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Notes 21 Introduction to Antennas ECE 3317 Applied Electromagnetic Waves Prof. David R. Jackson Fall 2020 Notes 21 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) 2 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 2 Power loss from antenna broadcast: 1/r (always better for very large r) − α Power loss from waveguiding system: e 2 r r A 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 ≈ λ0 /2 (resonant) Current . Very simple At resonance : Zin =73 [ Ω] . Moderate bandwidth . Low directivity . Omnidirectional in azimuth . Most commonly fed by a twin-lead transmission line . Linear polarization (Eθ , assuming wire is along z axis) . The antenna is resonant when the length is about one-half free-space wavelength 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. Compatible with TV twin lead At resonance : Zin =292 [ Ω] Z0 =300 [ Ω] 8 Introduction to Antennas (cont.) Monopole Wire Antenna h h ≈ λ0 /4 At resonance : Zin =36.5 [ Ω] 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 conducting 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 Prof. Yagi 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 11 Introduction to Antennas (cont.) Yagi Antenna (cont.) UHF Yagi UHF Yagi UHF Yagi VHF Log-periodic 12 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 Current 1 λ0 W L ≈=λd /2 2 ε r L x h 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.) 20 Introduction to Antennas (cont.) Dielectric Resonator Antenna (DRA) The dielectric resonator antenna was invented by our very own Prof. Long! ε r 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 21 Introduction to Antennas (cont.) Dielectric Resonator Antenna (cont.) GPS antenna 22 Introduction to Antennas (cont.) Leaky-Wave Antenna y Slot Air x Rectangular waveguide The slot allows the wave to radiate (“leak”) from the slot. 23 Antenna Radiation We consider here the radiation from an arbitrary antenna. S z rr( ,,θφ) r +- y "far field" x r →∞ The far-field radiation acts like a plane wave going in the radial direction. 24 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. rr( ,,θφ) r 2D2 +- r > λ0 A derivation is given in the Antenna Engineering book: C. A. Balanis, Antenna Engineering, 4th Ed., 2016, Wiley. 25 Antenna Radiation (cont.) ˆˆ The far-field has the following form: EE=θφθφ + E ˆˆ HH=θφθφ + H z H S Eθ =η0 TMz y Hφ E x z S Eφ y = −η0 TEz E Hθ H x Depending on the type of antenna, either or both polarizations may be radiated (e.g., a vertical wire antenna radiates only TM polarization). z 26 Antenna Radiation (cont.) The far-field Poynting vector is now calculated: 1 S= EH × * 2 E 1 ˆˆ ˆ ˆ θ =η =(θφEEθφ +×) ( θ HH θ + φ φ) 0 2 Hφ 1 =** − rˆ ( EHθφ EH φθ) Eφ 2 = −η0 ** Hθ 1 Eθ Eφ = rEˆ θφ+ E 2 ηη 00 2 2 1 E Eφ =rˆ θ + 2 ηη 00 27 Antenna Radiation (cont.) Hence we have 2 2 1 =ˆ + S rE( θφ E ) 2η0 or 2 E Sr= ˆ η 2 0 Note: In the far field, the Poynting vector is pure real (no reactive power flow). 28 Radiation Pattern The far field always has the following form: − jk0 r e F Er( ,,θφ) = E (θφ,) r E F (θφ, ) ≡ Normalizedf ar - field electric field In dB: E F (θφ, ) dB(θφ ,) = 20log 10 F θφ E ( mm, ) (θφmm, ) = direction of maximum radiation 29 Radiation Pattern (cont.) The far-field pattern is usually shown vs. the angle θ (for a fixed angle φ) in polar coordinates. E F (θφ, ) dB(θφ ,) = 20log 10 F θφ E ( mm, ) A “pattern cut” z (shown for a dipole antenna) θ 30° 30° φ = 0 60° 60° 0 dB -10 dB o θm = 90 -20 dB -30 dB 120° 120° 150° 150° 30 Radiated Power The Poynting vector in the far field is F 2 E (θφ, ) 1 Sr( ,,θφ) = rˆ 2η r2 0 The total power radiated is then given by 2 22ππ ππE F (θφ, ) P=⋅=( Srrˆ) 2 sinθ dd θφ sinθdd θφ rad ∫∫ ∫∫2η 00 000 Hence we have 2ππ 1 2 = F θφ θ θ φ Prad ∫∫ E( ,) sin dd 2η0 00 31 Directivity The directivity of the antenna in the directions (θ, φ) is defined as S (θφ, ) θφ ≡ r →∞ Dr( , ) 2 Prrad /4( π ) 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, DDdB (θφ,) = 10log10 (θφ , ) Note: The directivity is sometimes referred to as the “directivity with respect to an isotropic radiator”. 32 Directivity (cont.) z z θ θ 30° 30° 30° 30° 60° 60° 60° 60° 0 0 dB dB -9 -6 -3 -9 -6 -3 120° 120° 120° 120° 150° 150° 150° 150° Antenna with moderate directivity Antenna with high directivity (e.g., dipole) (e.g., horn or dish) 33 Directivity (cont.) Resonant half-wavelength dipole: θπ= /2 D = 1.643 m DD=max = D(πφ/ 2, ) z z θ h 30° 30° Dipole lh= 2 60° 60° Feed y 0 dB -9 -6 -3 x 120° 120° −h 150° 150° 34 Beamwidth The beamwidth measures how narrow the beam is. (The narrower the beamwidth, the higher the directivity). HPBW = half-power beamwidth 35 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 36 Gain and Efficiency The radiation efficiency of an antenna is defined as Prad Prad = power radiated by antenna er ≡ = power input to antenna Pin Pin The gain of an antenna in the directions (θ, φ) is defined as G(θφ,,) ≡ eDr ( θφ) In dB, we have GGdB (θφ,) = 10log10 (θφ , ) 37 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 S (θφ, ) θφ ≡ r →∞ Dr( , ) 2 Prrad /4( π ) Therefore, in the far field: θφ= π2 θφ Sr ( ,) Prad /4( rD) ( ,) θφ= π2 θφ Sr ( ,) ePr in /4( r) D( ,) θφ= π2 θφ Sr ( ,) Pin /4( rG) ( ,) 38 Receive Antenna The Thévenin equivalent circuit of a wire antenna being used as a receive antenna is shown below.
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