Lecture VI Antennas & Propagation -1- Antennas & Propagation Mischa Dohler King’s College London Centre for Telecommunications Research

Lecture VI Antennas & Propagation -2- Overview of Lecture VI - Review of Lecture V - Antenna Analysis and Synthesis - Uda-Yagi Antenna - Turnstile Antenna - Loop Antenna - Helical Antenna - Quadrifilar Helix Antenna

Lecture VI Antennas & Propagation -3- Review

Lecture VI Antennas & Propagation -4- Mutual Impedance Approximated current distribution: Electromagnetic Field in the Near Field:

Lecture VI Antennas & Propagation -5- Linear Antenna Array x y z P(r, ,  )   mixed elevation & azimuth pure elevation d

Lecture VI Antennas & Propagation -6- Principle of Pattern Multiplication Individual Pattern (of 1 antenna element) ARRAY FACTOR (different for each Array) Constant (similar to all structures)

Lecture VI Antennas & Propagation -7- Uniform Array Array Factor Maximum (Main Beam) for u = 0 : Broadside ArrayEndfire Array

Lecture VI Antennas & Propagation -8- Array Polynomial Nulls on unity circle indicate no radiation in that particular direction! u=0 u=  /2 1 Walking along the circle is like walking around the array Feeding Current: x

Lecture VI Antennas & Propagation -9- Pattern Synthesis

Lecture VI Antennas & Propagation -10- Odd Array - Odd Array with N = 2m + 1 Feeding Current: x N = 2*2 + 1  m=2

Lecture VI Antennas & Propagation -11- Fourier Coefficients - Symmetric feeding: - Trigonometric Series with &

Lecture VI Antennas & Propagation -12- Synthesis Procedure 1. Specify the Array Factor f(  ) either graphically or analytically 2. Find the Fourier series expansion coefficients of f(  ) 3. Relate the coefficients to the feeding current amplitude and phase. Example, see blackboard.

Lecture VI Antennas & Propagation -13- Uda-Yagi Antenna

Lecture VI Antennas & Propagation element Uda-Yagi x y z Driver Endfire Regime reflector 5% longer Reflector Director d2d2 d1d1 director 5% shorter d=d1=d2: Directivity 9dB highly frequency sensitive significant backlobe radiation

Lecture VI Antennas & Propagation element antenna 1 driven element  1 parasitic element (reflector/director) Pattern Multiplication Principle (although not strictly applicable)

Lecture VI Antennas & Propagation -16- E-field in the Azimuth-plane: Maximum Radiation corresponding to ReflectorDirector Reflector - Director

Lecture VI Antennas & Propagation -17- Maximum Radiation corresponding to ReflectorDirector Reflector – Director Length  insensitive to d/  d 1 = d 2 = d sensitive to L/  Reflector 5% longer  Director 5% shorter

Lecture VI Antennas & Propagation -18- Reflector Driver

Lecture VI Antennas & Propagation -19- Director Driver

Lecture VI Antennas & Propagation Element Uda-Yagi DirectorDriverReflector

Lecture VI Antennas & Propagation -21- Application of Uda-Yagi The Uda-Yagi is the most popular receiving antenna in VHF-UHF due to: 1. Simple feeding system design 2. Low cost 3. Light weight 4. Relatively high gain

Lecture VI Antennas & Propagation -22- Application of Uda-Yagi Higher frequencies cause higher propagation losses. Thus higher gains with more directors are required. FM-Radio(88MHz-108MHz)3 element UY TV (low)(54MHz-88MHz)3 element UY TV (high)(174MHz-216MHz)5-6 element UY TV (470MHz-890MHz)10-12 element UY VHF UHF

Lecture VI Antennas & Propagation -23- Practical Design Criteria 1. Closer spacing between elements results in higher front-to-back ratio with a broader main beam. 2. Wider spacing yields the opposite. 3. Wider spacing has a greater bandwidth. 4. Uda-Yagi has broader bandwidth if reflector is longer than optimum and director shorter. 5. Folded dipole as driven element to gain more radiation power and broader bandwidth. 6. To broaden bandwidth reflector should be replaced by flat sheet (or wire grid). 7. Tilted fan dipole for broader bandwidth.

Lecture VI Antennas & Propagation -24- VHF TV Receive Antenna Man-made noise was found to be preferably vertical polarised.  TV broadcast is horizontally polarised! 5-6 Directors Folded Dipole Driver Sheet Reflector Feeding Mast

Lecture VI Antennas & Propagation -25- Corner Reflector

Lecture VI Antennas & Propagation -26- Application of Corner Reflector Tilted Dipole in the Corner Reflector produces an elliptically polarised wave. Application - Communication through ionosphere (Faraday Rotation) - Minimises clutter echoes from raindrops

Lecture VI Antennas & Propagation -27- Turnstile Antenna

Lecture VI Antennas & Propagation -28- Turnstile Antenna Small Cross-Dipole with quadrature current feeding: x r y z P dL A B

Lecture VI Antennas & Propagation -29- Polarisations x-z plane (  = 0°)  Linearly Polarised x-y plane (  = 90°)   = 0° Linear Polarisation  0° <  < 90° Elliptical   = 90° Circular y-z plane (  = 90°)   = 0° Linear Polarisation  0° <  < 90° Elliptical   = 90° Circular

Lecture VI Antennas & Propagation -30- Radiation Pattern 3-D Pattern of infinitesimal Turnstile Antenna x y z 2-D x-z plane Field Pattern of Turnstile Antennas x z x z Infinitesimal Turnstile Finite Length Turnstile Radiation in all directions!

Lecture VI Antennas & Propagation -31- Application 1. Circular polarisation in Broadside direction:  Satellite Communication  Radar Application 2. Communication of unstabilised space-crafts due to radiation property in all directions. 3. In x-z plane almost circular radiation pattern:  TV-broadcast transmit antenna

Lecture VI Antennas & Propagation -32- Loop Antenna

Lecture VI Antennas & Propagation -33- Loop Antennas (rectangular, loop) Circular Loop x r y z P a Radius of wire: b Loop coefficients B 0, B n see graph. Small Circular Loop The Loop pattern has exactly the same shape as that of a Hertzian Dipole, where the electric and magnetic fields are interchanged.

Lecture VI Antennas & Propagation -34- Parameters of the Loop Radiation intensity U Radiation Power P Radiation Resistance R r Directive Gain g Radiation Efficiency e

Lecture VI Antennas & Propagation -35- Application 1. Bad transmitter, but spatially very compact:  Low Frequency AM receiver (HiFi) Connection to high impedance to give high induced voltage. Ferrite as kernel will give even better performance. Multiple loop turns to increase radiation resistance 2. Directional Finder (combined with dipole) : x y z x y - + Dipole Loop Resultant Pattern

Lecture VI Antennas & Propagation -36- Helical Antenna

Lecture VI Antennas & Propagation -37- Helical Antenna x z The Helical Antenna was invented by John Kraus in (see his books) Diameter D Turn spacing S Circumference C Pitch Angle  Operational Modes Normal Mode Radiation Axial Mode Radiation Ground Plane > /2 Number of turns N

Lecture VI Antennas & Propagation -38- Normal Mode Radiation x z Diameter D Entire Helix Length L y Normal Mode Radiation (broadside) appears if: D << entire L << Current is sinusoidal along wire, thus radiation from a loop

Lecture VI Antennas & Propagation -39- Axial Mode Radiation preferred mode x z y Circumference C Axial Mode Radiation (endfire) appears if: 3/4 < C/ < 4/3 1. Narrow Mainbeam with minor sidelobes 2. HPBW  1/(Number of turns) 3. Circular Polarisation (orientation  helix orientation) 4. Wide Bandwidth 5. No coupling between elements 6. Supergain Endfire Array

Lecture VI Antennas & Propagation -40- Parameter of Axial Mode Radiation HPBW Gain Input Impedance Axial Ratio (Polarisation)

Lecture VI Antennas & Propagation -41- Application 1. High gain, large bandwidth, simplicity, circular polarisation in AXIAL MODE:  Space Communication ( MHz) 2. Arrays of Helixes with higher gain (they hardly couple!)

Lecture VI Antennas & Propagation -42- Quadrifilar Helix Antenna

Lecture VI Antennas & Propagation -43- Quadrifilar Helix Antenna The Helical Antenna was invented by Kilgus in (see his papers) 1. Used for communication between mobile user and non- geostationary satellite systems 2. Gives Circular Polarisation in all directions, thus becomes independent of elevation angle of satellite.