CHAPTER 5MICROWAVE ANTENNAS

5.1.1:INTRODUCTION OF

MICROWAVE ANTENNA

RADIO WAVE PROPAGATION INTRODUCTION

EM waves travel in straight lines, unless acted upon by some outside force. They travel faster through a vacuum than through any other medium.

As EM waves spread out from the point of origin, they decrease in strength in what is described as an inverse square relationship.

For example: a signal 2 km from its starting point will be only 1/4 as strong as that 1 km from the source. A signal 3 km from the source will be only 1/9 that at the 1 km point.

RADIO WAVES

Electromagnetic radiation comprises both an Electric and a Magnetic Field. The two fields are at right-angles to each other and the direction of propagation is at right-

angles to both fields. The Plane of the Electric Field defines the Polarisation of the wave.

z

x

y

Electric Field, E

Magnetic Field, H

Direction of Propagation

POLARIZATION

The polarization of an antenna is the orientation of the electric field with respect to the Earth's surface and is determined by the physical structure of the antenna and by its orientation.

Radio waves from a vertical antenna will usually be vertically polarized.

Radio waves from a horizontal antenna are usually horizontally polarized.

Direction of Propagation

Direction of Propagation

Direction of Propagation

Direction of Propagation

ECM 717 - BY DR MOHD TARMIZI ALI

Horizontally polarized directional yagi antenna

Vertically polarized omnidirectional dipole antenna ECM 717 - BY DR MOHD

TARMIZI ALI

What is an ‘antenna’?

An antenna is a device that is made to efficiently radiate and receive radiated electromagnetic waves.

An antenna is an electrical conductor or system of conductors. Transmission - radiates electromagnetic

energy into space Reception - collects electromagnetic energy

from space.

In two-way communication, the same antenna can be used for transmission and reception.

Transmitter

Receiver

5.1.2:TYPES OF

MICROWAVE ANTENNA

Antenna Types Horn Antenna Parabolic Antenna Slot Antenna Dipole Antenna Dielectric Antenna Printed Antenna Phase Array Antenna

Horn Antenna

Horn antennas are very popular at UHF (300 MHz-3 GHz) and higher frequencies.

Horn antennas often have a directional radiation pattern with a high antenna gain, which can range up to 25 dB in some cases, with 10-20 dB being typical.

Horn antennas have a wide impedance bandwidth, implying that the input impedance is slowly varying over a wide frequency range.

Horn Antenna

The gain of horn antennas often increases as the frequency of operation is increased.

This is because the size of the horn aperture is always measured in wavelengths, a higher frequency has a smaller wavelength.

Since the horn antenna has a fixed physical size, the aperture is more wavelengths across at higher frequencies.

Horn antennas have very little loss, so the directivity of a horn is roughly equal to its gain.

Types of horn antenna

Figure 1. E-plane horn antenna.Figure 2. H-Plane horn antenna.

Figure 3. Pyramidal horn antenna

Parabolic Antennas

Parabolic Antenna The most well-known reflector antenna is the

parabolic reflector antenna, commonly known as a satellite dish antenna.

Examples of this dish antenna are shown in the following Figures.

Figure 2. A random direcTV dish antenna on a roof.

Figure 1. The "big dish" antenna of Stanford University.

Parabolic Antenna Parabolic reflectors typically have a very high gain (30-

40 dB is common) and low cross polarization.

They also have a reasonable bandwidth, with the fractional bandwidth being at least 5% on commercially available models, and can be very wideband in the case of huge dishes.

The smaller dish antennas typically operate somewhere between 2 and 28 GHz.

The large dishes can operate in the VHF region (30-300 MHz), but typically need to be extremely large at this operating band.

Slot Antenna A slot antenna consists of a metal surface,

usually a flat plate, with a hole or slot cut out.

When the plate is driven as an antenna by a driving frequency, the slot radiates electromagnetic waves in similar way to a dipole antenna.

Often the radio waves are provided by a waveguide, and the antenna consists of slots in the waveguide.

Slot antennas are used typically at frequencies between 300 MHz and 24 GHz.

Slot Antenna Slot antennas are often used

at UHF and microwave frequencies instead of line antennas when greater control of the radiation pattern is required.

Widely used in radar antennas, for the sector antennas

used for cell phone base stations.

Often found in standard desktop microwave sources used for research purposes.

Slot Antenna

Dipole Antenna Dipole antenna with a very thin radius is considered.

For very small dipole antennas, the input impedance is capacitive, which means the impedance is dominated by a negative reactance value.

As the dipole gets larger, the input resistance increases, along with the reactance.

At slightly less than 0.5 the antenna has zero imaginary component to the impedance, and the antenna is said to be resonant.

If the dipole antenna's length becomes close to one wavelength, the input impedance becomes infinite.

Dipole antenna basics

Dipole antenna consists of two terminals or "poles" into which radio frequency current flows.

This current and the associated voltage causes and electromagnetic or radio signal to be radiated.

Dipole is generally taken to be an antenna that consists of a resonant length of conductor cut to enable it to be connected to the feeder.

The basic half wave dipole antenna

The current distribution along a dipole is roughly sinusoidal.

falls to zero at the end and is at a maximum in the middle.

voltage is low at the middle and rises to a maximum at the ends.

It is generally fed at the centre, at the point where the current is at a maximum and the voltage a minimum.

This provides a low impedance feed point which is convenient to handle.

High voltage feed points are far less convenient and more difficult to use.

When multiple half wavelength dipoles are used, they are similarly normally fed in the centre.

Here again the voltage is at a minimum and the current at a maximum.

Theoretically any of the current maximum nodes could be used.

Dipole polar diagram

The polar diagram of a half wave dipole antenna that the direction of maximum sensitivity or radiation is at right angles to the axis of the RF antenna.

The radiation falls to zero along the axis of the RF antenna as might be expected.

Polar diagram of a half wave dipole in free space

If the length of the dipole antenna is changed then the radiation pattern is altered.

As the length of the antenna is extended it can be seen that the familiar figure of eight pattern changes to give main lobes and a few side lobes.

The main lobes move progressively towards

the axis of the antenna as the length increases.

The dipole antenna is a particularly important form of RF antenna which is very widely used for radio transmitting and receiving applications.

The dipole is often used on its own as an RF antenna, but it also forms the essential element in many other types of RF antenna.

As such it is the possibly the most important form of RF antenna.

Radiation pattern and gain

Dipoles have a radiation pattern, shaped like a toroid (doughnut) symmetrical about the axis of the dipole.

The radiation is maximum at right angles to the dipole, dropping off to zero on the antenna's axis.

The theoretical maximum gain of a Hertzian dipole is 10 log 1.5 or 1.76 dBi.

The maximum theoretical gain of a λ/2-dipole is 10 log 1.64 or 2.15 dBi.

Dipole types

Short dipole Half-wave dipole Quarter-wave monopole Folded dipole

A short dipole is a physically feasible dipole formed by two conductors with a total length L very small compared with the wavelength λ.

The two conducting wires are fed at the center of the dipole.

The current is assumed to be maximal at the center (where the dipole is fed) and to decrease linearly to zero at the ends of the wires.

Typically a dipole antenna is formed by two quarter-wavelength conductors or elements placed back to back for a total length of L = λ/2.

The larger the differential voltage, the greater the current between the elements.

Short dipole Half-wave dipole

The quarter-wave monopole antenna is a single-element antenna fed at one end, that behaves as a dipole antenna.

The quarter-wave conductor and its image together form a half-wave dipole that radiates only in the upper half of space.

A folded dipole is a half-wave dipole with an additional wire connecting its two ends.

the antenna are folded back until they almost meet at the feed point, such that the antenna comprises one entire wavelength.

This arrangement has a greater bandwidth than a standard half-wave dipole.

Quarter-wave monopole Folded dipole

Dielectric Antenna

Dielectric Antenna An antenna in the form of a section of dielectric rod

excited by a radio wave guide or the post of a coaxial cable.

A surface wave is generated in the rod of the antenna and propagates along the axis of the rod.

Dielectric antennas are essentially traveling-wave antennas, consisting of elementary electric and magnetic dipoles.

The radiation maximum coincides with the axis of the rod, as does the maximum of any traveling-wave antenna.

The type of radiation of a dielectric antenna depends on the phase velocity of propagation of the surface wave, which decreases with an increase in the diameter of the dielectric rod and in the dielectric constant of its material.

The lower the phase velocity, the greater the length of

the rod.

As the phase velocity decreases, or as it approaches the speed of light in the surrounding medium (air), the dielectric rod begins to lose its wave-guide properties.

This leads to an abrupt decrease in the field intensity near the end of the rod, an increase of radiation into the medium surrounding the antenna (directly from the open end of the wave guide), and a decrease in the antenna’s efficiency.

The rods of dielectric antennas are made from dielectric materials with low attenuation of electromagnetic waves.

Dielectric antennas are used mainly in aircraft radio equipment, which operates on centimeter or decimeter wavelengths.

low cost alternative to free space high gain antenna designs such as Yagi-Uda and horn antennas, which are often more difficult to manufacture at these frequencies

Printed Antenna

Printed antenna technology used for wireless system

Printed antenna application are:- Arrays for low or medium directivity- Efficient radiators- Planar antenna

Printed Antenna Originated from the use of planar microwave

technologies.

The begin antenna printed in the mid 1970.

The layered structure with 2 parallel conductors separated by a thin dielectric substrate and the lower conductor acting as a ground plane.

Printed belongs to the class or resonant antennas. Printed antennas have found use in most classical microwave applications.

Operates typically from 1- 100 GHz.

Printed Antenna

Phase Array Antenna

A phased array is an array of antennas in which the relative phases of the respective signals feeding the antennas are varied in such a way that the effective radiation pattern of the array is reinforced in a desired direction and suppressed in undesired directions.

An antenna array is a group of multiple active antennas coupled to a common source or load to produce a directive radiation pattern.

Usually, the spatial relationship of the individual antennas also contributes to the directivity of the antenna array.

Phase Array Antenna Use of the term "active antennas" is

intended to describe elements whose energy output is modified due to the presence of a source of energy in the element or an element in which the energy output from a source of energy is controlled by the signal input.

One common application of this is with a standard multiband television antenna, which has multiple elements coupled together.

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5.1.3: TYPES OF FEEDER FOR PARABOLIC ANTENNA

Feeder Types Gregorian Cassegrain Horn feed Omnidirectional

Antenna Feed Antenna feed refers to the components of an antenna which

feed the radio waves to the rest of the antenna structure, or in receiving antennas collect the incoming radio waves, convert them to electric currents and transmit them to the receiver.

Antennas typically consist of a feed and additional reflecting or directive structures whose function is to form the radio waves from the feed into a beam or other desired radiation pattern.

Feed consists of a dipole driven element, which converts the radio waves to an electric current, and a coaxial cable or twin lead transmission line which conducts the received signal from the antenna into the house to the television receiver.

Gregorian Antenna

It is a type of parabolic antenna used at earth station

Its design is similar to Cassegrain antenna (secondary reflector is convex), only Gregorian antenna’s secondary reflector is concave

Gregorian antenna In another word, Gregorian antenna is a

double reflector antenna with the second reflector located at a distance greater than the main reflector

In modern world, Gregorian is less compact than Cassegrain in term of design but with a smaller second reflector thus the advantages of beam blocking result in more efficient antenna

Antenna diagram

Cassegrain Antenna.This is example

of cassegrain

antenna

Cassegrain antennas are a subcategory of reflector antennas. Reflector antennas have been used from discovery of electromagnetic wave propagationOnwards.Cassegrain antenna consists of two reflectors (primary and secondary) and a feeder. The main characteristics of Cassegrain antennas is their high directivity.The bigger diameter of antenna reflector is used, the bettergain is achieved.

Application Cassegrain Antenna.

RADAR. SPACE COMMUNICATION RADIO ASTRONOMY WIRELESS COMMUNICATION.

Operation cassegrain antenna In telecommunications and radar, a Cassegrain antenna is a parabolic antenna in

which the feed radiator is mounted at or behind the surface of the concave main parabolic reflector dish and is aimed at a smaller convex secondary reflector suspended in front of the primary reflector. The beam of radio waves from the feed illuminates the secondary reflector, which reflects it back to the main reflector dish, which reflects it forward again to form the desired beam.

Illustrated of

operation

cassegrain antenna

Cassegrain antenna. Closeup of the convex secondary reflector in a large satellite communications antenna in Pleumeur-Bodou, France

Cassegrain satellite communication antenna in

Sweden. The convex secondary reflector can be seen suspended above the dish, and the feed horn is visible projecting from the

center of the dish.

RADIATION PATTERN

Another important aspect of the antenna is the front-to-back ratio. It measures the directivity of the antenna. It is a ratio of energy which antenna is directing in a particular direction, which depends on its radiation pattern to the energy which is left behind the antenna or

wasted. The higher the gain of the antenna, the higher the front-to-back ratio is. A good antenna front-to-back ratio is normally 20 dB.

Advantage of cassegrain antenna

The feed antennas and associated waveguides and "front end" electronics can be located on or behind the dish, rather than suspended in front where they block part of the outgoing beam.

the feed antenna is directed forward, rather than backward toward the dish as in a front-fed antenna, the spillover side lobes caused by portions of the beam that miss the secondary reflector are directed upwards toward the sky rather than downwards towards the warm earth.

Dual reflector shaping: The presence of a second reflector in the signal path allows additional opportunities for tailoring the radiation pattern for maximum performance.

increase the focal length of the antenna, to improve the field of view Parabolic reflectors used in dish antennas have a large curvature and short focal length, to locate the focal point near the mouth of the dish, to reduce the length of the supports required to hold the feed structure or secondary reflector.

Disadvantage of cassegrain antenna

A disadvantage of the Cassegrain is that the feed horn(s) must have a narrower beamwidth (higher gain) to focus its radiation on the smaller secondary reflector, instead of the wider primary reflector as in front-fed dishes. The angular width the secondary reflector subtends at the feed horn is typically 10° - 15°, as opposed to 120° - 180° the main reflector subtends in a front-fed dish. Therefore the feed horn must be longer for a given wavelength.

Horn antenna Also known as microwave horn, it is an

antenna which is the waveguide is horn-shaped

This is to direct radio waves into a beam Usually used for UHF and microwave

frequency, which is above 300MHz They used as a feeder (called feed horn)

for larger structure such as parabolic antenna

Feed horn It is a small horn antenna used to convey

radio waves between transmitter and receiver, and also the reflector

Generally used in parabolic antenna Usually used in SHF

Operation In transmitting antenna, it convert the

radio frequency alternating current from transmitter to radio waves, which is then feed the radio waves to the antenna to be focused into beam

Operation In receiving antenna, the received radio

waves is gathered and focused by the antenna’s reflector to the feed horn, which is then converted into radio frequency amplified by the receiver

Omnidirectional antenna

This is example of

omnidirectional

antennna.

 An omnidirectional antenna is an antenna that has a non-directional pattern (circular pattern) in a given plane with a directional pattern in any orthogonal plane. Examples of omnidirectional antennas are dipoles and collinear antennas. In radio communication, an omnidirectional antenna is an antenna which radiates radio wave power uniformly in all directions in one plane, with the radiated power decreasing with elevation angle above or below the plane, dropping to zero on the antenna's axis. This radiation pattern is often described as "doughnut shaped".

Application of omnidirectional antenna

Cell phones Fm radio Walkie talkie Wireless computer network. Cordless phone. Gps

Operation of omnidirectional antenna.

The omnidirectional antenna radiates or receives equally well in all directions. It is also called the "non-directional" antenna because it doesnot favor any particular direction. The pattern for an omnidirectional antenna, with the four cardinal signals. This type of pattern is commonly associated with verticals, ground planes and other antenna types in which the radiator element is vertical with respectto the Earth's surface. Illustrate block diagram antenna omnidirectional antenna

Radiation pattern omnidirectional antenna

Omnidirectional antennas have a similar radiation pattern. These antennas provide a 360 degree horizontal radiation pattern. These are used when coverage is required in all directions (horizontally) from the antenna with varying degrees of vertical coverage. Polarization is the physical orientation of the element on the antenna that actually emits the RF energy. An omnidirectional antenna, for example, is usually a vertical polarized antenna.

Advantage of omnidirectional antenna

An omni directional antenne receives or transmits signals from all directions as in 360 degrees.

advantage

disadvantage

Signal strength is uniform with omnidirectional antennas.If you use an omnidirectional antenna all signals are noise sources look into thesame antenna gain. There is neither increase of the desired signal, nor suppression of undesired signals. On a crowded band, the omnidirectional antenna confers no SNRadvantage.

5.2:CHARACTERISTIC OF HORN & PARABOLIC

ANTENNA

Antenna Radiation Patterns

An antenna radiation pattern or antenna pattern is defined as a mathematical function or a graphical representation of the radiation properties of the antenna as a function of space coordinates.

In most cases, the radiation pattern is determined in the far field region and is represented as a function of the directional coordinates.

IdealizedPoint Radiator Vertical Dipole Radar Dish

Isotropic Omnidirectional Directional

Types of Radiation Patterns

Antenna Radiation Pattern

Main beam / side lobe region

The peak of the main beam represents the highest level of field strength and approximately 70% of the radiated energy is enclosed.

The side lobe region represents a potential source of interference into the communication link, and for this reason is generally required to be of low level.

Antenna Side lobes

While most of the power radiated by an antenna is contained in the ”main lobe", a certain amount of power can be transmitted, (or received), in off-axis directions.

Side lobes are an intrinsic property of antenna radiation and cannot be completely eliminated.

However, side lobes are also due to antenna defects that can be minimized with proper design.

Horn Antenna Radiation Pattern

This antenna is simulated using a commercial solver.

The radiation pattern at 2 GHz is shown in Figure 1.

The gain of the horn is 18.1 dB in the +z-direction.

Figure 1

Parabolic Antenna Radiation Pattern This example will be for a parabolic dish reflector

with the diameter of the dish D equal to 11 wavelengths.

A circular horn antenna will be used as the feed.

The maximum gain from the physical aperture is the actual gain is 29.3 dB = 851, so we can conclude that the overall efficiency is 77%.

Gain, directivity and efficiency

Gain and directivity are quantities which define the ability to concentrate energy in a particular direction and are directly related to the antenna radiation pattern.

It includes all ohmic and dissipative losses arising from conductivity of metal and dielectric loss.

Antenna efficiency is a coefficient that accounts for all the different losses present in an antenna system.

Antenna Gain

• The fields across the aperture of the parabolic reflector is responsible for this antenna's radiation.

• Directivity or gain is the ratio of the power radiated by an antenna in its direction of maximum radiation to the power radiated by a reference antenna in the same direction.

• Is measured in dBi (dB referenced to an isotropic antenna) or dBd (dB referenced to a half wavelength dipole).

Antenna Gain Parabolic Antenna Gain,

G = 6D2/2

where D = diameter

Horn Antenna Gain

G = 10A/2

A =flange area

Antenna Efficiency

Antenna efficiency is affected by:

1.The sub reflector and supporting structure blockage.2.The main reflector rms surface deviation.3.Illumination efficiency, which accounts for the non uniformity of the illumination, phase distribution across the antenna surface, and power radiated in the side lobes.4.The power that is radiated in the side lobes.

The radiation efficiency is the usual efficiency that deals with ohmic losses.

Horn antennas are often used as feeds, and these have very little loss.

Parabolic reflector is typically metallic with a very high conductivity, this efficiency is typically close to 1 and can be neglected.

Radiation Efficiency

Effective Aperture

The effective antenna aperture is the ratio of the available power at the terminals of the antenna to the power flux density of a plane wave incident upon the antenna, which is polarization matched to the antenna.

If there is no specific direction chosen, the direction of maximum radiation intensity is implied.

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