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Antenna
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What is antenna?
• Antenna is an electrical device which converts electric
power into radio waves, and vice versa.
• In transmission, a radio transmitter supplies an electric current
oscillating at radio frequency (i.e. a high frequency alternating
current (AC)) to the antenna's terminals, and the antenna radiates the
energy from the current as electromagnetic waves (radio waves).
In reception, an antenna intercepts some of the power of an
electromagnetic wave in order to produce a tiny voltage at its
terminals, that is applied to a receiver to be amplified.
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Transceiver
block diagram
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Applications
(radio broadcasting)
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Applications (radars& cell phones)
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Applications
(satellite communication)
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Door opener
Applications
Bluetooth
Microphone
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Operation
• Typically an antenna consists of an arrangement of
metallic conductors (elements), electrically connected (often
through a transmission line) to the receiver or transmitter. An
oscillating current of electrons forced through the antenna by a
transmitter will create an oscillating magnetic field around the
antenna elements, while the charge of the electrons also creates an
oscillating electric field along the elements. These time-varying fields
radiate away from the antenna into space as a
moving transverse electromagnetic field wave.
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Operation
• During reception, the oscillating electric and magnetic fields of
an incoming radio wave exert force on the electrons in the
antenna elements, causing them to move back and forth,
creating oscillating currents in the antenna.
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Categories
• Omnidirectional:
monopole antenna is typically connected to the inner
conductor of a coaxial transmission line (or a matching
network); the shield of the transmission line is connected
to ground. In this way, the ground (or any large conductive
surface) plays the role of the second conductor of a
dipole, thereby forming a complete circuit.
• Directional
The dipole antenna, which is the basis for most antenna
designs, is a balanced component, with equal but
opposite voltages and currents applied at its two terminals
through a balanced transmission line (or to a coaxial
transmission line through a so-called balun). 11
Dipole antenna fields
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Phased array
• phased array consists of two or more simple antennas which
are connected together through an electrical network. This often
involves a number of parallel dipole antennas with a certain
spacing.
Greater directionality can be obtained using
beam-forming techniques such as
a parabolic reflector or a horn. Since high
directivity in an antenna depends on it being
large compared to the wavelength, narrow
beams of this type are more easily achieved
at UHF and microwave frequencies.
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Reciprocity
• It is a fundamental property of antennas that the electrical
characteristics of an antenna described in the next section,
such as gain, radiation pattern, impedance, bandwidth,
resonant frequency and polarization, are the same whether the
antenna is transmitting or receiving.
Most materials used in antennas meet these conditions, but some
microwave antennas use high-tech components such
as isolators and circulators, made of nonreciprocal materials such
as ferrite. These can be used to give the antenna a different behavior
on receiving than it has on transmitting, which can be useful in
applications like radar.
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Resonant antennas
• Standing waves on a half wave
dipole driven at its resonant frequency.
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Bandwidth
• The frequency range or bandwidth over which an antenna
functions well can be very wide (as in a log-periodic antenna) or
narrow (in a resonant antenna); outside this range the antenna
impedance becomes a poor match to the transmission line and
transmitter (or receiver).
• Instead, it is often desired to have an antenna whose impedance does
not vary so greatly over a certain bandwidth. It turns out that the
amount of reactance seen at the terminals of a resonant antenna
when the frequency is shifted, say, by 5%, depends very much on the
diameter of the conductor used.
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Gain
Gain is a parameter which measures the degree of directivity of the
antenna's radiation pattern. A high-gain antenna will radiate most of its
power in a particular direction, while a low-gain antenna will radiate over a
wider angle. The antenna gain, or power gain of an antenna is defined as
the ratio of the intensity (power per unit surface area) radiated by the
antenna in the direction of its maximum output, at an arbitrary distance,
divided by the intensity radiated at the same distance by a
hypothetical isotropic antenna which radiates equal power in all directions.
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Gain
• High-gain antennas have the advantage of longer range and
better signal quality, but must be aimed carefully at the other
antenna. An example of a high-gain antenna is a parabolic
dish such as a satellite television antenna.
• Low-gain antennas have shorter range, but the orientation of
the antenna is relatively unimportant. An example of a low-gain
antenna is the whip antenna found on portable radios
and cordless phones.
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Radiation pattern
• The radiation pattern of
an antenna is a plot of
the relative field strength
of the radio waves
emitted by the antenna
at different angles.
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Types (isotropic)
• An isotropic
antenna (isotropic radiator)
is a hypothetical antenna
that radiates equal signal
power in all directions.
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Types
(dipole)
•Corner
reflector UHF TV
antenna
•Log-periodic dipole array covering 140-470 MHz
•Two-element turnstile antenna for
reception of weather satellite data,
137 MHz. Has circular polarization.
"Rabbit ears" dipole variant
for VHF television reception
This antenna radiates maximally in
directions perpendicular to the
antenna's axis, giving it a small directive
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gain of 2.15 dBi
Types
(dipole)
Patch (microstrip)
A type of antenna with elements consisting of metal sheets mounted over a ground
plane. Similar to dipole with gain of 6 - 9 dBi. Integrated into surfaces such as
aircraft bodies. Their easy fabrication using PCB techniques have made them
popular in modern wireless devices. Often used in arrays.
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Monopole
•VHF ground
plane antenna
•Mast radiator antenna
of medium wave AM
radio station, Germany
Quarter-wave whip
antenna on an FM radio for
88-108 MHz
Rubber Ducky antenna on
UHF 446 MHz walkie
talkie with rubber cover
removed.
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Arrays
•Sector antennas (white
bars) on cell phone tower.
Collinear arrays of dipoles, these
radiate a flat, fan-shaped beam.
•108 MHz reflective
array antenna of AN-270 radar
used during WW2.
•Reflective array UHF TV
antenna, with bowtie dipoles
to cover the UHF 470890 MHz band
•VHF collinear
array of folded
dipoles
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Arrays
•Batwing VHF television
broadcasting antenna
•Crossed-dipole FM radio
broadcast antenna
•US Air Force PAVE PAWS phased
array radar antenna for ballistic
missile detection, Alaska. The two
circular arrays are each composed
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of 2677 crossed dipole antennas.
•Flat microstrip array antenna for satellite TV reception.
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PART 2
Antenna
measurement
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ANTENNA MEASUREMENT
• Testing of antennas to ensure that the antenna meets specifications:
• Gain
• Radiation pattern
• Beamwidth
• Polarization
• Impedance
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ANTENNA PATTERN
Is the response of the antenna to a plane wave incident from a
given direction or the relative power density of the wave
transmitted by the antenna in a given direction.
 far-field range
 near-field
 compact range
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Far-field range (FF)
• The far-field range was the original antenna measurement technique,
and consists of placing the AUT a long distance away from
the instrumentation antenna. Generally, the far-field distance
or Fraunhofer distance, d, is considered to be
Separating the AUT and the instrumentation antenna by this distance
reduces the phase variation across the AUT enough to obtain a
reasonably good antenna pattern.
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Far-field range (FF)
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Near-field range (NF)
Planar near-field range
Planar near-field measurements are conducted by scanning a small probe antenna over a planar surface. These
measurements are then transformed to the far-field by use of a Fourier transform, or more specifically by applying a
method known as stationary phase to the Laplace transform . Three basic types of planar scans exist in near field
measurements.
Rectangular planar scanning
The probe moves in the Cartesian coordinate system and its linear movement creates a regular rectangular sampling
grid with a maximum near-field sample spacing of Δx = Δy = λ /2.
Polar planar scanning
More complicated solution to the rectangular scanning method is the plane polar scanning method.
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Free-space ranges
The formula for electromagnetic radiation dispersion and information is:
Where D=Distance, P=Power, and S=Speed
What this means is that double the communication distance requires four times the power. It also
means double power allows double communication speed (bit rate). Double power is approx. 3dB
(10 log(2) to be exact) increase.
Of course in the real world there are all sorts of other phenomena which enter in, such as
Fresnel canceling,
path loss,
background noise
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Antenna parameters
• Except for polarization, the SWR is the most easily measured of
the parameters above.
• Impedance can be measured with specialized equipment, as it
relates to the complex SWR.
• Measuring radiation pattern requires a sophisticated setup
including significant clear space (enough to put the sensor into
the antenna's far field, or an anechoic chamber designed for
antenna measurements), careful study of experiment geometry,
and specialized measurement equipment that rotates the
antenna during the measurements.
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Radiation pattern
• The radiation pattern is a graphical depiction of the relative field strength
transmitted from or received by the antenna, and shows sidelobes and backlobes.
As antennas radiate in space often several curves are necessary to describe the
antenna. If the radiation of the antenna is symmetrical about an axis (as is the
case in dipole, helical and some parabolic antennas) a unique graph is sufficient.
• Each antenna supplier/user has different standards as well as plotting formats.
Each format has its own advantages and disadvantages. Radiation pattern of
an antenna can be defined as the locus of all points where the emitted power
per unit surface is the same. The radiated power per unit surface is
proportional to the squared electrical field of the electromagnetic wave. The
radiation pattern is the locus of points with the same electrical field. In this
representation, the reference is usually the best angle of emission. It is also
possible to depict the directive gain of the antenna as a function of the direction.
Often the gain is given in decibels.
• The graphs can be drawn using cartesian (rectangular) coordinates or a polar
plot. This last one is useful to measure the beamwidth, which is, by convention,
the angle at the -3dB points around the max gain. The shape of curves can be
very different in cartesian or polar coordinates and with the choice of the limits
of the logarithmic scale.
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Efficiency
• Efficiency is the ratio of power actually radiated by an antenna to the
electrical power it receives from a transmitter. A dummy load may have
an SWR of 1:1 but an efficiency of 0, as it absorbs all the incident power,
producing heat but radiating no RF energy; SWR is not a measure of an
antenna's efficiency. Radiation in an antenna is caused by radiation
resistance which cannot be directly measured but is a component of the
total resistance which includes the loss resistance. Loss resistance
results in heat generation rather than radiation, thus reducing efficiency.
Mathematically, efficiency is equal to the radiation resistance divided by
total resistance (real part) of the feed-point impedance.
• Efficiency is defined as the ratio of the power that is radiated to the total
power used by the antenna;
• Total power = power radiated + power loss.
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Bandwidth
• IEEE defines bandwidth as "The range of frequencies within which
the performance of the antenna, with respect to some
characteristic, conforms to a specified standard.“
• In other words, bandwidth depends on the overall effectiveness of the
antenna through a range of frequencies, so all of these parameters
must be understood to fully characterize the bandwidth capabilities of
an antenna.
• in practice, bandwidth is typically determined by measuring a
characteristic such as SWR or radiated power over the frequency
range of interest. For example, the SWR bandwidth is typically
determined by measuring the frequency range where the SWR is less
than 2:1. Another frequently used value for determining bandwidth for
resonant antennas is the -3dB Return Loss value.
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Directivity
Antenna directivity is the ratio of maximum radiation intensity (power
per unit surface) radiated by the antenna in the maximum direction
divided by the intensity radiated by a hypothetical isotropic
antenna radiating the same total power as that antenna.
For example, a hypothetical antenna which had a radiated pattern of
a hemisphere (1/2 sphere) would have a directivity of 2. Directivity is
a dimensionless ratio and may be expressed numerically or
in decibels (dB).
Directivity is identical to the peak value of the directive gain; these
values are specified without respect to antenna efficiency thus
differing from the power gain (or simply "gain") whose
value is reduced by an antenna's efficiency.
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GAIN
Gain as a parameter measures the directionality of a given antenna. An antenna with a low gain
emits radiation in all directions equally, whereas a high-gain antenna will preferentially radiate in
particular directions.
Specifically, the Gain or Power gain of an antenna is defined as the ratio of the intensity (power
per unit surface) radiated by the antenna in a given direction at an arbitrary distance divided by
the intensity radiated at the same distance by an hypothetical isotropic antenna:
As an example, consider an antenna that radiates an
electromagnetic wave whose electrical field has an
amplitude E at a distance r
. This amplitude is given by:
• I is the current fed to the antenna and
• A is a constant characteristic of each antenna.
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CALCULATION OF ANTENNA PARAMETERS
• The gain in any given direction and the impedance at a
given frequency are the same when the antenna is used in
transmission or in reception.
• using the reciprocity theorem, it is possible to prove that the
Thévenin equivalent circuit of a receiving antenna is:
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CALCULATION OF ANTENNA PARAMETERS
• Va is the Thévenin equivalent circuit tension.
• Za is the Thévenin equivalent circuit impedance and is the same as the
antenna impedance.
• Ra is the series resistive part of the antenna impedance .
• Ga is the directive gain of the antenna (the same as in emission) in the
direction of arrival of electromagnetic waves.
• λ is the wavelength.
• Eb is the magnitude of the electrical field of the incoming
electromagnetic wave.
• ψ is the angle of misalignment of the electrical field of the incoming
wave with the antenna. For a dipole antenna, the maximum induced
voltage is obtained when the electrical field is parallel to the dipole. If this
is not the case and they are misaligned by an angle ψ, the induced
voltage will be multiplied by cos ψ.
• Z0 is a universal constant called vacuum impedance or impedance of
free space.
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CALCULATION OF ANTENNA PARAMETERS
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CALCULATION OF ANTENNA PARAMETERS
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