Antenna and It's Parameters by YEASIN NEWAJ

Microwave Engineering
Presented By
YEASIN NEWAJ
BSc. in Electrical and Electronics Engineering

Introduction
 The first antennas were built in 1888 by German physicist Heinrich Hertz in his pioneering
experiments to prove the existence of electromagnetic waves predicted by the theory of James
Clerk Maxwell.
 Hertz placed dipole antennas at the focal point of parabolic reflectors for both transmitting and
receiving. He published his work in Annalen der Physik und Chemie (vol. 36, 1889).

Introduction
 An antenna is an electrical device which converts electric currents into radio waves, and vice versa. It is usually used with a
radio transmitter or radio receiver.
 In transmission, a radio transmitter applies an oscillating radio frequency electric current to the antenna's terminals, and the
antenna radiates the energy from the current as electromagnetic waves (radio waves).
 Transmitting Antenna: Any structure designed to efficiently radiate electromagnetic radiation in a preferred direction is called a
transmitting antenna.
 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.
 Receiving Antenna: Any structure designed to efficiently receive electromagnetic radiation is called a receiving antenna
 An antenna can be used for both transmitting and receiving.

BASIC
STRUCTURE
 It is a metallic conductor system capable of radiating and receiving EM waves.
 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 electromagnetic
field wave.

BASIC STRUCTURE
 Conversely, 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.
 Antenna reciprocity: can be used as transmitter and receiver. In two way communication same antenna can be used as
transmitter and receiver.
 Antennas may also contain reflective or directive elements or surfaces not connected to the transmitter or receiver, such as
parasitic elements, parabolic reflectors or horns, which serve to direct the radio waves into a beam or other desired radiation
pattern.
 Antennas can be designed to transmit or receive radio waves in all directions equally (omnidirectional antennas), or transmit
them in a beam in a particular direction, and receive from that one direction only ( directional or high gain antennas).

WHY ANTENNAS ?
 Need of antenna arisen when two person wanted to communicate between them when separated by some distance and wired
communication is not possible.
 Antennas are required by any radio receiver or transmitter to couple its electrical connection to the electromagnetic field.
 Radio waves are electromagnetic waves which carry signals through the air (or through space) at the speed of light with almost
no transmission loss.
 Radio transmitters and receivers are used to convey signals (information) in systems including broadcast (audio) radio,
television, mobile telephones , point-topoint communications links (telephone, data networks), satellite links.
 Radio waves are also used directly for measurements in technologies including Radar, GPS, and radio astronomy.
 In each and every case, the transmitters and receivers involved require antennas, although these are sometimes hidden (such as
the antenna inside an AM radio or inside a laptop computer equipped with wi-fi).

WHERE USED?
 Antennas are used in systems such as radio and television broadcasting, point to point radio
communication, wireless LAN, radar and space exploration
 Antennas are most utilized in air or outer space
 But can also be operated under water or even through soil and rock at certain frequencies for short
distances

TYPES OF ANTENNAS
According to their applications and technology available, antennas generally fall in one of two categories:
 Omnidirectional or only weakly directional antennas which receive or radiate more or less in all
directions. These are employed when the relative position of the other station is unknown or arbitrary.
They are also used at lower frequencies where a directional antenna woul be too large, or simply to cut
costs in applications where a directional antenna isn't required.
 Directional or beam antennas which are intended to preferentially radiate or receive in a particular
direction or directional pattern.

TYPES OF ANTENNAS
According to length of transmission lines available, antennas generally fall in one of two categories:
 Resonant Antennas – is a transmission line, the length of which is exactly equal to multiples of half
wavelength and it is open at both ends.
 Non-resonant Antennas – the length of these antennas is not equal to exact multiples of half
wavelength. In these antennas standing waves are not present as antennas are terminated in correct
impedance which avoid reflections. The waves travel only in forward direction .Non-resonant antenna
is a unidirectional antenna.

ANTENNA PARAMETERS
The Fundamental parameters of antenna are
given below
 Frequency
 Frequency Bands
 Radiation Patterns
 Field Regions
 Directivity
 Efficiency & Gain
 Beams width & Side lobes
 Impedance
 Bandwidth Polarization of Waves
 Polarization of Antennas
 Effective Aperture
 Fiirs Transmission Formula
 Antenna Temperature

Frequency
 Frequency is one of the most important concepts in the universe and to antenna theory.
 Antennas function by transmitting or receiving electromagnetic (EM) waves.
 The equation that relates frequency, wavelength and the speed of light is given by;
 Basically, the frequency is just a measure of how fast the wave is oscillating.
 And since all EM waves travel at the same speed, the faster it oscillates the shorter the wavelength.
And a longer wavelength implies a slower frequency

Frequency Bands
 In general, waveforms are not made up of a discrete number of frequencies, but rather a continuous
range of frequencies.
 No matter what information you want to send, that waveform can be represented as the sum of a range
of frequencies.
 Since every piece of information in the universe can be decomposed into sine and cosine components
of varying frequencies by the use of ‘Mathematical Technique Fourier Transforms’, we always discuss
antennas in terms of the wavelength it operates at or the frequency we are using.

Radiation Pattern
 A radiation pattern defines the variation of the power radiated by an antenna as a function of the
direction away from the antenna. This power variation as a function of the arrival angle is observed in
the antenna's far field.
 As an example, consider the 3-dimensional radiation pattern in figure , plotted in decibels (dB) .

Radiation Pattern
 This is an example of a donut shaped or toroidal radiation pattern. In this case, along the z-axis, which
would correspond to the radiation directly overhead the antenna, there is very little power transmitted.
In the x-y plane (perpendicular to the z-axis), the radiation is maximum. These plots are useful for
visualizing which directions the antenna radiates.
 Standard spherical coordinates are used, where ‘θ’ is the angle measured off the z-axis, and ‘Ø’ is the
angle measured counterclockwise off the x-axis as shown in Figure.

Radiation Pattern
 While the radiation pattern is actually three-dimensional, it is common however to describe this
behavior with two planar patterns, also called the principal plane patterns. They can be obtained from
the spatial radiation characteristics by looking at a cut-plane - usually through the origin and the
maximum of radiation.
 The horizontal pattern shows the field strength as a function of the azimuth angle ϕ with a fixed ϑ
(usually ϑ = 90°).
 The vertical pattern shows the field strength as a function of ϑ for a fixed ϕ (usually ϕ = +/- 90 ° or
0°/180°)

Radiation Pattern
 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.
 Some antennas may also be described as "omnidirectional", which for an actual antenna means that the
radiation pattern is isotropic in a single plane. Examples of omnidirectional antennas include the dipole
antenna and the slot antenna.
 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.

Radiation Pattern
 In a directional antenna designed to project radio waves in a particular direction, the lobe in that
direction is designed larger than the others and is called the "main lobe".
 The other lobes usually represent unwanted radiation and are called “sidelobes". The axis through the
main lobe is called the "principle axis" or “boresight axis.

Field Regions
 The fields surrounding an antenna are divided into 3 principle regions:
1. Reactive Near Field.
2. Radiating Near Field or Fresnel Region.
3. Far Field or Fraunhofer Region.
 The far field region is the most important, as this determines the antenna's radiation pattern.
 Also, antennas are used to communicate wirelessly from long distances, so this is the region of
operation for most antennas.

Field Regions
Far Field (Fraunhofer) Region
 The far field is the region far from the antenna, as you might suspect. In this region, the radiation
pattern does not change shape with distance (although the fields still die off as 1/R, so the power dies
off as 1/R^2).
 Also, this region is dominated by radiated fields, with the E- and H-fields orthogonal to each other and
the direction of propagation as with plane waves.
 If the maximum linear dimension of an antenna is D, then the following 3 conditions must all be
satisfied to be in the far field region:

Field Regions
 The first and second equation above ensure that the power radiated in a given direction from distinct
parts of the antenna are approximately parallel (see figure a).
 This helps ensure the fields in the far-field region behave like plane waves. Note that >> means "much
greater than“ and is typically assumed satisfied if the left side is 10 times larger than the right side.
 The Rays from any Point on the Antenna are
Approximately Parallel in the Far Field as shown in figure

Field Regions
 Near a radiating antenna, there are reactive fields that typically have the E-fields and H fields die off
with distance as ‘1/R2’ and ‘1/R3’.
 The third equation above ensures that these near fields are gone, and we are left with the radiating
fields, which fall off with distance as ‘1/R’.
 The far-field region is sometimes referred to as the Fraunhofer region, a carryover term from optics.

Field Regions
Reactive Near Field Region
 In the immediate vicinity of the antenna, we have the reactive near field. In this region, the fields are
predominately reactive fields, which means the E- and H- fields are out of phase by 90 degrees to each
other (recall that for propagating or radiating fields, the fields are orthogonal (perpendicular) but are in
phase).
 The boundary of this region is commonly given as:

Field Regions
Radiating Near Field (Fresnel) Region:
 The radiating near field or Fresnel region is the region between the near and far fields.
 In this region, the reactive fields are not dominate; the radiating fields begin to emerge. However,
unlike the far field region, here the shape of the radiation pattern may vary appreciably with distance.
 The region is commonly given by:

Directivit
y
 Directivity is a fundamental antenna parameter. It is a measure of how 'directional' an antenna’s
radiation pattern is. 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). Directivity is technically
a function of angle, but the angular variation is described by its radiation pattern.
 An antenna's normalized radiation pattern can be written as a function in spherical coordinates:
 Mathematically ‘directivity’ formula is given by:
 The numerator is the maximum value of F (the magnitude of the radiation pattern), and the
denominator just represents the "average power radiated over all directions"

Antenna Efficiency and
Gain
Antenna Efficiency:
 The efficiency of an antenna relates the power delivered to the antenna and the power radiated or dissipated
within the antenna. The losses associated within an antenna are typically the conduction losses (due to finite
conductivity of the antenna) and dielectric losses (due to conduction within a dielectric which may be
present within an antenna).
 The antenna efficiency (or radiation efficiency) can be written as the ratio of the radiated power to the input
power of the antenna:
 Equation [7] is sometimes referred to as the antenna's radiation efficiency. The total efficiency of an antenna
is the radiation efficiency multiplied by the impedance mismatch loss of the antenna.
 ML = antenna's loss due to impedance mismatch, ‘εR’ is the antenna's radiation efficiency.

Antenna Efficiency and
Gain
Antenna Gain
 The term ‘Gain’ describes how much power is transmitted in the direction of peak radiation to that of
an isotropic source. Gain is more commonly quoted in a real antenna's specification sheet because it
takes into account the actual losses that occur.
 Gain (G) can be related to directivity (D) by:

Beam widths and Side lobe Levels
 In addition to directivity, the radiation patterns of antennas are also characterized by their beam widths
and side lobe levels.
 Consider the radiation pattern given by:
 This pattern is actually fairly easy to generate using ‘Antenna Arrays’. The 3-dimensional view of this
radiation pattern is given in Figure .

 The polar angle measured off z-axis plot is given by:
 The main beam is the region around the direction of maximum radiation. The main beam in Figure 2 is
centered at 90 degrees. The side lobes are smaller beams that are away from the main beam.
 These side lobes are usually radiation in undesired directions which can never be completely
eliminated. The side lobes in Figure 13 occur at roughly 45 and 135 degrees.

Beam widths and Side lobe
Levels

 The Half Power Beam width (HPBW) is the angular separation in which the magnitude of the
radiation pattern decrease by 50% (or -3 dB) from the peak of the main beam. From figure 13, the
pattern decreases to -3 dB at 77.7 and 102.3 degrees. Hence the HPBW is 102.3-77.7 = 24.6 degrees.
 Another commonly quoted beam width is the ‘Null to Null Beam width’. This is the angular separation
from which the magnitude of the radiation pattern decreases to zero (negative infinity dB) away from
the main beam. From Figure 13, the pattern goes to zero (or minus infinity) at 60 degrees and 120
degrees. Hence, the Null-Null Beam width is 120-60=60 degrees.
 Finally, the ‘Side lobe Level’ is another important parameter used to characterize radiation patterns.
The side lobe level is the maximum value of the side lobes (away from the main beam). From Figure
13, the side lobe level (SLL) is -14.5 dB.

Antenna Impedance
 An antenna's impedance relates the voltage to the current at the input to the antenna. If impedance of
an antenna is given by:
 The real part of an antenna's impedance ‘x’ represents power that is either radiated away or absorbed
within the antenna.
 The imaginary part of the impedance ‘y’ represents power that is stored in the near field of the antenna
(non-radiated power).
 An antenna with a real input impedance (zero imaginary part) is said to be resonant.

Bandwidth
 Bandwidth is another fundamental antenna parameter. Bandwidth describes the range of frequencies
over which the antenna can properly radiate or receive energy. Often, the desired bandwidth is one of
the determining parameters used to decide upon an antenna. For instance, many antenna types have
very narrow bandwidths and cannot be used for wideband operation.
 Bandwidth is typically quoted in terms of VSWR. For instance, an antenna may be described as
operating at 100-400 MHz with a VSWR<1.5. This statement implies that the reflection coefficient is
less than 0.2 across the quoted frequency range. Hence, of the power delivered to the antenna, only 4%
of the power is reflected back to the transmitter. Alternatively, the return loss S11=20*log10(0.2)=-
13.98 dB.
 The bandwidth is often specified in terms of its Fractional Bandwidth (FBW). The FBW is the ratio of
the frequecny range (highest frequency minus lowest frequency) divided by the center frequency.

Polarization of antenna
 The polarization of an antenna is determined by the direction of the electric field 𝐸⃗ . A distinction
must be made between the following types of polarizations:
 Linear polarization: The 𝐸⃗ field vector changes in magnitude only.
 Circular polarization: The magnitude of the 𝐸⃗ field vector is constant, but the direction changes and
rotates around the direction of propagation.
 Elliptical polarization: The magnitude and the direction of the 𝐸⃗ field vector changes and its peak
position can be described by an elliptical equation.

Polarization of
antenna
 The polarization of an antenna is the polarization of the radiated fields produced by an antenna,
evaluated in the far field.
 Hence, antennas are often classified as "Linearly Polarized" or a "Right Hand Circularly Polarized
Antenna".
 This simple concept is important for antenna to antenna communication.
 First, a horizontally polarized antenna will not communicate with a vertically polarized antenna. Due
to the reciprocity theorem, antennas transmit and receive in exactly the same manner.
 Hence, a vertically polarized antenna transmits and receives vertically polarized fields.
 Consequently, if a horizontally polarized antenna is trying to communicate with a vertically polarized
antenna, there will be no reception.

 Polarization mismatch occurs when the polarization of the receiving antenna is not equal to the
polarization of the incoming wave.
 Figure 7 gives an overview of the polarization mismatch and the related loss imposed on the received
signal.
 Note that V means vertical, H horizontal, LHC left-hand circular and RHC right-hand circular
polarization

 The losses that occur when trying to receive a linearly polarized signal with a circularly polarized
antenna amounts to 3 dB (same vice versa) - this can usually be tolerated.
 Most critical is the case where the orthogonal antenna polarization is used, because the attenuation
increases beyond all limits theoretically.
 In practice, most antennas have a limited polarization decoupling, so that the loss in reality will never
reach infinity.

Effective Area (Effective Aperture)
 A useful parameter calculating the receive power of an antenna is the effective area or effective
aperture. Assume that a plane wave with the same polarization as the receive antenna is incident upon
the antenna. Further assume that the wave is travelling towards the antenna in the antenna's direction of
maximum radiation (the direction from which the most power would be received).
 Then the effective aperture parameter describes how much power is captured from a given plane wave.
Let ‘W’ be the power density of the plane wave (in W/m^2). If ‘P’ represents the power at the antennas
terminals available to the antenna's receiver, then:
 Hence, the effective area simply represents how much power is captured from the plane wave and
delivered by the antenna. This area factors in the losses intrinsic to the antenna (ohmic losses, dielectric
losses, etc.).
 A general relation for the effective aperture in terms of the peak gain (G) of any antenna is given by:

Friis Transmission Formula
 The Friis Transmission Equation is used to calculate the power received from one antenna (with gain
G1), when transmitted from another antenna (with gain G2), separated by a distance R, and operating
at frequency f or wavelength lambda.
Derivation of Friis Transmission Formula
 To begin the derivation, consider two antennas in free space (no obstructions nearby) separated by a
distance R:

 Assume that “PT” Watts of total power are delivered to the transmit antenna. For the moment, assume
that the transmit antenna is omnidirectional, lossless, and that the receive antenna is in the far field of
the transmit antenna.
 The power ‘P’ of the plane wave incident on the receive antenna a distance ‘R’ from the transmit
antenna is given by:
 If the transmit antenna has a gain in the direction of the receive antenna given by “GT” , then the power
equation above becomes:

 The gain term factors in the directionality and losses of a real antenna. Assume now that the receive
antenna has an effective aperture given by ‘AER’. Then the power received by this antenna (PR) is given
by:
 Since the effective aperture for any antenna can also be expressed as:
 The resulting received power can be written as:
 This is known as the ‘Friis Transmission Formula’. It relates the free space path loss, antenna gains and
wavelength to the received and transmit powers.

 Another useful form of the Friis Transmission Equation is given in Equation [2]. Since wavelength and
frequency f are related by the speed of light c ,we have the Friis Transmission Formula in terms of
frequency:
 Above Equation shows that more power is lost at higher frequencies. This is a fundamental result of
the Friis Transmission Equation. This means that for antennas with specified gains, the energy transfer
will be highest at lower frequencies. The difference between the power received and the power
transmitted is known as path loss. Said in a different way, Friis Transmission Equation says that the
path loss is higher for higher frequencies

 The importance of this result from the Friis Transmission Formula cannot be overstated. This is why
mobile phones generally operate at less than 2 GHz. There may be more frequency spectrum available at
higher frequencies, but the associated path loss will not enable quality reception. As a further
consequence of Friss Transmission Equation, suppose you are asked about 60 GHz antennas. Noting that
this frequency is very high, you might state that the path loss will be too high for long range
communication - and you are absolutely correct. At very high frequencies (60 GHz is sometimes
referred to as the mm (millimeter wave) region), the path loss is very high, so only point-to-point
communication is possible. This occurs when the receiver and transmitter are in the same room, and
facing each other.
 As a further corollary of Friis Transmission Formula, do you think the mobile phone operators are happy
about the new LTE (4G) band, that operates at 700MHz? The answer is yes: this is a lower frequency
than antennas traditionally operate at, but from the path loss will therefore be lower as well. Hence, they
can "cover more ground" with this frequency spectrum, and a Verizon Wireless executive recently called
this "high quality spectrum", precisely for this reason. Side Note: On the other hand, the cell phone
makers will have to fit an antenna with a larger wavelength in a compact device (lower frequency =
larger wavelength), so the antenna designer's job got a little more complicated!

Antenna Temperature
 Antenna Temperature ‘TA’ is a parameter that describes how much noise an antenna produces in a given
environment.
 This temperature is not the physical temperature of the antenna. Moreover, an antenna does not have an
intrinsic "antenna temperature" associated with it; rathe the temperature depends on its gain pattern and
the thermal environment that it is placed in.
 Antenna temperature is also sometimes referred to as Antenna Noise Temperature. An antenna’s
temperature will vary depending on whether it is directional and pointed into space or staring into the
sun.
 For an antenna with a radiation pattern given by ‘R(θ,Ø)’ the noise temperature is mathematically
defined as:

Antenna Temperature
 This states that the temperature surrounding the antenna is integrated over the entire sphere, and
weighted by the antenna's radiation pattern. Hence, an isotropic antenna would have a noise temperature
that is the average of all temperatures around the antenna; for a perfectly directional antenna (with a
pencil beam), the antenna temperature will only depend on the temperature in which the antenna is
"looking".
 The noise power received from an antenna at temperature ‘TA’ can be expressed in terms of the
bandwidth (B) the antenna (and its receiver) are operating over:
 In the above, K is Boltzmann's constant (1.38 * 10^-23 [Joules/Kelvin = J/K].

Why do Antennas Radiate?
 All radiation is caused by accelerating charges which produce changing electric fields. And due to
Maxwell's Equations, changing electric fields give rise to changing magnetic fields, and hence we have
electromagnetic radiation.
 The subject of antenna theory is concerned with transferring power from your receiver (the energy is
contained in voltages and currents) into electromagnetic radiation (where the energy is contained in the
E- and H-fields) travelling away from the antenna.
 This requires the impedance of your antenna to be roughly matched to your receiver, and that the
currents that cause radiation add up in-phase (that is, they don't cancel each other out as they would in a
transmission line).

Common Questions
 If all accelerating electric charges radiate, then the wires that connect my computer to the wall should
be antennas, correct? The charges on them are oscillating at 60 Hertz as the current travels so this should
yield radiation, correct?
 Answer: Yes. Your wires do act as antennas. However, they are very poor antennas. The reason (among
other things), is that the wires that carry power to your computer are a transmission line - they carry
current to your computer (which travels to one of your battery's terminals and out the other terminal) and
then they carry the current away from your computer (all current travels in a circuit or loop). Hence, the
radiation from one wire is cancelled by the current flowing in the adjacent wire (that is travelling the
opposite direction).

Common Questions
 If its so simple, then everything could be an antenna. Why don't I just use a metal paper clip as an
antenna, hook it up to my receiver and then forget all about antenna theory?
 Answer: A paper clip could definitely act as an antenna if you get current flowing on the antenna.
However, it is not so simple to do this. The impedance of the paper clip will control how much power
your receiver or transmitter could deliver to the paper clip (i.e. whether or not you could get any current
flowing on the paper clip at all). The impedance will depend on what frequency you are operating at.
Hence, the paper clip will work at certain frequencies as an antenna. However, you will have to know
much more about antennas before you can say when and it may work in a given situation.

Antenna Arrays
 An antenna array (often called a 'phased array') is a set of 2 or more antennas. The signals from the
antennas are combined or processed in order to achieve improved performance over that of a single
antenna. The antenna array can be used to
increase the overall gain
provide diversity reception
cancel out interference from a particular set of directions
"steer" the array so that it is most sensitive in a particular direction
determine the direction of arrival of the incoming signals
to maximize the Signal to Interference Plus Noise Ratio (SINR)
 Drawback: the increased cost, size, and complexity.

Antenna Factor
 The Antenna Factor is used by RF or EMC antenna engineers to describe the required electric field
strength that produces 1 Volt at the terminals of an antenna. Alternatively, the Antenna Factor concept
specifies what the received voltage is in the presence of an electric field. It is defined mathematically as:
 For instance, if the terminals of the antenna are short circuited, the received voltage is always zero, so
the Antenna Factor is not defined.
 Hence, the Antenna Factor has an implied impedance associated with the antenna terminals, most
commonly 50 Ohms. However, sometimes an "open circuit" antenna factor is discussed, which is the
available voltage for an antenna with an open circuit (no receiver or load attached). The basic concept of
antenna factor with a terminal (port, receiver or load) impedance is shown in Figure

Antenna Factor
 In the above Figure, the E-field is shown as part of a propagating wave (which isn't necessarily the case).
The antenna receives the field at a voltage shows up at its terminals, the circles shown in Figure.
 The receiver impedance (or the load, or a measuring device such as a network analyzer) is shown
connected to the antenna terminals.
 The ratio of the incident field strength to the output voltage is the Antenna Factor.

Electric Field (E-Field)
 Electromagnetic waves are made up of Electric Fields (often called the E-field) and magnetic fields.
 Technically, the E-field at a point in space is a measure of how strong the force would be on a unit point
charge (a small sphere with an electric charge of 1 Coulomb on it). Hence, the units of the E-field are
Newtons/Coulomb [N/C]. These units are equivalent to Volts/meter [V/m], which is what the E-field is
commonly quoted in (for instance, 10 V/m).
 The E-field is a vector quantity - this means at every point in space it has a magnitude and a direction.
For instance, lets say an E-field exists in space given by:
 This is the E-field of a plane wave travelling in the +z-direction, and the E-field is linearly polarized and
'points' in the y-direction (k is the wavenumber). The amplitude of the wave is A Volts/meter.

Magnetic Field (H-Field)
 The H-field is a vector quantity (has a magnitude and direction) and is measured in Amps/Meter [A/m].
Recall that the E-field points away from a positive point charge. An H-field curls (or wraps) around a
wire of moving charge, as shown in Figure. Hence, H-fields are associated with moving electric charges.
 There are no isolated magnetic charges, so an H-field can't be defined as a force per unit magnetic
charge in the way an E-field can be defined. However, magnetic dipoles do exist (magnets) which have a
positive and negative end (or North and South). The magnetic field lines travel away from the North side
and terminate on the south side.

S-Parameters
 S-parameters describe the input-output relationship between ports (or terminals) in an electrical system.
For instance, if we have 2 ports (intelligently called Port 1 and Port 2), then S12 represents the power
transferred from Port 2 to Port 1. S21 represents the power transferred from Port 1 to Port 2. In general,
SNM represents the power transferred from Port M to Port N in a multi-port network.
 A port can be loosely defined as any place where we can deliver voltage and current. So, if we have a
communication system with two radios (radio 1 and radio 2), then the radio terminals (which deliver
power to the two antennas) would be the two ports. S11 then would be the reflected power radio 1 is
trying to deliver to antenna 1. S22 would be the reflected power radio 2 is attempting to deliver to
antenna 2. And S12 is the power from radio 2 that is delivered through antenna 1 to radio 1. Note that in
general S-parameters are a function of frequency (i.e. vary with frequency).
 As an example, consider the following two-port network:

S-Parameters
 In the above Figure, S21 represents the power received at antenna 2 relative to the power input to
antenna 1. For instance, S21=0 dB implies that all the power delivered to antenna 1 ends up at the
terminals of antenna 2. If S21=-10 dB, then if 1 Watt (or 0 dB) is delivered to antenna 1, then -10 dB
(0.1 Watts) of power is received at antenna 2.
 In practice, the most commonly quoted parameter in regards to antennas is S11. S11 represents how
much power is reflected from the antenna, and hence is known as the reflection coefficient (sometimes
written as gamma or return loss. If S11=0 dB, then all the power is reflected from the antenna and
nothing is radiated. If S11=-10 dB, this implies that if 3 dB of power is delivered to the antenna, -7 dB is
the reflected power. The remainder of the power was "accepted by" or deliverd to the antenna. This
accepted power is either radiated or absorbed as losses within the antenna. Since antennas are typically
designed to be low loss, ideally the majority of the power delivered to the antenna is radiated.

Resonant
 An antenna is said to be resonant if its input impedance is entirely real, i.e. Zin = R + j*0.
 In this case the voltage and current are in phase at the antenna's terminals. This property makes the
impedance matching of an antenna to a transmission line and receiver easier, as the imaginary part of the
impedance does not need tuned out.
 In addition, when viewing the frequency plot of S11 for an antenna, there is often a large decrease in the
magnitude of S11 around the resonant frequency, indicating that power is radiated well around this
frequency.

Axial Ratio
 The axial ratio is the ratio of orthogonal components of an E-field. A circularly polarized field is made
up of two orthogonal E-field components of equal amplitude (and 90 degrees out of phase). Because the
components are equal magnitude, the axial ratio is 1 (or 0 dB).
 The axial ratio for an ellipse is larger than 1 (>0 dB). The axial ratio for pure linear polarization is
infinite, because the orthogonal components of the field is zero.
 Axial ratios are often quoted for antennas in which the desired polarization is circular. The ideal value of
the axial ratio for circularly polarized fields is 0 dB. In addition, the axial ratio tends to degrade away
from the main beam of an antenna, so the axial ratio may be indicated in a spec sheet (data sheet) for an
antenna as follows: "Axial Ratio: <3 dB for +-30 degrees from main beam". This indicates that the
deviation from circular polarization is less than 3 dB over the specified angular range.

Required Equipment in Antenna Measurements
 The required equipment for antenna measurements include:
 A source antenna and transmitter - This antenna will have a known pattern that can be used to illuminate
the test antenna
 A receiver system - This determines how much power is received by the test antenna
 A positioning system - This system is used to rotate the test antenna relative to the source antenna, to
measure the radiation pattern as a function of angle.

Required Equipment in Antenna Measurements
 The Source Antenna should of course radiate well at the desired test frequency. It must have the desired
polarization and a suitable beam width for the given antenna test range. Source antennas are often horn
antennas, or a dipole antenna with a parabolic reflector.
 The Transmitting System should be capable of outputting a stable known power. The output frequency should
also be tunable (selectable), and reasonably stable (stable means that the frequency you get from the
transmitter is close to the frequency you want).
 The Receiving System simply needs to determine how much power is received from the test antenna. This can
be done via a simple bolometer, which is a device for measuring the energy of incident electromagnetic
waves. The receiving system can be more complex, with high quality amplifiers for low power measurements
and more accurate detection devices.
 The Positioning System controls the orientation of the test antenna. Since we want to measure the radiation
pattern of the test antenna as a function of angle (typically in spherical coordinates), we need to rotate the test
antenna so that the source antenna illuminates the test antenna from different angles. The positioning system
is used for this purpose.

Anechoic Chambers
 Anechoic chambers are indoor antenna ranges. The walls, ceilings and floor are lined with special
electromagnetic wave absorbering material.
 Indoor ranges are desirable because the test conditions can be much more tightly controlled than that of
outdoor ranges.
 The material is often jagged in shape as well, making these chambers quite interesting to see.
 The jagged triangle shapes are designed so that what is reflected from them tends to spread in random
directions, and what is added together from all the random reflections tends to add incoherently and is
thus suppressed further.
 A picture of an anechoic chamber is shown in the following picture, along with some test equipment:

Anechoic Chambers
 The drawback to anechoic chambers is that they often need to be quite large. Often antennas need to be
several wavelengths away from each other at a minimum to simulate far-field conditions. Hence, it is
desired to have anechoic chambers as large as possible, but cost and practical constraints often limit their
size.

Antenna and It's Parameters by YEASIN NEWAJ

Antenna and It's Parameters by YEASIN NEWAJ

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Antenna and It's Parameters by YEASIN NEWAJ