Prepared by MOHD TAUFIK BIN JUSOH @ TAJUDINAntenna & WaveguidesBasic Antenna Operationy Size of antenna is inversely proportional to frequencyy Relatively small antenna can efficiently radiate high frequency electromagnetic waves while low-frequency waves require relatively large antennas.y Every antenna has directional characteristic and radiate more energy in certain direction.y Directional characteristic of antennas are used to concentrate radiation in desired direction or capture energy arriving from a particular direction.y Electromagnetic wave reception occurs in an antenna because the Electromagneticflux of the wave cuts across antenna conductortherefore inducing a voltage into the conductor that varies with time. (same manner)y The induced voltage represents energy that the antenna absorbs from the passing waveBasic Antenna Operation (cont)y Basic antenna operation is best understood by looking at the voltage standing wave patterns on a transmission line, shown in figure belowRadiation from a transmission line:(a) transmission-line radiation; (b) spreading conductors/dipole antenna; (c) Marconi antenna/ vertical monopole; (d) Hertz antenna / half wave dipole antennaAntenna Equivalent Circuity In radio system, transmitters are connected to receivers through transmission lines, antennasand free space.y Electromagnetic waves are coupled from transmit to receive antennas through free space in a manner similar to the way energy is coupled from the primary to the secondary of a transmitter.y Degree of coupling of antennas is much lower than transformers; since electromagnetic waves are involve rather than just a magnetic waves.Antenna Equivalent Circuit (cont)y Antenna coupling system can be represented with a four-terminal network as in Figure 2(a).y Electromagnetic energy must be transferred from the transmitting antenna to free space and then from free space to the receiving antenna.y Figure 2(b) equivalent cct for a transmit antennay Figure 2(c)- equivalent cct for a receive antenna Figure 2 : (a) Antenna as a four-terminal network; (b) transmit antenna equivalent circuit; (c) receive antenna equivalent circuitAntenna Reciprocityy A basic antenna is a passive reciprocal device passive in that is cannot actually amplify a signaly Transmit antenna must be capable of handling high powers and therefore must be constructed with materials that can withstand high voltages and currents, such as metal tubing.y Receive antenna produce very small voltages and currents and can be constructed from small-diameter wire.y Standard antennas - have no active a components (diodes, transistors, FETs), therefore they are passive and reciprocal. y In practice active antenna does not exists. Active antennas is actually a combination of a passive antenna and LNA. Active antennas are nonreciprocal. y In many radio communication system, same antenna is use for transmitting and receiving (reciprocal). Must be constructed from heavy duty material.y Isolation is needed, a coupling device known as diplexer is use to direct the transmit and receive signal.y Bear in mind, active or passive antenna produce power loss whether they are use for transmitting or receiving signals.Radiation Patterny Radiation pattern is a polar diagram or graph representing fields strength of power densities at various angular positions relative to an antenna.y Provide information that describes how an antenna directs the energy to radiatey Information presented in the form of a polar plot for both horizontal (azimuth) and vertical (zenith or elevation) sweeps.y Define quantitative aspects such as 3 dB beamwidth, directivity, side lobe, levels and front to back ratio.y The radiation pattern could be divided intoy Main lobesy Side lobesy Back lobesRadiation Pattern (cont)Figure 3 : Radiation patterns:(a) absolute (fixed power) radiation pattern; (b) relative (fixed distance) radiation pattern; (c) relative (fixed distance) radiation pattern in decibels; and (d) relative (fixed distance) radiation pattern in decibels for an omnidirectional (point source) antennaRadiation Pattern (cont)y Near fields refer to pattern that is close to the antenna y During one half of a cycle, power is radiated from an antenna where some of the power is stored temporarily in the near field.y During second half of a cycle , power in the near field is returned to the antenna. y also known as induction field.D = antenna uiametei wavelengthy Farfield refer to pattern that is far from the antennay Power that reaches far field continues radiate outward and is never returned to the antenna.y Also known as radiation field since the radiated power is more important.Radiation Pattern (cont)y Radiation resistance y All the power supplied to an antenna is not radiated.y Some of it is converted to heat and dissipated .y Radiation resistance is somewhat unreal in that it cannot be measured directly. It is an AC antenna resistance and is equal to the ratio of the power radiated by the antenna to the square of the current at its feedpointWhere ;Rr = radiation resistance (ohm)Prad = power radiated by antenna(watts)i = antenna current at the feedpoint (ampere)Radiation Pattern (cont)y Antenna Efficiencyy Is the ratio of the power radiated by an antenna to the sum of the power radiated and the power dissipated=antenna efficiencyPrad=radiated power (watts)Pin = input power (watts)=Prad + Pdy or the ratio of the power radiated by an antenna to the total input powerWhere;Prad=radiated power (watts)Pd = power dissipated in antenna (watts)Radiation Pattern (cont)y Simplified equivalent circuit for an antenna, see Figure 4Figure 4y Some of the input power is dissipated in the effective resistance (ground resistance, corona, imperfect dielectric, eddy currents etc) and the remainder is radiated.y Since the total antenna power is the sum of the dissipated and radiated power, therefore in terms of resistance and current, antenna efficiency is;eieRe effective antenna iesistanceAntenna Gainy Directive gain is the ratio of the power density radiated in a particular direction to the power density to the same point by a reference antenna (isotropic antenna, lossless), assuming both antenna radiate the same amount of powery The maximum directive gain is called directivity.Where ;D = directivity (unitless)P = power density at some point with a given antenna (watts/msqr)Pref = power density at some point with a reference antenna (watts/msqr)y If the antenna is lossless, it radiates 100 % of the input powery For an isotropic reference, the power in dB of a half-wave dipole is approximately 2.15 dBAntenna Gain (cont)y Isotropic radiator is a sphere shape that radiates power equally in all direction simultaneously.y Antennas do not increase the transmit power but shape of the radiation field to lengthen or shorten the distance of the propagated wave.y The higher the gain the farther the wave will travel concentrating its output wave more tightly.y 2 types of referencesy Isotropic antenna : gain is given in dBiy Half wave dipole antenna is given in dBdy Manufacturers often use dBi in their marketing (to show a slightlyhigher gain)Antenna Gain (cont)Isotropicpattern, 0 dBiDipole pattern, 0 dBi0 (dBd) = 2.15 (dBi) 3 (dBd) = 5.15 (dBi)Effective Isotropic Radiated Power (EIRP)y Effective Isotropic Radiated Power is define as an equivalent transmit power and is express mathematically;Where;Dt = transmit antenna directive gain (unitless)y EIRP also can be written using input power and power gain as ;Where;EIRP (cont)Example 1y For a transmit antenna with power gain At = 10, and an input power , Pin = 100Watt, determine;a) EIRP in Watt , dBm, and dBWb) Power density at a point 10 km from the transmit antennac) Power density had an isotropic antenna been used with the same input power an efficiencyExample 1 : Solutiona)b)Example 1 : Solutionc) It can be seen that the power density at a point 10 km from the transmit antenna is 10 times greater with the given antenna than it would be had an isotropic radiator been used. To achieve the same power density , the isotropic antenna would require an input power 10 times greater or 1000 WExample 2For a transmit antenna with radiation resistance Rr= 72 Ohms, an effective antenna resistance, Re = 8 ohms, a directive gain , D=20, and an input power Pin = 100 W. Determine a) Antenna efficiencyb) Antenna gain (absolute and dB)c) Radiated power in watts, dBm, and dBWd) EIRP in watts, dBm and dB WExample 2 : Solutiona)b)c)Example 2 : Solutiond)Antenna Polarizationy Polarization the polarization of an antenna simply refers to the orientation of the electric field radiated from it.y A radio wave is made of electric field and magnetic field which are perpendicular to each othery The sum of the two fields is called electro-magnetic fieldy Energy is transferred back and forth from one field to the other in the process known as oscillationy Polarization is the physical orientation of the antenna in a horizontal or vertical position y Horizontal Polarization electric field is parallel to the groundy Vertical Polarization the electric field is perpendicular to the groundy Antennas that are not polarized in the same way are not able to communicate with each other effectivelyPolarization (Cont)Linear PolarizationElliptical PolarizationCircular PolarizationAntenna Beamwidthy Antenna beamwidth is simply the angular separation between the two half-power (-3 dB) points on the major lobe of an antennas plane radiation pattern, usually taken in one of the principle planesy The beamwidth for the antenna whose radiation pattern is shown in Figure 5Figure 5Antenna Beamwidth (cont)y Angle formed between points A, X, and B (angle is the beamwidth for the particular antenna.y Point A and B are half-power points and antenna beamwidth is sometimes called -3 dB beamwidth or half-power beamwidth.y Antenna gain is inversely proportional to beamwidth. The higher gain the narrowerbeamwidth.y An omnidirectional (isotropic) radiates equally well in all directions . It has a gain of unity and a beamwidth of 360 degreey Typical antenna beamwidths between 30 degdreeand 60 degreeAntenna Bandwidthy Antenna Bandwidth is vaguely defined as the frequency range over which antenna operation is satisfactory y Bandwidth is normally taken as the difference between the half-power frequency (difference between the highest and lowest frequencies of operation) but sometimes refers to variations in the antennas input impedance. y Bandwidth of antenna = half-power bandwidth (HPBW)y Antenna bandwidth is often expressed as a percentage of the antennas optimum frequency operation.Example 3 Determine the percent bandwidth for an antenna with an optimum frequency of operation of 400 MHz and 3dB frequencies of 380 MHz and 420 MHz.VSWRy Occurs when there is mismatched impedance (resistance to current flow, measured in Ohms) btw devices in a RF systemy Caused by an RF signal reflected at a point of impedance mismatchin the signal path y VSWR causes return loss, or loss of forward energy through a systemy VSWR is a ratioy Typical value of VSWR would be 1.5 : 1y The first two numbers relate the ratio of impedance mismatch against a perfect impedance matchy The second number is always 1, representing the perfect match y Impedance mismatch will reduce system efficiencyTypes of Antennasy Some examples of antennas are;y Shape or geometries y Wire antennas : dipole, loop, helixy Aperture : horn, sloty Printed antennas, patch, printed sloty Gain y High gain : dishy Medium gain : horny Low gain : dipole, loop, slot, patchy Beam Shapes : y Omnidirectional : dipoley Pencil beam : dishy Fan beam : arrayy Bandwidth y Wide band : log, spiral, helixy Narrowband : patch, slotTypes of Antennas (cont)Half-wave dipoley The linear half-wave dipole is one of the most widely used antennas at frequencies above 2 MHz.y At frequency below 2 MHz, the physical length of a half-wavelength antenna is prohibitive.y The half-wave dipole generally referred to as a Hertz antenna after Heinrich HertzFigure6 Idealized voltage and current distributions along a half-wave dipoleHalf-wave DipoleFigure 7 Half-wave dipole radiation patterns:(a) vertical (side) view of a vertically mounted dipole; (b) cross-sectional view; (c) horizontal (top) viewGrounded Antennay A monopole (single pole ) an antenna one-quarter wavelength long, mounted vertically with the lower end either connected directly to ground or grounded through the antenna coupling network, is called a Marconi antennay The characteristics of the Marconi antenna are similar to those of the Hertz antenna because of the ground-reflected waves.Figure 8 Quarter-wave grounded antenna:(a) voltage and current standing waves; (b) radiation patternAntenna Arraysy An antenna array is form when two or more antenna elements are combined to form a single antennay An antenna element is an individual radiator, such as a half or quarter-wave dipole. y The elements are physically in such a way that their radiation fields interact with each other, producing a total radiation pattern.y The purpose of an array is to increase the directivity of an antenna system and concentrate the radiated power within a smaller geographic area.Antenna Arraysy In essence, there are TWO types of antenna elements y Driven elementy Parasitic element (non-driven)y Driven - elements are directly connected to the transmission line and receive power from or are driven by a the sourcey Parasitic - elements are not connected to the transmission line; they receive energy through mutual induction with a driven element or other parasitic element.y Parasitic element is longer than driven elementFigure 9Antenna arrayOther type of antennas arrayy Broadside Arrayy End-fire arrayy Nonresonant Array : The Rhombic antennaFigure 10 Broadside antenna:(a) broadside array; (b) radiation patternOther type of antennas array(cont)y End-fire arrayFigure 11 End-fire antenna:(a) end-fire array; (b) radiation pattern (side view)Other type of antennas array(cont)y Nonresonant Array : The Rhombic antennay Ideal for HFtransmission(range 3 MHz 30 MHz)Figure 12 Rhombic antenna:(a) rhombic array; (b) radiation patternEffect of Gain, Bandwidth, front-to-back ration for half-wave dipole by adding directors and reflectorsSpecial Purpose Antennasy Folded Dipole Antennay Yagi- Uda Antennay Turnstile Antennay Helical Antennay Log-periodic Antennay Loop Antennay Phased Array AntennaSpecial Purpose AntennasFolded Dipoley The folded dipole is essentially a single antenna made up of two elementsFigure 13 (a) Folded dipole; (b) three-element folded dipoleSpecial Purpose Antennas (cont)Yagi-UdaAntennay A widely used antenna that commonly uses a folded dipole as the driven element.y A Yagi Uda is a linear array consists of a dipole and two or more parasitic elements.y ONE (1) use for reflector and ONE (1) or more use for director.y The driven element is a half-wavelength folded dipole, connected to transmission line. Use for receiving only.y The reflector is a straight aluminium rod approximately 5 % longer than dipole (driven element).y The director is cut approximately 5 % shorter than the driven element.y The spacing between elements is generally between 0.1 and 0.2 wavelength. Special Purpose Antennas (cont)y Typical directivity of the Yagi Uda is between 7 dB and 9 dB. y Bandwidth of the Yagi can be increased by using more than one folded dipole, each cur slightly different length.y The Yagi commonly use for VHFtelevision reception because its wide bandwidth (54 MHz 216 MHz)Figure 14 Yagi-Uda antenna:(a) three-element Yagi; (b) radiation patternSpecial Purpose Antennas (cont)Turnstile Antennay Form by placing two dipoles at right angles to each other, 90 degree out of phasey This will produce nearly omni-directional patterny Gain can up more than 10 dBFigure 15 (a) Turnstile antenna; (b) radiation patternSpecial Purpose Antennas (cont)Helical Antennay Helical antenna is a broadband VHF and UHF antenna that is ideally suited for application for which radiating circular rather than horizontal or vertical polarized EM waves are required.y Can be used as a single-element antenna or stacked horizontally or vertically in an array to modify its radiation pattern by increasing the gain and decreasing the beamwidth of the primary lobe.Figure 16 End-fire helical antennaaveguideA waveguide is a special type of transmission line that consists of a conducting metallic tube through which high frequency elelctromagneticenergy is propagated. Efficiently interconnect high frequency electromagnetic waves between an antenna and a transceiver.Waveguide (cont)y Parallel-wire transmission line, including coaxial cables, cannot effectively propagate EM energy above approximately 20 GHz.y Parallel-wire impractical for many UHF and microwave applications. Several alternative , including fiber optic and waveguides.y In simplest form, a waveguide is a hollow conductive tube, usually rectangular in cross section but sometimes circular or elliptical. y Waveguides do not conduct current but it reflects EM energy from its surface.Rectangular Waveguidey Rectangular wave guide is the most common form of waveguide.y To understand how rectangular waveguides work, it is necessary to understand the basic behavior of waves reflecting from a conducting surface.y Important characteristics or TEM waves in order to propagate in waveguides.y Phase Velocity and Group Velocityy Cut off Frequency and Cut off wavelengthy Modes of Propagationy Characteristic Impedancey Impedance Matchingy Transmission Line to Waveguide CouplingRectangular Waveguide (cont)Figure 17 :Cross-sectional view of a rectangular waveguideRectangular Waveguide (cont)Figure 18Electromagnetic wave propagation in a rectangular waveguideOther Type of Waveguidesircular Waveguidey Used in Radar and microwave applicationFigure 19 Circular waveguide with rotational jointOther Type of WaveguidesRidged Waveguidey More expansive to manufacture than a standard rectangular waveguidey But it has more loss per unit length than a rectangular waveguideFigure 20 Ridged waveguide:(a) single ridge; (b) double ridgeOther Type of WaveguidesFlexible Waveguidey Consists of spiral-wound ribbon of brass or copper.y Use for interconnects when several transmitters and receivers are needed.Figure 21Flexible waveguide