1. Antennas & Propagation
Mischa Dohler
King’s College London
Centre for Telecommunications Research

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

3. Review

4. Wire Antennas Hertzian Dipole Finite Length Dipole Antenna Array
Uda-Yagi
Turnstile
Loop
Helix
Quadrifilar Helix

5. VHF TV Receive Antenna Uda-Yagi Antenna Feeding Mast 5-6 Directors
Sheet Reflector
Folded Dipole Driver
Feeding Mast
5-6 Directors

6. Helical Antenna 3/4 < C/ < 4/3
Axial Mode Radiation (endfire) appears if:
3/4 < C/ < 4/3
Narrow Mainbeam with minor sidelobes
HPBW  1/(Number of turns)
Circular Polarisation (orientation  helix orientation)
Wide Bandwidth
No coupling between elements
Supergain Endfire Array z y x
Circumference C

7. Frequency Independent Antennas

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.

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

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 sheet Log-periodic wire
Characteristic will be repeated at (discrete) nf1.

11. Log-periodic Dipole Array

12. Spiral Antenna

13. Fractal Antenna
                                             

14. Aperture Antennas

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

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

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

18. Parameter Rectangular Aperture
y-z plane:
x-z plane:

19. Polarisation in the far field is the same as in the aperture.
Circular Aperture y P r’ x r  z a
Polarisation in the far field is the same as in the aperture.
J1(x) is the first order Bessel Function of first kind.

20. Parameter Circular Aperture
y-z plane:
x-z plane:
Large Apertures:

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

22. Horn Antennas

23. Horn Antennas TE10 Excitation: TE10 mode
E-Plane sectoral horn
H-Plane sectoral horn
Pyramidal horn
Excitation: TE10 mode
Impedance Matching through flare
Gradual Transmission with minimised reflection

24. Specifications Directive Radiator
Primary feed for parabolic reflectors
High gain, wide bandwidth and simple
Particularly used in microwave region (>1GHz)
Fan radiation patterns

25. Slot Antennas

26. Slot Antennas z -x L y w
Bookers Principle:

27. Slot on Waveguide Walls
TE10 mode
Radiation is maximum at maximal interrupted current
Radiation
No Radiation

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

29. Microstrip (Patch) Antennas

30. Patch Structure Patch Substrate L t - - - - + + + + r d + + + +
Feed
Substrate L t r d

31. Patch Shapes Rectangular Dipole Circular Ring Elliptical Triangular
Analysing Methods
Transmission Line
Cavity
Maxwell Equations
Triangular

32. Application & Performance
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

33. Parabolic Reflector Antennas

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

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

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

37. Practical Considerations

38. Practical Considerations
- The Quality Factor Q
- Electrically Small Antennas
- Physically Small Antennas
- Imperfect Ground

39. Feeding

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 6th sense -