An Introduction to Antennas

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1 May 11, 010 An Introduction to Antennas 1 Outline Antenna definition Main parameters of an antenna Types of antennas Antenna radiation (oynting vector) Radiation pattern Far-field distance, directivity, efficiency, gain, etc. Friis equation Equivalent circuits 1

2 May 11, 010 Definition of an Antenna It is a component that converts an EM wave propagating on a transmission line to a plane EM wave propagating in free-space, or vice versa (transmission or reception) 3 Main arameters of an Antenna Radiation pattern: 3D plot of the power density (transmit or receive) Far-field: region where the radiated wave has the form of a plane wave Directivity: ability to transmit/receive in a given direction Efficiency: the ratio of the radiated power to the input power Gain: the product of efficiency and directivity Impedance: the driving-point impedance offered to the source or to the load 4

3 May 11, 010 Main arameters of an Antenna (cont) Bandwidth: usable frequency bandwidth (affected by matching networks) olarization: polarization of the electric field intensity vector of the radiated wave (vertical, horizontal, circular, etc.) Size/complexity: geometrical structure and size affect radiation pattern, gain, bandwidth, etc. Efficient operation implies a minimum size of λ/ 5 Types of Antennas Wire antennas: simplest type; used at lower frequencies (HF to UHF); low gain; lightweight Aperture antennas: based on an open-ended waveguide; used at microwave frequencies; low gain rinted antenna: compatible with planar technology; used at microwave frequencies; low gain Reflector antennas: high gain (focused radiation); used at microwave frequencies; bulky and heavy 6 3

May 11, 010 Types of Antennas (cont) reflector antenna with radome

horn antenna microstrip array antenna pyramidal horn antenna (D. M.

Yagi-Uda beam antenna Yagi array antenna (TV) dipole antenna (&

4 May 11, 010 Types of Antennas (cont) reflector antenna with radome multibeam reflector antenna parabolic antenna corrugated conical horn antenna microstrip array antenna pyramidal horn antenna (D. M. ozar, Microwave Engineering, Wiley, 005) 7 Wire of Antennas Yagi-Uda beam antenna Yagi array antenna (TV) dipole antenna (& loop antenna) rotatable logarithmic periodic antenna (Wikipedia, 010) 8 4

5 May 11, 010 Combined lanar/wire Antennas (Besser Associates, CST, 010) 9 TEM ropagation The electric and magnetic fields in the direction of propagation are zero (R. Ludwig and. Bretchko, RF Circuit Design, rentice Hall, 000) 10 5

6 May 11, 010 Antenna Radiation Electric field of an antenna in the far-field zone E E u + E u (V/m) where u and u are the unit vector in and directions The corresponding magnetic field is H H u + H u (A/m) H E /η0 H E /η0 where η Ω is the wave impedance of free-space 11 Antenna Radiation (cont) Electric field of an antenna in the far-field zone E where e [ F (, ) u + F (, ) u ] r F (,) and F (,) are the radiation pattern functions k 0 π/λ is the propagation constant in free-space λ c/f jk 0 r 1 6

7 May 11, 010 Antenna Radiation (cont) oynting vector Since and then E S E H * (W/m ) E u + E u H H u + Hu H /η 0 H E /η0 E 0 1 S η 0 ( E + E ) u r 13 Antenna Radiation (cont) Total average power radiated by the antenna 1 1 Re{ Re{ * rad S} ds (W) E H } ds Since and 1 S η0 then rad η ds ( rsend)( rd ) r ( E + E ) u r 1 π π E sendd e [ F (, ) u + F (, ) u ] r ( F + F ) sendd (W) 0 ϕ 0 0 jk 0 r 14 7

8 May 11, 010 Antenna Radiation attern It is the plot of F (,) or F (,) versus the position around the antenna The choice between F (,) and F (,) depends on the antenna polarization F (,) and F (,) can be plotted versus and Since F (,) and F (,) are proportional to voltage, the plot is calculated using 0log F(,) 15 Antenna Radiation attern Example E-plane radiation pattern of a small horn antenna (versus ) main beam ( 0, 0dB) sidelobes sidelobe at 30, 3.5dB (D. M. ozar, Microwave Engineering, Wiley, 005) 16 8

9 May 11, 010 Antenna Radiation attern Example (cont) E-plane radiation pattern of a small horn antenna (versus ) 3-dB beam width (D. M. ozar, Microwave Engineering, Wiley, 005) 17 Radiation attern of an Infinitesimal Dipole It is assumed that L << λ (L λ/50) (Lonngren et al., Fundamentals of Electromagnetics with Matlab, HI, 009) 18 9

10 May 11, 010 IFA Antenna Radiation attern Effects of the finite ground plane on the radiation pattern of an IFA antenna (Besser Associates, CST, 010) 19 IFA Antenna Radiation attern (cont) (Besser Associates, CST, 010) 0 10

11 May 11, 010 Far-Field Distance, R ff It is the distance at which the spherical wave radiated by the antenna becomes almost a plane wave It can be estimated by d R ff λ where d is the maximum dimension of the antenna aperture (D. M. ozar, Microwave Engineering, Wiley, 005) 1 Far-Field Distance Example A DBS parabolic reflector antenna has a plate with a diameter of 18 and operates at 1.4 GHz: d cm λ c/f (0.3Gm/s)/(1.4GHz).4 cm The far-field distance is R d λ (45.7).4 ff 17.7 m (D. M. ozar, Microwave Engineering, Wiley, 005) 11

May 11, 010 Radiating Distances Reactive region: S is imaginary, fields decay more rapidly than 1/r Near-field region: S is complex, radiation pattern depends on r, fields decay

r-field region: S is real, radiation pattern does not depend on r, fields decay with 1/r (d D) (J. F.

12 May 11, 010 Radiating Distances Reactive region: S is imaginary, fields decay more rapidly than 1/r Near-field region: S is complex, radiation pattern depends on r, fields decay more rapidly than 1/r Far-field region: S is real, radiation pattern does not depend on r, fields decay with 1/r (d D) (J. F. Aubin, Microwave Journal, 005) 3 Antenna Directivity, D It is the antenna ability to transmit/receive in a given direction, usually expressed in db, 10log 10 (D) It is defined as D 4πU U max / where U is the radiation intensity given by 1 U (, ) F + F (W per unit solid η 0 It can be estimated by 3,400 D avg ( ) angle) where 1 and are the beamwidths (in degrees) of two orthogonal planes of the main beam 1 4 1

13 May 11, 010 Antenna Radiation Efficiency, e rad It is the ratio of the radiated power to the input power e rad in loss rad 1 in in loss in 5 Antenna Gain, G It is the product of the antenna efficiency by the antenna directivity G e D It is usually expressed in db, 10log 10 (G) rad 6 13

14 May 11, 010 The Friis Equation Friis equation allows us to calculate how much power is received by an antenna r GtGrλ t (4πR) (W) (D. M. ozar, Microwave Engineering, Wiley, 005) 7 The Friis Equation Example 1 An earth station with a transmitter power of 10 W, a frequency of 6 GHz, and an antenna gain of 4 db transmits to a satellite repeater. The receiver antenna on the satellite has a gain of 31 db, and the satellite is in a synchronous orbit 35,900 Km above the earth. What is the received power, in dbm? (D. M. ozar, Microwave Engineering, Wiley, 005) 8 14

15 May 11, 010 The Friis Equation Example 1 An earth station with a transmitter power of 10 W, a frequency of 6 GHz, and an antenna gain of 4 db transmits to a satellite repeater. The receiver antenna on the satellite has a gain of 31 db, and the satellite is in a synchronous orbit 35,900 km above the earth. What is the received power, in dbm? r dbm 9.41 pw (D. M. ozar, Microwave Engineering, Wiley, 005) 9 The Friis Equation Example A microwave radio link at 4.9 GHz uses transmit and receive antennas with gains of 30 db. If the distance between the transmitter and the receiver is 7 km, and it is desired to have a minimum received power of 60 dbm, what is the required transmitter power, in dbm? (D. M. ozar, Microwave Engineering, Wiley, 005) 30 15

16 May 11, 010 Equivalent Circuits of an Antenna (S 11 ) The antenna impedance Z A can be approximated by a resonant circuit, Simplified equivalent circuits for the transmitter receiver VS Z S Matching Network Z A Transmitter Antenna 31 16

Notes 21 Introduction to Antennas

Notes 21 Introduction to Antennas 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