ELEC 425 Interference Control in Electronics Lecture 7(a) Introduction to Antennas: Terminology

Transcription

1 Dr. Gregory J. Mazzaro Fall 017 ELEC 45 Interference Control in Electronics Lecture 7(a) Introduction to Antennas: Terminology Chapter 9 THE CITADEL, THE MILITARY COLLEGE OF SOUTH CAROLINA 171 Moultrie Street, Charleston, SC 9409

2 Antenna: Definition antenna -- a transducer between a wave guided by a transmission line and an unbounded propagating EM wave -- or vice versa -- changes energy traveling in a TL circuit to energy traveling over-the-air reciprocity -- to transmit as efficiently as receive -- most practical antennas are reciprocal

3 Types of Antennas radiation sources -- currents vs. aperture fields -- ultimately all EM radiation is produced by time-varying currents 3

4 Types of Antennas radiation sources -- currents vs. aperture fields -- ultimately all EM radiation is produced by time-varying currents 4

5 Antenna Terminology E-field (antenna) polarization -- orientation of the E/H fields that are transmitted from (or received by) the antenna -- e.g. linear (horizontal, vertical), circular, elliptical (antenna) impedance -- the circuit quantity that models the transfer of energy from the feedline to unbounded EM waves (or vice versa) 5

6 Antenna Terminology radiation resistance, R rad -- a circuit model for the portion of EM energy that arrives at the antenna and is radiated into free space -- R loss is a model for the EM energy that is lost (as heat in the antenna; not radiated) -- X antenna is a model for the portion that is reflected back into the transmission-line feed a b rad I R rad RMS rad I R loss RMS loss R loss X antenna radiation efficiency, x -- the fraction of power that arrives at the antenna that is radiated: a t I rad R rad x rad Rrad R R t rad loss b 6

7 Example: Rad. Resistance & Efficiency The radiation resistance of a quarter-wave monopole antenna is 36.5 W. It is 85% efficient. The antenna is connected directly to a function generator with an internal impedance of 50 W. Determine the peak-to-peak (open-circuit) voltage of the source required to radiate an average power of mw from the antenna. Neglect reactance in the antenna. I R rad ant rad x rad Rrad R R t rad loss 7

8 Antenna Array: Biconical Log-eriodic 9

9 Antenna Terminology antenna factor (or directive gain ), AF -- one way of calculating transmitted/received signal -- calculated as the ratio of electric field strength (applied to or emanating from an antenna) to the voltage across the antenna s input terminals + V E AF E V effective area (or effective aperture), A eff -- another way of calculating received signal -- calculated as the ratio of power received ( intercepted ) to the power density incident on the antenna inc x 1 inc A eff inc inc 10

10 Antenna Array: Biconical Log-eriodic 11

11 Example: Antenna Factor An electric field of 60.0 dbmv/m is incident on an antenna, as depicted. The electric field oscillates along the antenna axis. The antenna factor for this antenna is db/m. Determine the power input to the spectrum analyzer, in decibels referenced to 1 milliwatt. AF E V AF E V db dbμv/m dbμv 1

12 Dr. Gregory J. Mazzaro Fall 017 ELEC 45 Interference Control in Electronics Lecture 7(b) Building-Block Antennas Chapter 9 THE CITADEL, THE MILITARY COLLEGE OF SOUTH CAROLINA 171 Moultrie Street, Charleston, SC 9409

13 Hertzian ( Short ) Dipole Hertzian (or short ) dipole -- a straight-line antenna -- the length of the antenna (l) is much smaller than the wavelength of the radiation transmitted/received (l) I l l Aligned with the z axis, the E-field pattern is cylindrically symmetric (i.e. it does not vary with f ) and is given by and it follows that the average power density is E short dipole jkr I l e j sin ˆ θ l R I l 1 sin 8 l R avg normalized short F sin dipole 15

14 Half-Wave Dipole Antenna half-wave dipole -- a straight-line antenna -- the length of the antenna (l) is equal (or very close) to one-half of the radiated wavelength (l) The E-field pattern is derived by integrating the short dipole from l = l/4 to l = +l/4 and the result is cos cos jkr l I e E ˆ dipole j θ sin R I I avg cos cos F l dipole sin I cos cos 8 r sin normalized x f z F(, f) y 16

15 Quarter-Wave Monopole Antenna quarter-wave monopole -- a straight-line antenna near a ground plane -- l = l/4 z 0 z 0 I I I The E-field pattern is found from image theory: it is the same E the half-wave dipole, but only above the ground plane. E l 4 mono cos cos jkr I e j ˆ θ z 0 sin R 0 z 0 cos cos 4 F l mono sin 17

16 Table of Antenna arameters E transmitted (, f) R rad ( = 0 ) G max short dipole, length = l jkr I l e j sin ˆ θ l R l 80 l 1.50 half-wave dipole, l = l/ cos cos jkr I e j ˆ θ sin R 73 W 1.64 quarter-wave monopole, l = l/4 cos cos jkr I e j ˆ θ, z 0 sin R 36.5 W 3.8 small loop, area = S jkr S e I sin ˆ f l R 30 S 4 l dipole, arbitrary length l l l jkr I e cos l cos cos l j ˆ θ R sin 18

17 Example: Antenna, ower Density A 1-m-long dipole is excited by a 5-MHz current with an amplitude of 5 A. Determine the maximum radiated power density at a distance of km. u p c f l r avg 1 ReE H E kˆ avg S avg ds E short dipole jkr I l e j sin ˆ θ l R jkr cos cos I e l ˆ dipole j E θ R sin E arbitrary dipole 19 l l jkr I e cos l cos cos l j R sin θˆ

18 Dr. Gregory J. Mazzaro Fall 017 ELEC 45 Interference Control in Electronics Lecture 7(c) Antenna Gain and the Friis Equation Chapter 9 THE CITADEL, THE MILITARY COLLEGE OF SOUTH CAROLINA 171 Moultrie Street, Charleston, SC 9409

19 Antenna Gain antenna (power) gain: G or G p ( unitless, or in dbi ) -- a measure of the concentration of radiated power in a given direction -- very closely related to radiation intensity (F) -- includes radiation efficiency (loss in the antenna) Gp, f 4 R avg f, t directive gain: G d ( unitless, or in dbi ) -- nearly the same as power gain -- referenced to radiated power ( rad ) instead of power fed to the antenna ( t ) -- does not include radiation efficiency (loss) Gd, f 4 R avg f, rad t rad x R G R R G rad rad x t rad loss G d p d G p

20 Radiation Intensity vs. Gain normalized radiation intensity: F f, avg f, max f, avg antenna (power) gain: Gp, f 4 R avg f, t -- indicates the direction(s) in which the antenna radiates -- maximum value = 1 (or 0 db) -- commonly used to calculate relative levels of power received/transmitted -- in practice, typically plotted as a function of angle -- indicates the direction(s) in which the antenna radiates -- may increase indefinitely (in theory) -- commonly used to calculate absolute levels of power received/transmitted -- in practice, typically plotted as a function of frequency 3

21 Radiation Intensity vs. Gain normalized radiation intensity: F f, avg f, max f, avg antenna (power) gain: Gp, f 4 R avg f, t ETS Lindgren horn antenna 4

22 The Friis Equation Friis Equation -- a power-budget equation, used to calculate power received/transmitted or the antenna properties necessary to establish/maintain communication -- generally written in two forms: efficiency & effective apertures OR antenna gains Derivation G t 4 R t A x eff rec rec r r t Transmitter (Tx) eff A t eff A r rec 1 G A x 4 R eff rec t r r t xt Receiver (Rx) xr substitute G t G r G G x G t t d,t x G r r d,r A eff l G d 4 eff eff xx t r At Ar l rec t t r t G G l R 4R 5

23 Example: Friis Equation Two antennas are separated by a distance of 00 l. The transmit antenna has a gain of 5 db and the receive antenna has a gain of 18 db. If 5 mw of power must be received, calculate the minimum required transmit power. 6 eff eff xx t r At Ar l rec t t r t G G l R 4R