Dipole Antennas Prof. Girish Kumar Electrical Engineering - PowerPoint PPT Presentation

Dipole Antennas Prof. Girish Kumar Electrical Engineering Department, IIT Bombay gkumar@ee.iitb.ac.in (022) 2576 7436

Infinitesimal Dipole An infinitesimally small current element is called the Hertz Dipole (Length L< λ/50) Assume an infinitesimal current element of length dl carrying an alternating current I o . The instantaneous current is 𝑗(𝑢) = 𝐽 𝑝 𝑓 𝑘𝑥𝑢 𝑗 𝑨 4𝜌 𝐽 𝑝 𝑒𝑚 𝑓 −𝑘𝑙𝑠 2π 𝑨 = 𝜈 w here, k = 𝑓 𝑘𝑥𝑢 𝐵 = 𝐵 𝑨 𝑨 Dipole and its field components λ 𝑠 in spherical polar co-ordinate

Uniform Current – Magnetic Vector Potential

E and H Fields from Magnetic Vector Potential

Uniform Current – E and H Fields

Uniform Current – Near and Far Fields Near Field Region Near Reactive Field Region Near Radiative Field Region Far Field Region where d is the maximum dimension r > of the antenna

Uniform Current - Radiation Pattern Directivity Far Field Region (kr>>1) U 3 𝐹 𝜄 = 𝑘𝜃 𝑙𝐽 𝑝 𝑚    max 4 D 0 4𝜌𝑠 sin𝜄 2 P rad 𝐼 𝜚 = 𝑘 𝑙𝐽 𝑝 𝑚 4𝜌𝑠 sin𝜄 E-plane radiation pattern 𝐹 𝜄 Impedance of = 𝜃 = 120𝜌 𝐼 𝜚 free-space 2 𝑚 𝑆 𝑠 = 80𝜌 2 𝜇 𝐹 𝑠 ≃ 𝐹 𝜚 = 𝐼 𝑠 = 𝐼 𝜄 = 0 3-D radiation pattern H-plane radiation pattern Note : Infinitesimal antenna is not an efficient radiator

Small Dipole Antenna A current element whose length is  /50 < l   /10 is called small dipole antenna A small dipole Antenna Approximate Triangular Current distribution

Small Dipole – Radiation Resistance Far Field Region (kr>>1) Small dipole current distribution 𝐹 𝜄 ≃ 𝑘𝜃 𝑙𝐽 0 𝑚𝑓 −𝑘𝑙𝑠 𝑏 𝑨 𝐽 0 1 − 2 sin𝜄 𝐽 𝑓 𝑦 ′ , 𝑧 ′ , 𝑨 ′ = 𝑚 𝑨 ′ , 0 ≤ 𝑨 ′ ≤ 𝑚 2 8𝜌𝑠 𝐹 𝑠 ≃ 𝐹 𝜚 = 𝐼 𝑠 = 𝐼 𝜄 = 0 𝑏 𝑨 𝐽 0 1 + 2 𝑚 𝑨 ′ , 𝑚 2 ≤ 𝑨 ′ ≤ 0 𝐼 𝜚 ≃ 𝑘 𝑙𝐽 0 𝑚𝑓 −𝑘𝑙𝑠 − sin𝜄 8𝜌𝑠 2 𝑆 𝑠 = 2𝑄 𝑠𝑏𝑒 𝑚 |𝐽 0 | 2 = 20𝜌 2 𝜇 Small dipole vector potential 0 𝑚 𝑨 ′ 𝑓 −𝑘𝑙𝑆 𝐵 𝑦, 𝑧, 𝑨 = 𝜈 𝐽 0 1 + 2 𝑒𝑨 ′ 4𝜌 𝑏 𝑨 For l = λ / 10, R r = 2 Ω 𝑆 − 𝑚 2 l = λ / 4, R r = 12.3 Ω 𝑚 2 𝑚 𝑨 ′ 𝑓 −𝑘𝑙𝑆 𝐽 0 1 − 2 𝑒𝑨 ′ + 𝑏 𝑨 𝑆 Dipoles also have reactive impedance 0

Input Impedance of Transmission Line l 𝑎 0 𝑎 L 𝑎 𝑀 = 0, → Z 𝑗𝑜 = 𝑘𝑎 0 tan𝛾𝑚 Case 1: 𝑎 0 𝑎 𝑀 = ∞, → Z 𝑗𝑜 = Case 2: 𝑘tan𝛾𝑚 𝑎 in 𝑎 𝑀 = 𝑎 0 , → Z 𝑗𝑜 = 𝑎 0 Case 3: 2𝜌 Where, 𝛾 = 𝜇 𝑗𝑔 𝑚 < 𝜇 For Short-circuit, Z L = 0, Z in is inductive, so T-Line represents inductance 4 → tan𝛾𝑚 = +𝑤𝑓 Open-circuit, Z L = , Z in is capacitive, so T-Line represents capacitance 𝜇 4 < 𝑚 < 𝜇 2 → tan𝛾𝑚 = −𝑤𝑓

Half wavelength Dipole Electric and magnetic fields of Directivity of half-wavelength dipole a half-wavelength dipole 𝐸 0 = 4𝜌 𝑉 max cos 𝜌 ≃ 1.643 2 cos𝜄 𝐹 𝜄 ≃ 𝑘𝜃 𝐽 0 𝑓 −𝑘𝑙𝑠 𝑄 𝑠𝑏𝑒 2𝜌𝑠 sin𝜄 cos 𝜌 D = 2.1 dB 2 cos𝜄 𝐼 𝜚 ≃ 𝑘 𝐽 0 𝑓 −𝑘𝑙𝑠 2𝜌𝑠 sin𝜄 Note: Input impedance for λ /2 dipole is 73+j42.5 Ω . λ /2 Dipole Radiation 𝑆 𝑠 = 2𝑄 𝑠𝑏𝑒 |𝐽 0 | 2 ≃ 73 To make imaginary part equal to zero, the antenna Resistance length is reduced until the input impedance becomes real. Design of Dipole Antenna Real Input impedance is < 68 Ω . 𝑚 + 𝑒 = 0.48𝜇 , where, d is the diameter of wire and d< 𝜇 /10

Current Distribution of Dipole Antenna for Different Lengths

Radiation Pattern of Dipole Antenna for Different Lengths

Dipole Antenna Radiation Pattern for l = 1 . 25 λ Two Dimensional Three Dimensional Directivity is maximum for a thin dipole of length l = 1.25λ

Dipole Antenna Resistance and Directivity D 0 = 3.25 R r

Flat Dipole Antenna BW for |S 11 | < 10 dB is from 1.39 to 1.54 GHz (150 MHz, 10.2%) Length of each segment = 50 mm Width = 4mm, Gap = 2mm

Flat Dipole Antenna Pattern and Directivity Radiation Pattern at 1.5, 3.75 and 4.5 GHz Directivity of 4.8 dB is maximum at 3.75 GHz where length of dipole is approx. 1.25 λ

Printed Dipole Antenna Length of each segment = 50 mm Width = 4mm, Gap = 2mm FR4 substrate: ε r = 4.4, tan δ = BW = 1.14 to 1.28 GHz (140 MHz, 11.6%) 0.02, h = 1.6mm

Broadband Dipole Antenna Bandwidth of dipole antenna is directly proportional to its diameter Cylindrical dipole antenna Biconical dipole antenna (can use hollow pipe also) (can use wire grid also)

Balun Design Devices that can be used to balance inherently unbalanced systems by cancelling or choking the outside current, are known as baluns ( bal ance to un balance). 𝑚 𝑚 𝜇 4 𝜇 4 Metal Coaxial line Outer Inner conductor Shorted conductor of coax together of coax 𝜇 4 Coaxial Balun (1:1)- Narrow Bandwidth

Balun Design (Contd.) Ferrite core maintains high impedance levels over a wide frequency range. A good design can provide bandwidths of 10 to 1 whereas coil coaxial baluns can provide bandwidths of 2 or 3 to 1. Ferrite core Metal sleeve 𝑎 1 (unbalanced ) 𝑎 1 (balanced ) Shorted to coax’s Coaxial line outer conductor Ferrite core balun Sleeve or bazooka balun Wide BW Narrow BW

Microstrip Balun Dipole Antenna for GSM900 BW for |S 11 | < 10 dB is from 881 to 967 MHz Microstrip Balun Dipole Antenna (covers GSM900 band of 890 to 960 MHz) L = 127 mm, w = 4 mm FR4 substrate: ε r = 4.4, tan δ = 0.02, h = 1.6mm

Folded Dipole Antenna The impedance of the N fold folded dipole is N 2 times greater than that of an isolated dipole of the same length as one of its side. Impedance for 2-fold dipole antenna is 𝑎 𝑗𝑜 = 2 2 𝑎 𝑠 𝑎 𝑗𝑜 = 4𝑎 𝑠 2-fold dipole antennas are used in Yagi-Uda Antennas for TV reception using balanced line of Z 0 = 300 Ω

Dipole Antenna Applications Chip Compact Dipole Antenna for RFID Folded Broadband Dipole Antenna for RF Harvesting (Triangular shape for broadband and multi-fold gave Z in = 750 Ω )