Introduction to Antennas & Propagation Introduction to Antennas & Propagation -1- Antennas & Propagation Wu Qun

Introduction to Antennas & Propagation Introduction to Antennas & Propagation -2- Overview of Lecture VII - Review of Lecture VI - Frequency Independent Antennas - Basics of Aperture Antennas - Horn Antenna - Slot Antenna - Microstrip (Patch) Antenna - Parabolic Antenna - Antennas: Practical Considerations

Introduction to Antennas & Propagation Introduction to Antennas & Propagation -3- Review

Introduction to Antennas & Propagation Introduction to Antennas & Propagation -4- Wire Antennas 1. Hertzian Dipole 2. Finite Length Dipole 3. Antenna Array 4. Uda-Yagi 5. Turnstile 6. Loop 7. Helix 8. Quadrifilar Helix

Introduction to Antennas & Propagation Introduction to Antennas & Propagation -5- VHF TV Receive Antenna Uda-Yagi Antenna 5-6 Directors Folded Dipole Driver Sheet Reflector Feeding Mast

Introduction to Antennas & Propagation Introduction to Antennas & Propagation -6- Helical Antenna 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

Introduction to Antennas & Propagation Introduction to Antennas & Propagation -7- Frequency Independent Antennas

Introduction to Antennas & Propagation Introduction to Antennas & Propagation -8- Rumsey’s Principle All antenna characteristics so far were always scaled with respect to. Thus, changing changes the characteristic. The impedance and pattern properties of an antenna will be frequency independent if the antenna shape is specified only in terms of angles and the antenna itself is infinite.

Introduction to Antennas & Propagation Introduction to Antennas & Propagation -9- Rumsey’s Principle Scaling through angles  self-scaling Infinite size  problem of realisation Finite Bowtie Antenna Current should decay fast

Introduction to Antennas & Propagation Introduction to Antennas & Propagation -10- Log-periodic toothed Antenna Effectively infinite  current decays fast Current decays fast  introduce discontinuities Discontinuities  destroy self-scaling nature Self-scaling nature  log-periodic toothed antenna Log-periodic sheetLog-periodic wire Characteristic will be repeated at (discrete)  n f 1.

Introduction to Antennas & Propagation Introduction to Antennas & Propagation -11- Log-periodic Dipole Array

Introduction to Antennas & Propagation Introduction to Antennas & Propagation -12- Spiral Antenna

Introduction to Antennas & Propagation Introduction to Antennas & Propagation -13- Fractal Antenna

Introduction to Antennas & Propagation Introduction to Antennas & Propagation -14- Aperture Antennas

Introduction to Antennas & Propagation Introduction to Antennas & Propagation -15- Huygen’s Principle Any wavefront can be considered to be the source of secondary waves that add to produce distant wavefronts. x y z P J,  enen r’ r

Introduction to Antennas & Propagation Introduction to Antennas & Propagation -16- Aperture Plane Towards infinity Aperture Plane Closing Hemisphere - E-field vanishes on the Hemisphere at infinity. - Total field is derived from the knowledge of the field on the aperture plane.

Introduction to Antennas & Propagation Introduction to Antennas & Propagation -17- Rectangular Aperture x z P r’ r y b/2 a -a/2  Polarisation in the far field is the same as in the aperture.

Introduction to Antennas & Propagation Introduction to Antennas & Propagation -18- Parameter Rectangular Aperture y-z plane: x-z plane:

Introduction to Antennas & Propagation Introduction to Antennas & Propagation -19- Circular Aperture x z P r’ r y a  Polarisation in the far field is the same as in the aperture. J 1 (x) is the first order Bessel Function of first kind.

Introduction to Antennas & Propagation Introduction to Antennas & Propagation -20- Parameter Circular Aperture y-z plane: x-z plane: Large Apertures:

Introduction to Antennas & Propagation Introduction to Antennas & Propagation -21- Directivity Rectangular Aperture: Circular Aperture: Real Physical Area Definition Thus, for the uniform rectangular and circular aperture the physical area is equal to the effective area. Non-uniform apertures or fields: … Aperture Efficiency Aperture Antennas: 30-90% Horn Antennas: 50%

Introduction to Antennas & Propagation Introduction to Antennas & Propagation -22- Horn Antennas

Introduction to Antennas & Propagation Introduction to Antennas & Propagation -23- Horn Antennas E-Plane sectoral horn H-Plane sectoral horn Pyramidal horn Excitation: TE 10 mode TE 10 Impedance Matching through flare Gradual Transmission with minimised reflection

Introduction to Antennas & Propagation Introduction to Antennas & Propagation -24- Specifications 1. Directive Radiator 2. Primary feed for parabolic reflectors 3. High gain, wide bandwidth and simple 4. Particularly used in microwave region (>1GHz) 5. Fan radiation patterns

Introduction to Antennas & Propagation Introduction to Antennas & Propagation -25- Slot Antennas

Introduction to Antennas & Propagation Introduction to Antennas & Propagation -26- Slot Antennas -x z y w L Bookers Principle:

Introduction to Antennas & Propagation Introduction to Antennas & Propagation -27- Slot on Waveguide Walls TE 10 mode Radiation is maximum at maximal interrupted current Radiation No Radiation

Introduction to Antennas & Propagation Introduction to Antennas & Propagation -28- Applications 1. Slot Antennas are used in fast-moving vehicles. 2. The slot-length is usually /2 3. Particularly used in microwave region (>1GHz)

Introduction to Antennas & Propagation Introduction to Antennas & Propagation -29- Microstrip (Patch) Antennas

Introduction to Antennas & Propagation Introduction to Antennas & Propagation -30- Patch Structure Substrate Patch Feed rr L t d

Introduction to Antennas & Propagation Introduction to Antennas & Propagation -31- Patch Shapes RectangularDipole Elliptical Circular Ring Triangular Analysing Methods - Transmission Line - Cavity - Maxwell Equations

Introduction to Antennas & Propagation Introduction to Antennas & Propagation -32- Application & Performance 1. It is applied where small antennas are required:  aircrafts, mobiles, etc 2. Due to shape variations they are versatile in polarisation, pattern, impedance, etc. 3. They have a low efficiency, spurious feed radiation and a narrow bandwidth 4. They usually operate in broadside regime 5. /3 < L < /2 and 2 <  r < 12

Introduction to Antennas & Propagation Introduction to Antennas & Propagation -33- Parabolic Reflector Antennas

Introduction to Antennas & Propagation Introduction to Antennas & Propagation Uda-Yagi:15dB 2. Helical Antenna:15dB 3. Antenna Arrays high gains  many elements 4. Horn: high gains  large size Large Gains Complicated Feeding Artificially increase size - (re-) transmitted waves are in phase - (re-) transmitted waves are as parallel as possible Aperture increasing Reflector

Introduction to Antennas & Propagation Introduction to Antennas & Propagation -35- Parabolic Reflector Feed Parabolic Dish Parallel and in-phase waves Non-uniform fields due to aperture blocking etc … Aperture Efficiency = 80% r - Dish has to be 100% parabolic - Feeder shouldn’t block too much

Introduction to Antennas & Propagation Introduction to Antennas & Propagation -36- Applications 1. Used where high gains are required:  Cosmic Radiation, etc. 2. Navigation 1. Beam is slightly steerable 2. Deviation from perfect surface can be made <1mm 3. Diameters are usually 100m-300m

Introduction to Antennas & Propagation Introduction to Antennas & Propagation -37- Practical Considerations

Introduction to Antennas & Propagation Introduction to Antennas & Propagation -38- Practical Considerations - The Quality Factor Q - Electrically Small Antennas - Physically Small Antennas - Imperfect Ground

Introduction to Antennas & Propagation Introduction to Antennas & Propagation -39- Feeding

Introduction to Antennas & Propagation Introduction to Antennas & Propagation -40- ‘Exotic’ Antennas - Fractal Antennas - Light Antennas - Gravity Antennas Everything what propagates can be transmitted. Everything what can be transmitted can be received. - EM waves, sound, smell, light, gravity and maybe 6 th sense -