ECE 3317 Applied Electromagnetic Waves Prof. David R. Jackson Fall 018 Notes 1 Introduction to Antennas 1
Introduction to Antennas Antennas An antenna is a device that is used to transmit and/or receive an electromagnetic wave. Note: The antenna itself can always transmit or receive, but it may be used for only one of these functions in an application. Examples: Cell-phone antenna (transmit and receive) TV antenna in your home (receive only) Wireless LAN antenna (transmit and receive) FM radio antenna (receive only) Satellite dish antenna (receive only) AM radio broadcast tower (transmit only) GPS position location unit (receive only) GPS satellite (transmit only)
Introduction to Antennas (cont.) Antennas are often used for a variety of reasons: For communication over long distances, to have lower loss (see below) Where waveguiding systems (e.g., transmission lines) are impractical or inconvenient When it is desired to communicate with many users at once Power loss from antenna broadcast: Power loss from waveguiding system: 1/r (always better for very large r) r e α A r B 3
Introduction to Antennas (cont.) Main properties of antennas: Radiation pattern Beamwidth and Directivity (how directional the beam is) Sidelobe level Efficiency (power radiated relative to total input power) Polarization (linear, CP) Input Impedance Bandwidth (the useable frequency range) 4
Introduction to Antennas (cont.) Reflector (Dish) Antenna Ideally, the dish is parabolic in shape. Very high bandwidth Medium to high directivity (directivity is determined by the size) Linear or CP polarization (depending on how it is fed) Works by focusing the incoming wave to a collection (feed) point 5
Introduction to Antennas (cont.) Dipole Wire Antenna L λ / 0 Current (resonant) Very simple Moderate bandwidth Low directivity Omnidirectional in azimuth At resonance : Z = Ω 73 [ ] Most commonly fed by a twin-lead transmission line Linear polarization ( E θ, assuming wire is along z axis) The antenna is resonant when the length is about one-half free-space wavelength in 6
Introduction to Antennas (cont.) Dipole Wire Antenna (cont.) The bow-tie antenna has flared dipole arms, which increases the bandwidth. 7
Introduction to Antennas (cont.) Folded Dipole Antenna The folded dipole is a variation of the dipole antenna. It has an input impedance that is 4 times higher than that of the regular dipole antenna. At resonance : [ ] Z = 9 Ω in Compatible with TV twin lead Z 0 = 300 [ Ω] 8
Introduction to Antennas (cont.) Monopole Wire Antenna h h λ /4 0 At resonance : Z = Ω in 36.5 [ ] Feeding coax This is a variation of the dipole, using a ground plane instead of a second wire. Similar properties as the dipole Mainly used when the antenna is mounted on a conducing object or platform Usually fed with a coaxial cable feed 9
Introduction to Antennas (cont.) Monopole Wire Antenna (cont.) 10
Introduction to Antennas (cont.) Yagi Antenna This is a variation of the dipole, using multiples wires (with one reflector and one or more directors. Low bandwidth Moderate to high directivity Commonly used as a UHF TV antenna Prof. Yagi 11
Introduction to Antennas (cont.) Yagi Antenna (cont.) UHF Yagi UHF Yagi UHF Yagi VHF Log-periodic 1
Introduction to Antennas (cont.) Log-Periodic Antenna Beam This consists of multiple dipole antennas of varying lengths, connected together. High bandwidth Moderate directivity Commonly used as a VHF TV antenna 13
Introduction to Antennas (cont.) Log Periodic Antenna (cont.) 14
Introduction to Antennas (cont.) Typical Outdoor TV Antenna UHF Yagi VHF Log-periodic 15
Introduction to Antennas (cont.) Horn Antenna It acts like a loudspeaker for electromagnetic waves. High bandwidth Moderate directivity Commonly used at microwave frequencies and above Often used as a feed for a reflector antenna 16
Introduction to Antennas (cont.) Horn Antenna (cont.) Arno A. Penzias and Robert W. Wilson used a large horn antenna to detect microwave signals from the big bang (Nobel Prize, 1978). 17
Introduction to Antennas (cont.) Horn Antenna (cont.) This is a variety called the hoghorn antenna (a combination of horn+reflector). 18
Introduction to Antennas (cont.) Microstrip (Patch) Antenna y h W L Current ε r x L λ /= d 1 λ 0 ε r It consists of a printed patch of metal that is on top of a grounded dielectric substrate. Low bandwidth Low directivity (unless used in an array) Low-profile (h can be made very small, at the expense of bandwidth) Can be made by etching Easily fed by microstrip line or coaxial cable Can be made conformable (mounted on a curved surface) Commonly used at microwave frequencies and above 19
Introduction to Antennas (cont.) Microstrip (Patch) Antenna (cont.) 0
Introduction to Antennas (cont.) Dielectric Resonator Antenna (DRA) ε r The dielectric resonator antenna was invented by our very own Prof. Long! Cylindrical DRA It consists of a dielectric material (such as ceramic) on top of a grounded dielectric substrate. Moderate to large bandwidth Low directivity (unless used in an array) Commonly used at microwave frequencies and above 1
Introduction to Antennas (cont.) Dielectric Resonator Antenna (cont.) GPS antenna
Introduction to Antennas (cont.) Leaky-Wave Antenna y Slot π β = k 1 < k 0 0 ka 0 The wave is a fast wave. vp > c Air Rectangular waveguide x This allows the wave to radiate from the slot. Note: vp ω ω = > = c β k 0 3
Introduction to Antennas (cont.) Leaky-Wave Antenna (cont.) y k θ 0 b z β < k 0 k = k cosθ = β z 0 0 cos θ = β / k 0 0 A narrow beam is created at angle θ 0. 4
Antenna Radiation We consider here the radiation from an arbitrary antenna. S z r r ( r, θφ, ) x + - y "far field" r The far-field radiation acts like a plane wave going in the radial direction. 5
Antenna Radiation (cont.) How far do we have to go to be in the far field? Sphere of minimum diameter D that encloses the antenna. r r ( r, θφ, ) + - r > D λ 0 A derivation is given in the Antenna Engineering book: C. A. Balanis, Antenna Engineering, 4 th Ed., 016, Wiley. 6
Antenna Radiation (cont.) The far-field has the following form: z H y E x z S E = ˆ θ E + ˆ φe θ φ H = ˆ θ H + ˆ φh θ E H θ φ = η 0 φ TM z x E H y S E H φ θ = η 0 TE z Depending on the type of antenna, either or both polarizations may be radiated (e.g., a vertical wire antenna radiates only TM z polarization. 7
Antenna Radiation (cont.) The far-field Poynting vector is now calculated: 1 * S = E H 1 = ˆ + ˆ ˆ + ˆ 1 ˆ ( * * = r EH EH ) θ φ φ θ ( θ E E ) ( H H ) θ φ φ θ θ φ φ 1 E E r ˆ φ θ = Eθ + Eφ η0 η 0 1 E r ˆ θ = + η 0 0 * * E φ η E H E H θ φ φ θ = η 0 = η 0 8
Antenna Radiation (cont.) Hence we have ( ) 1 S = rˆ Eθ + Eφ η 0 or S E = rˆ η 0 Note: In the far field, the Poynting vector is pure real (no reactive power flow). 9
Radiation Pattern The far field always has the following form: jk r e = r 0 F (, θφ, ) E ( θφ, ) E r F E (, ) θφ Normalized far - field electric field In db: ( θφ, ) ( θ, φ ) F E db ( θφ, ) = 0log10 F E m m (, ) θ φ = direction of maximum radiation m m 30
Radiation Pattern (cont.) The far-field pattern is usually shown vs. the angle θ (for a fixed angle φ) in polar coordinates. z ( θφ, ) ( θ, φ ) F E db ( θφ, ) = 0log10 F E m m A pattern cut 30 θ 30 60 φ = 0 60-0 db -10 db 0 db θ m -30 db 10 10 150 150 31
Radiated Power The Poynting vector in the far field is (, θφ, ) S r F E ( θφ, ) 1 rˆ = η 0 r The total power radiated is then given by ππ ππ F E ( θφ, ) P ( ˆ rad = S r) r sinθdθdφ = sinθdθdφ η 0 0 0 0 0 Hence we have ππ 1 F Prad = E ( θφ, ) sinθdθdφ η 0 0 0 3
Directivity The directivity of the antenna in the directions (θ, φ) is defined as D ( θφ, ) P S rad r ( θφ, ) ( π r ) / 4 r The directivity in a particular direction is the ratio of the power density radiated in that direction to the power density that would be radiated in that direction if the antenna were an isotropic radiator (i.e., one that radiates equally in all directions). In db, D ( θφ, ) = 10log D( θφ, ) db 10 Note: The directivity is sometimes referred to as the directivity with respect to an isotropic radiator. 33
Directivity (cont.) The directivity is now expressed in terms of the far field pattern. Hence we have D ( θφ, ) P S rad r ( θφ, ) ( π r ) / 4 r F E ( θφ, ) 1 η 0 r D( θφ, ) 4πr r ππ 1 F = η 0 0 0 E ( ) θφ, sinθdθdφ Therefore, D ( θφ, ) = ππ 0 0 E ( ) ( θφ) F 4 πe, F θφ, sinθdθdφ 34
Directivity (cont.) Resonant half-wavelength dipole: Short dipole: D = 1.5 Feed z h l = h D = 1.643 Short dipole ( ) = ˆsin F E θ θ θ y 60 30 θm max = π / ( /, ) D= D = D π φ z θ 30-9 -6-3 Short dipole 60 0 db x h 10 150 150 10 35
Beamwidth The beamwidth measures how narrow the beam is. (The narrower the beamwidth, the higher the directivity). HPBW = half-power beamwidth 36
Sidelobes The sidelobe level measures how strong the sidelobes are. In this example the sidelobe level is about -13 db Main beam Sidelobe level Sidelobes 13dB 37
Gain and Efficiency The radiation efficiency of an antenna is defined as e r P rad P in P P rad in = = power radiated by antenna power input to antenna The gain of an antenna in the directions (θ, φ) is defined as G ( θφ, ) e D( θφ, ) r In db, we have G ( θφ, ) = 10log G( θφ, ) db 10 38
Gain and Efficiency (cont.) The gain tells us how strong the radiated power density is in a certain direction, for a given amount of input power. Recall that D ( θφ, ) P S rad r ( θφ, ) ( π r ) / 4 r Therefore, in the far field: ( θφ, ) /( 4 π ) ( θφ, ) S P r D r = rad ( θφ, ) /( 4 π ) ( θφ, ) Sr = ep r in r D r ( θφ, ) /( 4 π ) ( θφ, ) = in S P r G 39
Receive Antenna The Thévenin equivalent circuit of a wire antenna being used as a receive antenna is shown below. inc Pd incident power density W/m = inc E + - V Th Z Th = Z in 73[ ] ( ) Z = Ω resonant half - wavelength dipole in Z Th P inc d inc E = W/m η 0 V Th + - 40
Receive Antenna (cont.) The effective area determines the Thévenin voltage. Assume an optimum conjugate-matched load: P L = power absorbed by load Z Th V Th + - Z L = Z * Th P = A P inc L eff d A eff = effective area of antenna inc Pd incident power density W/m = 41
Receive Antenna (cont.) We have the following general formula*: Aeff λ0 θφ, = G θφ, 4π ( ) ( ) G ( θφ, ) = gain of antenna in the direction of the incident signal This assumes that the incoming signal is polarized in the optimum direction. *A poof is given in the Antenna Engineering book: C. A. Balanis, Antenna Engineering, 4 th Ed., 016, Wiley. 4
Receive Antenna (cont.) Effective area of a lossless resonant half-wave dipole antenna: Assuming the incident electric field is aligned along the wire and θ = 90 o : inc E l + - V Th Aeff Hence λ = 4π o o 0 ( 90, φ) G( 90, φ) λ0 = 1.643 4 π ( l) = 1.643 4π ( l = λ / 0 ) Aeff ( ) 90, φ = 0.530l o 43
Receive Antenna (cont.) Example Find the receive power in wireless system shown below, assuming that the receiver is connected to an optimum conjugate-matched load. z f P in [ ] 10 [ W] [ ] = 1 GHz = r = 1 km ( λ 0 = 9.979 [ cm] ) P in [ W] Transmit inc Pd W/m Receive o θ = 90 r o θ = 90 x P rad = P in Z L = Z = 73 [ Ω] * Th Assume lossless antennas (G = D =1.643) 44
Receive Antenna (cont.) P = A P inc L eff d Recall: A eff Gain of receive antenna Gain of transmit antenna λ P 4π 4πr o 0 inc rad ( 90, φ ) = 1.643, Pd = ( 1.643) Hence P L λ P 4π 4πr 0 rad = 1.643 ( 1.643) The result is P L = 8 1.54 10 [W] 45
Receive Antenna (cont.) Effective area of dish antenna 4π G( θφ, ) = A eff ( θφ, ) λ0 In the maximum gain direction: A e Aeff = Aphyeap phy ap = = physical area of dish aperture efficiency The aperture efficiency is usually less than 1 (less than 100%). 46