ICE5108084 Antenna Design Lecture 2 Antenna Radiation and

ICE-5108084 Antenna Design Lecture 2: Antenna Radiation and Impedance 2. 1 Antenna Radiation 2. 2 Antenna Impedance 2. 3 Antenna Resonance 2. 4 Reflection Coefficient 2. 5 Smith Chart 2. 6 Half-wave Dipole Antenna 1

2. 1 Antenna Radiation § Radiation (방사): Time-varying (시변) current → Time-varying electric field (E) & magnetic field (H) → Electromagnetic (EM) wave propgation (전파) § Radiowave receiving (전파수신): Time-varying electric field (E) & magnetic field (H) → Time-varying current induced (유도) on the antenna conductor - Tx = transmitter (송신기) - Rx = receiver (수신기) § Radiated electric field: Figure: Radiation and reception of electromagnetic wave using antennas 2

§ Electric field radiated by a small dipole antenna: - Contour plot - Magnetic field is normal to the electric field. A dipole antenna (left), its E-field contour plot (center), and electric and magnetic field (E H B = H) (right) 3

2. 2 Antenna Impedance § Impedance: - AC voltage phasor divided by current phasor - A sinusoidal (정현파) voltage applied to antenna terminal or port (단자) - Antenna excitation (여기) - Antenna terminal voltage: Vi - Antenna terminal current: Ii - Characteristic impedance (특성임피던스) Z 0 of feed line (급전선) - Antenna input impedance (입력임피던스) or antenna impedance Zi = Vi / Ii = Ri + j. Xi Antenna input impedance (left) and equivalent circuit (right) 4

§ Impedance plot: - Resistance (저항) and reactance (리액턴스) vs. frequency (주파수) Input impedance of a half-wave dipole antenna vs. frequency 5

2. 3 Antenna Resonance § Resonance (공진): X( ) = 0 at = r = 2πfr = resonant frequency (공진주파수) R( r) = Rr = resonant resistance (공진저항) - Resonant frequency (공진주파수): 70 -MHz dipole antenna (left) and its resonances (right) 6

2. 4 Reflection Coefficient § Reflectioin coefficient (반사계수): Z 0 = 50 Ω (coaxial cable) § Scattering parameter (산란계수): Input impedance (top) and reflection coefficient (bottom) of a half-wave dipole antenna |S 11|(d. B) = 10 d. B → | Г |2 = 0. 1 → Prad = 0. 9 P+ 7

§ VSWR (voltage standing wave ratio) (전압정재파비) or SWR (정재파비): § Antenna impedance bandwidth (대역폭): | Γ | ≤ ‒ 10 d. B at f 1 ≤ f 2 f 0 = ( f 2 + f 1 ) / 2 (center frequency) Δf = f 2 ‒ f 1 (absolute bandwidth) 200 -MHz bandwidth f 2 : f 1 or f 2 / f 1 (ratio bandwidth) Decade bandwidth, octave bandwidth, 10: 1 bandwidth % BW = (Δf / f 0) × 100 30% bandwidth 8

§ Bandwidth example: Figure: Bandwidth of a half-wave dipole antenna f 0 = (1. 6504 + 1. 5139) / 2 = 1. 5822 GHz Δf = 1. 6504 ‒ 1. 5139 = 0. 1365 GHz = 136. 5 MHz % BW = (0. 1365 / 1. 5822) × 100 = 8. 6% 9

2. 5 Smith Chart § Smith chart (스미스 도표): - Reflection coefficient vs. normalized impedance (정규화된 임피던스) - Reflection coefficient: real & imaginary parts form or polar form - Reflection coefficient plane - Constant r circles - Constant x circles Smith chart (top) and impedance trace on a Smith chart (bottom) 10

§ Impedance locus of a half-wave dipole antenna on the Smith chart: Input impedance (left) and Smith chart impedance trace (right) of a half-wave dipole antenna § Special points on the Smith chart: - Center: Match, | Γ | = 0 - Rightmost: z = ∞ (open circuit) (개방회로) - Leftmost: z = 0 (short circuit) (단락회로) - On the circular boundary: z = jx (reactive impedance) 11

2. 6 Half-wave Dipole Antenna § Half-wave dipole antenna: - Simple structure → Easy to make - Resonant resistance: 72 (thin wire), 30 -50 (thick wire) - Direct feeding with 50 - cable Structure (upper left), photo (lower left), and radiation pattern (right) of a half-wave dipole antenna 12

§ Dipole antenna current distribution: - Zero current at the tip - Sinusoidal distribution of current - Impedance changes with frequency Current distribution of a dipole antenns vs. dipole arm length 13

§ Dipole antenna input impedance: - Wire dipole: radius a = 0. 5 mm, length = 1 m - Ribon dipole: width w = 4 a - Resonant frequency = 142 MHz (0. 473 wavelength) Input impedance of a half-wave dipole antenna 14

§ Folded dipole antenna: - L = λ/2 at resonance - ZA = 4 Zd = 200 300 at resonance § Properites of the folded dipole antenna: - High input resistance Structure of a folded half-wave dipole antenna - Larger bandwidth than the dipole § Applications of the folded dipole antenna: - Yagi antenna feeder - TV, FM transmission and reception - VHF radar array antenna 15

§ Dipole antenna applications: - Test & measurements - LPDA (log-periodic dipole array) - UHF (915 MHz) RFID tag - VHF radar - FM/TV broadcasting 16

ICE-5108084 Antenna Design Lecture 2: Antenna Radiation and Impedance 17