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Summer 2013 © Tim Pratt REU June 2013 Antennas Tim Pratt, Instructor
Topics Antenna types Antenna gain and beamwidth Antenna patterns Yagi, horns and helix Reflector antennas Phased array principles Fixed beam phased array antennas Electronically steered phased arrays Summer 2013 © Tim Pratt
Summer 2013 © Tim Pratt Antennas All antennas serve several purposes Generate controlled beam Provide gain Interface from waveguide or feed line to medium (air) Antennas are often the limiting factor in a radio communication system Definition of antenna gain: Increase in power radiated in one direction relative to an antenna that radiates equally in all directions
Fig 10.1 Antenna Gain Summer 2013 © Tim Pratt Isotropic or omni-directional antenna G = 1 or 0 dB Directional antenna Axis or boresight G >> 1 Dish
Summer 2013 © Tim Pratt Fig 10.2 Low Gain, Wide Beam Antennas Dipole Monopole Helix Open ended waveguide Small hornGain 0 to 3 dB Patch /4 /2 Ground plane
Cellular Handset Antennas Antenna must be nearly omni-directional Simplest antenna is a monopole Length is one quarter wavelength At 2000 MHz, = 0.15 m = 15 cm = 6 inches Quarter wavelength = /4 = 3.75 cm or about 1.5 ins Older cell phones had antenna on top Newer phones bury antenna inside case Ground plane is user’s hand Wider bandwidth requires a more sophisticated antenna GPS signals are at 1575 MHz – a folded F may be used Summer 2013 © Tim Pratt
Summer 2013 © Tim Pratt Transmission Lines Antennas must be fed by transmission lines Open wire pair - rarely used now, low frequencies only Coaxial line - available as rigid or flexible Impedance is set by d 1, d 2, dielectric constant Impedances typically 50 ohms or 75 ohms Waveguide – hollow rectangular or circular tube Electric field in waveguide controls radiation
Summer 2013 © Tim Pratt Fig 10.3 Rectangular Waveguide TE 10 is the dominant mode in rectangular waveguide Waveguide has dimensions a = 0.6 to 0.9 b < 0.5 E-filed is zero parallel to a conducting surface TE 10 E-field distribution b a
Summer 2013 © Tim Pratt Fig 10.4 Circular Waveguide TE 11 is the dominant mode in circular waveguide Waveguide has dimensions D ~ 1 E field must terminate normal to a conducting surface TE 11 E-field distribution
Summer 2013 © Tim Pratt Fig 10.5 Planar Transmission Lines Stripline and microstrip w t Dielectric, Impedance is set by w, t, Thickness of dielectric must be controlled carefully at high microwave frequencies Microstrip Substrate
Summer 2013 © Tim Pratt Medium Gain Antennas Higher gain can be obtained with horns and traveling wave antennas Horns are aperture antennas Flare out waveguide to make aperture Field in aperture is same as in waveguide Must be a plane wave for maximum gain Traveling wave antennas are long – helix and Yagi
Summer 2013 © Tim Pratt Waveguide horn with large aperture Maximum gain ~ 25 dB Phase front in horn aperture is curved Phase center E field In waveguide Fig 10.6 Waveguide Horn
Summer 2013 © Tim Pratt Traveling Wave Antennas Yagi Helix These antennas have small aperture, medium gain Gain of Yagi antenna is approximately equal to the number of elements (not in dB) E.g. 20 elements, G ~ 13 dB Elements are spaced about / 4 Reminder: dB value = 10 log (P2 / P1) Never 20 log [ ]
Summer 2013 © Tim Pratt Reflector Dipole Directors (passive) Feed line End fire array Gain is roughly equal to the number of directors (not in dB) Fig 10.7 Yagi (-Uda) Ante nna
Summer 2013 © Tim Pratt High Gain Antennas In order of popularity: Reflector with feed Phased array Lens with feed (rare) Phased arrays are generally much more expensive than reflector antennas for a given gain Phased arrays allow electronic steering of beam – used mainly in radars
Summer 2013 © Tim Pratt Reflector (Dish) Waveguide horn feed Edge illumination - 10 to –13 dB Feed pattern Fig 10.8 Front Fed Reflector Antenna Parallel rays
Unit 2 Analysis of Antennas Gain and beamwidth Antenna patterns Examples Beam squint Summer 2013 © Tim Pratt
Summer 2013 © Tim Pratt Antenna Gain Accurate formula for Gain (not in dB) G = A 4 A / 2 = 4 A e / 2 A = physical area of aperture A e = effective area of aperture = wavelength A = aperture efficiency Aperture efficiency is typically 65% in well designed antenna For an antenna with a circular aperture diameter D G = A ( D / ) 2
Summer 2013 © Tim Pratt Approximation for Antenna Gain Approximate formula for G [empirical] G = 33,000 / ( a b ) [not dB] where a and b are antenna beamwidths in degrees in two orthogonal planes E.g. Azimuth and Elevation Equal beamwidths: G = 33,000 / 2 This formula is empirical and not particularly accurate It is useful for finding gain when beamwidths are known Example: Satellite transmit antenna creates 3 o x 4 o beam Gain is approximately 33,000 / 12 = 2750 or 34.4 dB
Summer 2013 © Tim Pratt Plane wave aperture Main beam sidelobes Aperture is derived from concept in optics Same effect when a plane wave is created by an antenna Fig 10.9 Creation of Antenna Pattern with an Aperture
Antenna Pattern Antenna pattern shows the distribution of transmitted power with angle for axis of beam (where gain is maximum) - see Fig Usually plotted in Cartesian coordinates with relative gain on vertical axis and maximum gain set to 0 dB Beamwidth is defined between half power (-3 dB) angles Pattern has main beam and sidelobes Gain is highest and sidelobes largest with uniform illumination – constant field across aperture Tapered illumination has lower field at aperture edge Gain is lower, beam is broader and sidelobes are lower Summer 2013 © Tim Pratt
Summer 2013 © Tim Pratt Relative Gain dB 0o0o Uniform illumination Tapered illumination Fig Antenna Patterns 0 dB dB axis -3 dB 3 dB beamwidth angle First sidelobeMain beam First null
Reciprocity in Antennas Gain and Pattern of any antenna is always same for transmit and receive We usually think of antenna pattern of transmitting antenna Same pattern applies to antenna when receiving Principle is called reciprocity Summer 2013 © Tim Pratt
Summer 2013 © Tim Pratt Rayleigh Range Antenna requires uniform phase across aperture Field radiates from antenna, diffracts in far field (long distance from antenna) Forms far field pattern at R > D 2 / Pattern is constant beyond Rayleigh Range R = 2 D 2 / Beamwidth in far field is set by aperture dimension D and illumination Typical 3 dB beamwidth for reflector antenna is 75 / D Example: Antenna with 5 m aperture diameter at 12 GHz = m, 3 dB B/W = 75 x / 5 = o
Summer 2013 © Tim Pratt Antenna Beamwidths and Sidelobes Beamwidth depends on aperture dimension D and illumination of aperture Examples: Uniform illumination = 51 D / degrees Tapered illumination > 57 D / degrees Depends on taper - heavier taper, wider beam First sidelobe peak: Uniform illumination dB linear, dB circular Tapered illumination << dB
Summer 2013 © Tim Pratt Antenna Example Large antenna at 6 GHz with 30 m aperture, 65 % aperture efficiency G = 10 log ( 0.65 x ( D / ) 2 ) At 6 GHz, = 0.05 m G = 10 log ( 0.65 x ( x 30 / 0.05) 2 ) = 10 log ( 0.65 x (1885) 2 ) = 63.6 dB This is the upper limit for gain with a reflector antenna Holding surface accuracy of paraboloidal dish for diameters > 30 m is difficult
Summer 2013 © Tim Pratt Antenna Example 3 dB Beamwidth Typical value is 75 / D degrees For D = 30 m, = 0.05 m 3 dB Beamwidth = degrees Antenna will have to be steered to follow any signal source that moves more than 0.03 degrees, such as a GEO satellite Typical cost: $ 5 M
Summer 2013 © Tim Pratt Antenna Imperfections Blockage of aperture causes null filling, higher sidelobes Phase errors caused by surface errors also cause null filling Asymmetric phase errors cause coma distortion Sidelobes on one side of beam are higher than on other Seen when feed is displaced transverse from focus of a reflector antenna Displacing feed gives beam squint
Summer 2013 © Tim Pratt ReflectorOff-axis feed Distorted wavefront phase Fig Beam Squint with Off-axis Feed Focus Center ray
Summer 2013 © Tim Pratt Gain, dB 0 angle Coma distortion Fig Antenna Pattern with Off-axis Feed 0 Nulls filled Low sidelobes High sidelobes
Summer 2013 © Tim Pratt Reflector Antenna Configurations Front feed: Feed blocks aperture Offset front feed: Feed below aperture, avoids blockage Cassegrain : Dual reflector - sub reflector inside focus of main reflector Gregorian: Dual reflector - sub reflector outside focus of main reflector
Summer 2013 © Tim Pratt Reflector (Dish) Waveguide horn feed Edge illumination - 10 to –13 dB Feed pattern Fig Front Fed Reflector Antenna Parallel rays
Summer 2013 © Tim Pratt Offset Reflector Feed Feed pattern Fig Offset Front Fed Reflector Parallel rays This configuration is used by Directv and Dish network for direct to home satellite TV reception. The feed and LNB are below the antenna beam, avoiding blocking.
© Tim Pratt January 2013 Fig DBS-TV receive antennas Offset front fed dish Summer 2013
© Tim Pratt Main Reflector Paraboloid Feed Fig Cassegrain Antenna Secondary Reflector (sub-reflector) Hyperboloid Parallel rays Cassegrain antennas are used mainly for large earth stations that need high gain antennas Main reflector focus
Summer 2013 © Tim Pratt Fig Large Cassegrain antenna - fully steerable
Summer ECE 4644Telecomms - II Copyright Tim Pratt 2013 Fig Echostar transmit / receive station © Tim Pratt 2013
Summer 2013 © Tim Pratt Main Reflector Paraboloid Feed Fig Gregorian Antenna Sub-reflector Paraboloid Gregorain antennas are used for mid size transportable earth stations on satellite trucks Main reflector focus
Summer 2013 © Tim Pratt Fig Mobile earth station with Gregorian antenna
Summer 2013 © Tim Pratt Reflector Antennas Much of the development of antennas came from radar requirements Surveillance radar needs high gain antenna Beam scans horizon to warn of approaching aircraft, ships, missiles Scan rate is typically 6 to 10 rpm Tracking antenna: radar locks to target Needs smaller antenna with high slew rate, tracking beams
Summer 2013 © Tim Pratt Phased Arrays Uses same aperture illumination principles as reflector antenna Can be a fixed array or an electronically steered array Electronic steering is used mainly in radar antennas Beam is scanned electronically with variable phase shifters Multiple beams are possible Random scan possible (defeat ECM) Nulling of jammers is possible Complex and expensive
Summer 2013 © Tim Pratt Radar Antennas The ideal radar antenna is a phased array Advantages: Great flexibility Multiple functions Disadvantages: Cost Complexity
Summer 2013 © Tim Pratt Fig Planar Phased Array with Square Aperture a a Aperture area = a 2 Element spacing = d N elements, with N = ( a/d ) 2 d Element
Summer 2013 © Tim Pratt Phased Array Antennas Phased array is made up of many active elements Element is a dipole, helix, open ended waveguide – any small antenna with gain ~ 0 dB Element spacing is typically d = 0.6 wavelengths Look at number of elements needed: For gain of G = 36 dB = 4000 = A 4 A / 2 Assume A = 0.6 A / 2 = 531 and a = = 23 For d = 0.6 N = 38 elements per side Total number of elements in array is N 2 = 38 2 = 1444
Summer 2013 © Tim Pratt Phased Array Antennas If beamwidth = 1.4 o Length of side increases to 46 Area increases to 4 x = Gain increases to = 42 dB N = 76 elements per side Total number of elements in the array is N 2 76 x 76 = 5776 Making and driving 5776 microwave devices is expensive
Summer 2013 © Tim Pratt Phased Array Antennas Phased array design and cost are dominated by requirement for thousands of active elements Useful number of elements is in range 1000 to 10,000 for array scanned in two planes Example: 5,000 elements at $1000 each gives phased array cost of $ 5M A reflector antenna will cost much less Electronic scanning in one plane only reduces cost
Summer 2013 © Tim Pratt Phased Array Antennas High cost of an electronically steered phased array antenna can be justified when: Antenna performs multiple functions Antenna must have good ECCM capability Random beam pointing is required Rapid beam steering is required Variable dwell time is needed Radar has been the primary user of phased arrays
Summer 2013 © Tim Pratt Fig Pave Paws phased array antenna Long range VHF radar for detection of ICBMs
Summer 2013 © Tim Pratt Fig A/N SPY-1 phased array radar antenna
Summer 2013 © Tim Pratt Fig A/N SPY-1 phased array radar antenna
Summer 2013 © Tim Pratt Fig Phased array using slotted elements Flat panel is scanned mechanically
Summer 2013 © Tim Pratt One Dimensional Phased Array Simplest phased array is a linear (1-D) array Elements are in a straight line, spaced d wavelengths apart Element spacing d must be in range 0.5 to 1.0 wavelengths Used to create omni directional antennas with a narrow beam in the vertical plane
Summer 2013 © Tim Pratt Broadside beam Dipoles with uniform phase Fig Omni directional dipole array Transmitted RF wave is vertically polarized
Summer 2013 © Tim Pratt Summer 2013 © Tim Pratt Beam is depressed to improve coverage of ground Dipoles with progressive phase shift Fig Omni directional dipole array Transmitted RF wave is vertically polarized 10 o 0o0o 20 o 30 o 40 o Phase
Summer 2013 © Tim Pratt Equal time delays TT TT TT TT dipoles wavefront wavelets Fig Series Fed Linear Array TT Beam direction load
Cellular Phone Base Station Antennas Cell phone base stations use linear array antennas like the one shown in Fig The linear array makes a narrow beam in the horizontal plane that is tilted down a little for best coverage over the earth’s surface Directional elements can be used to make the beam cover a sector Cell phone towers with a large number of antennas are covering several frequency bands and providing sector coverage to increase the number of users Summer 2013 © Tim Pratt
Summer 2013 © Tim Pratt Fig Cell phone tower with sector and omni antennas. Small dishes link tower to cell phone HQ and other towers.
Summer 2013 © Tim Pratt Fig Cell phone tower with sector antennas
Summer 2013 © Tim Pratt Fig Cell phone tower disguised as a tree
Summer 2013 © Tim Pratt Progressive time delays TT dipoles wavefront wavelets Fig Parallel Fed Linear Array TT TT TT Splitters TT Beam direction
Summer 2013 © Tim Pratt Beam Steering Beam is steered by changing relative phase between elements Fixed time (phase) delays give fixed beam Cell phone tower antenna has fixed beams Electronically controlled phase shifter gives movable beam Used mainly in military radars because of high cost Elements must be excited with correct amplitude distribution to control sidelobes
Summer 2013 © Tim Pratt Beam Steering Time delays are difficult to achieve Phase delay is used instead Relative phasing between elements determines beam direction Analyze for transmit case … apply reciprocity for receive Consider case of two adjacent elements One element has phase shift 0 o Adjacent element has phase shift Beam direction is to the array normal where = (2 / ) d sin
Summer 2013 © Tim Pratt o0o d d x x = d sin = (2 / ) d sin Fig Setting Beam Angle Broadside Beam angle
Summer 2013 © Tim Pratt To form beam at angle we need progressive phase delay along array of 0 o, , 2 , 3 , 4 , 5 … Eventually N > 360 o Reset phase to N o Beam is correctly pointed only in steady state Beam transient occurs as wavelets emerge from elements Beam Steering
Summer 2013 © Tim Pratt Beam Steering Example #1: Steer beam to 30 o from normal (broadside) Element spacing d = 0.6 = (2 / ) d sin = 2 x 0.6 x sin 30 o = 1.2 x ½ = 0.6 = 108 o We must insert 108 o phase shift between elements
Summer 2013 © Tim Pratt Phase Shifts in degrees wavefronts Fig Parallel fed linear array scanned 30 o 108 Splitters 108 o Beam direction 30 degrees
Summer 2013 © Tim Pratt Beam Steering Example #2: Steer beam to 45 o from normal Element spacing d = 0.6 = (2 / ) d sin = 2 x 0.6 x sin 45 o = 1.2 x ½ = = 153 o We must insert 153 o phase shift between elements
Summer 2013 © Tim Pratt Phase shifts in degrees wavefronts Fig Linear array scanned 45 o 153 Splitters Beam direction 45 degrees o
Conclusion All radio transmitters and receivers need antennas Simplest antenna is a monopole or dipole Omni-directional radiation pattern, low gain Used in mobile and portable radios – e.g. cell phone Reflector and phased array antennas provide high gain and narrow beam Used mainly for microwave links, satellite comms and radars Summer 2013 © Tim Pratt
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