Antennas Dr. John S. Seybold November 9, 004 IEEE Melbourne COM/SP AP/MTT Chapters Introduction The antenna is the air interface of a communication system An antenna is an electrical conductor or system of conductors that performs; 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 with the use of suitable receiver protection The principle of reciprocity states that the transmit and receive characteristics of an antenna are identical Copyright August 00, John S. Seybold, All Rights Reserved
Types of Antennas Isotropic antenna (idealized) Radiates power equally in all directions Dipole antennas Half-wave dipole antenna (or Hertz antenna) Quarter-wave vertical antenna (or monopole antenna) Aperture antennas Parabolic reflective antenna Horn antenna Cassegrain antenna Lens antenna Directive beam antenna Many variations, folded dipoles, end-fed dipoles, loop, inverted Vee, phased array,. Copyright August 00, John S. Seybold, All Rights Reserved 3 Dipole Antennas 4 4 The half-wave dipole consists of two radiators, each a quarter wavelength long Other lengths may be used, with different radiation patterns, gains and radiation resistances Has a nominal broadside gain of.14 db Reduced gain off the ends of the dipole Forms the basis for the standard quarter wave antenna Antenna gains may be expressed in db (dbi) or dbd Copyright August 00, John S. Seybold, All Rights Reserved 4
Quarter-Wave Antennas Essentially one-half of a dipole antenna Ideally relies on a reflective ground plane to provide an image of the antenna to complete the dipole (monopole antenna) Real-world antennas tend to use counterpoises instead Some antennas may be shorter than a quarter wavelength but are electrically equivalent to a quarter wave (rubber duck) The gain of a quarter-wave antenna varies considerably depending upon its deployment A rubber duck is typically 3dBi When handheld near the head, a quarter wave antenna will have a nominal gain of 10 dbi Copyright August 00, John S. Seybold, All Rights Reserved 5 Monopoles Radiators Traps Radiators Insulating Supports Insulator Electrical Counterpoise s Conductive Support Matching Network Shielded Coaxial Feedline Insulator Shielded Coaxial Feedline Copyright August 00, John S. Seybold, All Rights Reserved 6
Aperture Antennas Asymmetrical gain patterns can be achieved by using asymmetrical apertures or illumination functions Tapering the aperture illumination reduces side lobe levels at the expense of main lobe broadening and gain (efficiency) reduction There is a Fourier transform relationship between the aperture illumination taper function and the antenna radiation pattern Copyright August 00, John S. Seybold, All Rights Reserved 7 Yagi Antenna Driven Element Reflector Direction of maximum gain Directors Copyright August 00, John S. Seybold, All Rights Reserved 8
Reflector Antennas Reflector LNB Lens Main Reflector LNB Feed Horn Sub Reflector Feed Support Sub Reflector Supports Offset Feed Reflector Antenna Cassegrain Antenna Copyright August 00, John S. Seybold, All Rights Reserved 9 Radiation Patterns Radiation pattern Graphical representation of radiation properties of an antenna Depicted as two-dimensional cross section May be plotted in rectangular or polar coordinates Reception pattern Receiving antenna s equivalent to radiation pattern Copyright August 00, John S. Seybold, All Rights Reserved 10
Antenna Pattern ( ) 0 Antenna Radiation Pattern 10 0 Gain (db) 30 40 50 60 70 80 30 5 0 15 10 5 0 5 10 15 0 5 30 Angle (deg) Beamwidth Front-to-back ratio Side lobe level Copyright August 00, John S. Seybold, All Rights Reserved 11 Antenna Pattern Parameters Beam width (or half-power, 3 db, beam width) Measure of directivity of antenna Usually assumed to be the half-power or 3 db width of the pattern measured in degrees or milliradians Front-to-back ratio The ratio of the gain at 0 degrees to the gain at 180 degrees Provides a measure of how well unwanted signals from the rear can be rejected Side lobe level Usually taken to be the peak gain of the first side lobe relative to the main lobe in db Has a significant impact on a systems ability to eliminate spatially diverse sources of interference Copyright August 00, John S. Seybold, All Rights Reserved 1
Circular Aperture Antenna Pattern Copyright August 00, John S. Seybold, All Rights Reserved 13 Antenna Directivity and Gain The directivity of an antenna is a metric of its radiation coverage Power density at d in max direction D = mean power density at d When the antenna losses are included, this becomes the antenna gain Power Density at d in max direction G =η P T 4 π d P T is the power applied to the antenna terminals 4πd is the area of a sphere with radius d η is the total antenna efficiency, which includes resistive and taper losses of the antenna (η = η T η R ) Copyright August 00, John S. Seybold, All Rights Reserved 14
Antenna Gain Antenna gain Power output, in a particular direction, compared to that produced in any direction by a perfect omnidirectional antenna (isotropic antenna) Can be expressed in dbi, decibels relative to an ideal isotropic radiator or dbd, decibels relative to an ideal dipole Effective area Related to physical size and shape of antenna A e = η A p Where A p is the physical area of the antenna Copyright August 00, John S. Seybold, All Rights Reserved 15 Calculating Antenna Gain If specific information on antenna gain is not available, the gain of an aperture antenna can be estimated using 4π A P ηtηr 4π A G = = Where η is often assumed to be approximately 0.6 Works best for aperture antennas, difficult to apply to wire antennas or beams (effective height) If the physical area is not known, but the azimuth and elevation beam widths are known, we can use a rule of thumb 0,000 G θ θ AZ EL Where the 3 db beam widths are expressed in degrees (Skolnik, Introduction to Radar Systems) Copyright August 00, John S. Seybold, All Rights Reserved 16 e
Aperture Antenna Gain Example Given a circular aperture of 30 cm diameter, what is the gain at 39 GHz? A 30 cm diameter circular aperture has a physical area of 0.0707 m. Assuming an aperture efficiency of 60% yields A e = 0.044 m Using the expression for antenna gain and the wavelength of 7.69 mm yields G = 9005 Or expressed as decibels relative to an isotropic radiator as 10log(9005), or 39.5 dbi of gain. Copyright August 00, John S. Seybold, All Rights Reserved 17 Antenna Regions Far-Field (Fraunhoffer) Region Where D is the largest linear dimension of the antenna This is the region where the wavefront becomes approximately planer The apparent gain of the antenna is a function only of the angle (i.e. the antenna pattern is completely formed) D r > Radiating Near-Field (Transition region) The region between near and far field The antenna pattern is taking shape but is not fully formed Gain measurements will vary with distance D < r < π Reactive Near-Field Region where E and H are not orthogonal, Anything within this region will couple with the antenna and distort the pattern Gain is not a meaningful parameter here r < π Copyright August 00, John S. Seybold, All Rights Reserved 18
Antenna Radiation Regions Reactive Near-Field d π Radiating Near- Field Far-Field Region d=d / Copyright August 00, John S. Seybold, All Rights Reserved 19 Radiation Region Example How much separation is required between a 140 MHz quarter-wave monopole antenna and a 450 MHz quarter-wave monopole antenna if both are mounted on the roof of an automobile and near-field coupling is to be avoided? To avoid mutual, near-field coupling, each antenna must be outside of the reactive near-field of the other. The reactive near-field of the 140 MHz antenna is larger than that of the 440 MHz antenna, so it determines the minimum required separation. d min π where =.14 m The result is that the minimum required separation is d min = 0.341 m Copyright August 00, John S. Seybold, All Rights Reserved 0
Antenna Impedance Every antenna will present a certain amount of radiation resistance (impedance) to its source This impedance may be a function of frequency For best operation, the transmitter and receiver should be matched to the antenna impedance Mismatched components result in signal reflection, SWR and reduced power transfer (loss) Many antennas are 50 ohms as that is relatively standard in RF and microwave work Sometimes to achieve a match, loading coils or matching networks are used Copyright August 00, John S. Seybold, All Rights Reserved 1 Typical Characteristics for Various Aperture Illuminations Type of distribution, z < 1 Relative gain Half-power beamwidth Intensity of first sidelobe In degrees db below maximum intensity Uniform; A(z) = 1 1 51/d 13. Cosine; A(z) = cos n (πz/) n = 0 1 51/d 13. n = 1 0.810 69/d 3 n = 0.667 83/d 3 n = 3 0.575 95/d 40 n=4 0.515 111/d 48 Parabolic; A(z) = 1 -(1 - )z = 1.0 1 51/d 13. = 0.8 0.994 53/d 15.8 = 0.5 0.970 56/d 17.1 = 0 0.833 66/d 0.6 Triangular; A(z) = 1 - z 0.75 73/d 6.4 Circular; A(z) = (1-z ) 0.5 0.865 58.5/d 17.6 Cosine-squared plus pedestal; 0.33 + 0.66 cos (πz/) 0.88 63/d 5.7 0.08 + 0.9 cos (πz/), Hamming 0.74 76.5/d 4.8 d = aperture width (diameter) = wavelength Source: Introduction to Radar Systems, Merrill I. Skolnik, McGraw-Hill 1980 Copyright August 00, John S. Seybold, All Rights Reserved
Antenna Polarization Electromagnetic waves can be characterized by their polarization Elliptical polarization is a generalized polarization that encompasses linear and circular polarization Linear polarization means the the electric field portion of the electromagnetic wave exists in a plane (normal to the direction of propagation)- usually vertical or horizontal Circular polarization can be mathematically represented as the sum of a vertical and a horizontally polarized wave, 90 degrees out of phase Copyright August 00, John S. Seybold, All Rights Reserved 3 Circular Polarization z y E y y E Ey x E x Tilt Angle τ E 1 x The axial ratio is an important antenna parameter that describes the shape of the polarization ellipse. The axial ratio is defined as the ratio of the major axis to minor axis of the polarization ellipse when the phase angle between the linear polarization components, δ, is +90. By Definition, the axial ratio is always > 1 (0 db) Since it is a ratio of amplitudes (not powers), it can be expressed in db using 0log(AR). E x Copyright August 00, John S. Seybold, All Rights Reserved 4
Polarization Loss Linear antennas that are misaligned in orientation Circular antennas that are not truly circular Loss actually occurs between the wave and the receiving antenna, when circular polarization is nonideal For linear polarization, the polarization loss factor (power) is F = cos (τ) For nearly cirular polarization, the polarization loss factor is ( 1+ ARw )( 1+ ARr ) + 4ARw ARr + ( 1 ARw )( 1 ARr ) cos( [ τ w τ r ]) F = ( 1+ ARw )( 1+ ARr ) Copyright August 00, John S. Seybold, All Rights Reserved 5 Polarization Loss Loss Factor (db) 1.3 1. 1.1 1 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0. 0.1 Polarization Loss Factor vs. Axial Ratio 0 0 1 3 4 5 Antenna Axial Ratio (db) Incident Wave Axial Ratio 5 db 4 db 3 db db 1 db 0 db Polarization loss factor versus axial ratio for several different axial ratios (one for wave, the other for Rx antenna) Copyright August 00, John S. Seybold, All Rights Reserved 6
Pointing Loss When directional antennas are used, the link analysis should allow for some pointing loss (or tracking loss for dynamic systems) Antennas Aligned Antennas Misaligned Copyright August 00, John S. Seybold, All Rights Reserved 7 Some Antenna References 1. M. I. Skolnik, Introduction to Radar Systems, 3 rd Ed., McGraw-Hill, New York, 001. C. A. Balanis, Antenna Theory, Analysis and Design, nd Ed., Wiley, New York, 1997 3. K. Siwiak, Radiowave Propagation and Antennas for Personal Communications, nd ed., Artech House, Norwood, 1998 4. W. L. Stutzman, G. A. Thiele, Antenna Theory and Design, nd Ed., Wiley, Hoboken, 1998 5. J. D. Kraus, R. J. Marhefka, Antennas for All Applications, 3 rd Ed., McGraw- Hill, 00 6. J.Liberti, Jr., T. S. Rappaport, Smart Antennas for Wireless Communications: IS-95 and Third Generation CDMA Applications, Prentice-Hall, Upper Saddle River, 1999 7. J. S. Hollis, T. J. Lyon, L. Clayton, Microwave Antenna Measurements, nd Ed., Scientific Atlanta, Atlanta, 1970, Chapter 3 8. ARRL Antenna Handbook 9. R. C. Johnson, Antenna Engineering Handbook,McGraw-Hill, 199 Copyright August 00, John S. Seybold, All Rights Reserved 8
Summary Antenna gain is the gain over an isotropic radiator in the direction of maximum intensity usually expressed in dbi Antenna gain for an aperture antenna can be estimated from 4π A G = e The antenna pattern is not well formed until the far-field region is reached D r > Beam width is generally the angular distance between the 3 db points on the antenna pattern main lobe Antenna reception and radiation patterns are identical Polarization mismatch between Tx and Rx antennas reduces the received signal level Copyright August 00, John S. Seybold, All Rights Reserved 9