1. Introduction to Antennas
Dr Costas Constantinou
School of Electronic, Electrical & Computer Engineering
University of Birmingham W: E:

2. Recommended textbook
Constantine A. Balanis, Antenna Theory: Analysis and Design, 3rd Edition, Wiley-Interscience, 2005; ISBN: X
Chapters 1 & 2

3. Antennas
An antenna can be thought of as a transition / transducer device
Two ways of describing antenna operation
Field point of view
Circuit point of view

4. Antenna examples
Wire antennas
Monopoles
Dipoles
Arrays

5. Antenna examples Aperture Antennas Reflectors Lenses Horns Patches
Planar inverted F

6. Antennas Most antennas are resonant structures
Narrowband
Size is inversely proportional to frequency of operation
Travelling wave antennas also important
Wideband
Size dictates lowest frequency of operation
1000 ft diameter; 50 MHz to 10 GHz
chip size = 2 x 1 mm2; 60 GHz antenna

7. How does it work? – radiation
Imagine and electron (red) with its lines of force (green) moving from left to right.

8. How does it work? – radiation
If the electron moves with a constant velocity the lines “move” with it ...

9. How does it work? – radiation
... just like this.

10. How does it work? – radiation
What happens if the electron motion changes suddenly? Remember, change of motion is another word for acceleration.

11. How does it work? – radiationB A
Imagine a test charge A close to the electron. The delay is negligible, so it sees the electric field line inside the sphere. If the test charge (now at B) is far away, the delay means the “news” that the electric fields have changed has not reached the test charge yet. Lines cannot be discontinuous – they need to stretch. The lines of force acquire a “kink” because they can only start or finish on charges.
Sphere grows with time (i.e. delay increases with distance)

12. How does it work? – radiation
Here’s a picture of many lines of force: there is a spherical shell of stretched lines of force, moving outwards from the accelerated charge at the speed of light, dividing the region of space that “knows” about the change of motion of the charge (inside the sphere) from the region of space that hasn’t yet “found out”.

13. How does it work? – radiation
Here’s a movie of how it all works for all the electrons shaking in a short conductor, vertically positioned and the electric fields they generate.
Source: MIT Open Courseware

14. How does it work? – radiation
And here is a slightly simplified view showing how loops of line of force close in on themselves and become an entity in their own right, a wave carrying energy away from the antenna.
Source: MIT Open Courseware

15. Here’s a picture of the sensitivity pattern in 3D of your average television antenna. The “hotter” the colour the strongest the lines of force, or equivalently the energy transmitted, and by necessity (the principle of reciprocity applies), the receive sensitivity of the antenna to waves coming from each direction in space.
Antennas – TV aerial
Radiation of power in space can be controlled by carefully arranging the patterns of electron motion
This is the same as their sensitivity to received signals from different directions in space

16. Fundamental antenna parameters
Radiation pattern; radiation power density; radiation intensity
Beamwidth; directivity; sidelobe levels
Efficiency; gain
Polarisation
Impedance
Bandwidth
Vector effective length and equivalent area
Antenna temperature

17. Radiation pattern
A mathematical and/or graphical representation of the properties of an antenna, usually the radiation intensity vs. spatial direction coordinates sufficiently far from the antenna
Is polarisation specific
Spherical polar coordinates are always used
Source: C.A. Balanis©

18. Radiation pattern
Linear pattern
Polar pattern
Source: C.A. Balanis©

19. Radiation pattern Linear pattern E plane is plane of electric field
H plane is plane of magnetic field
If field direction not known, do not use E or H plane
Source: C.A. Balanis©

20. Omnidirectional antenna radiation pattern
H-plane
E-plane
λ/2 dipole antenna radiation pattern
Source: C.A. Balanis©

21. Radiation pattern definitions
Isotropic antenna
Radiates equally in all directions in space; physically unrealisable
Omnidirectional antenna
Radiates equally in all directions in one plane only; e.g. dipoles, monopoles, loops, etc.
Directional antenna
Radiates strongly in a given direction; has a principal or main lobe, the maximum of which point in the direction of the antenna’s boreside
Can you guess what is meant by front-to-back ratio?

22. Field regions Reactive near-field Radiative near-field (Fresnel)
Phases of electric and magnetic fields are often close to quadrature
High reactive wave impedance
High content of non-propagating stored energy near the antenna
Radiative near-field (Fresnel)
Fields are predominantly in-phase
Wavefronts are not yet spherical; pattern varies with distance
Radiative far-field (Fraunhofer)
Electric and magnetic fields are in-phase
Wavefront is spherical; field range dependence is e-jkr/r
Wave impedance is real (Eθ/Hφ = 120π = 377 Ω)
Power flow is real; no stored energy
Field regions have no sharp boundaries
Source: C.A. Balanis©

23. Reminder on angular units
Steradians
For the whole sphere,
Radians
Source: C.A. Balanis©

24. Radiation power intensity and density
Poynting vector
Time-averaged Poyting vector
Radiation power density
Radiation intensity
Total radiated power

25. Directivity

26. Directivity
Isotropic antenna
Current element L << λ

27. Directivity
Half wave dipole L = λ/2

28. Beamwidth Current element L << λ
The half-power angles in E-plane are given by,
Halfwave dipole – a similar numerical calculation for the two roots of

29. Beamwidth vs. directivity
The narrower the beamwidth of an antenna, the bigger its directivity
For a single main beam antenna where ΩA is the main lobe half power beam solid angle
Kraus approximation for non-symmetrical main lobes
Tai & Pereira approximation for non symmetrical main lobes
Source: C.A. Balanis©

30. Antenna efficiency, ηant
In an antenna, we experience reduction in radiated power due to
Reflection at the input terminals (impedance mismatch)
Ohmic conductor losses (c) in the antenna conductors
Dielectric losses (d) in the antenna dielectrics
The latter two are grouped under the term antenna radiation efficiency
Typical antenna efficiency values
Dipole ηant ~ 98%
Patch antenna ηant ~ 90%
Mobile phone PIFA ηant ~ 50%
Source: C.A. Balanis©

31. Antenna Gain
Antenna Absolute Gain

32. Bandwidth
Many properties vary with frequency and deteriorate in value from their optimum values:
Pattern bandwidth
Directivity/gain
Sidelobe level
Beamwidth
Polarisation
Beam direction
Impedance bandwidth
Input impedance
Radiation efficiency

33. Polarisation
Antenna polarisation refers to the orientation of the far-field radiated electric field vector from the antenna
A vertical dipole radiates a vertical electric field
A horizontal dipole radiates a horizontal electric field
A general (e.g. horn) antenna with a vertical aperture electric field radiates a vertical electric field in the E-plane and H-plane only; everywhere else the electric field vector is inclined to the vertical and changes with angular direction

34. Polarisation The polarisation of an electromagnetic wave can be
Linear (as in all previously discussed examples)
Circular (e.g. using a helical antenna to transmit)
Elliptical (e.g. circular after reflection from a lossy interface)
Circular and elliptical polarisations have a sense of rotation
Positive helicity (or right hand, clockwise)
Negative helicity
Source: C.A. Balanis©

35. Polarisation
Source: C.A. Balanis©

36. Polarisation Linearly polarised uniform plane wave (E0x and E0y real)
Circularly polarised uniform plane wave (+/- corresponding to positive/negative helicity)
Elliptically polarised uniform plane wave (+/- corresponding to positive/negative helicity; E0x and E0y real)

37. Polarisation
The radiation pattern performance of antennas is often specified in terms of its co-polar and cross-polar components
Detailed mathematical definition is Ludwig’s 3rd definition of cross-polarisation (A. Ludwig (1973), “The definition of cross polarization,” IEEE Transactions on Antennas and Propagation, 21(1))
Co-polar radiation pattern of an antenna is measured with a suitably polarised probe antenna which is sensitive to the “wanted” polarisation
Cross-polarised pattern is measured for linear polarisation by rotating the probe antenna by π/2 around the line joining the two antennas, or for circular/elliptical polarisation by changing the probe antenna helicity sign

38. Impedance Transmitting operation Receiving operation
generator
(Zg = Rg + jXg)
receiver
(Zrx) RL XA Rr Rg Xg Vg a b Ig RL
Thevenin equivalent circuit (suitable for electric radiators, e.g. monopole, dipole, etc.) a Va Ia
Rrx Rr
Xrx b XA Ig Gg Bg Gr GL BA a b
Norton equivalent circuit (suitable for magnetic radiators, e.g. loop, etc.)
Grx
Brx Gr GL BA a b Ia

39. Impedance The antenna operation is characterised by an impedance ZA
An equivalent radiation resistance, Rr
A loss (ohmic and dielectric) resistance, RL
A reactance, XA
When connected to a generator, usually via a transmission line, the usual transmission line and circuit theories apply
Radiated power
Maximum power transferred from generator to antenna (maximum power transfer theorem)
Half of generator power is consumed intenally, other half is shared between antenna losses and antenna radiation

40. Impedance

41. Radiation efficiency
We have come across radiation efficiency before, but now we express it in circuit theory equivalent terms
Describes how much power is radiated vs. dissipated in the antenna

42. Antenna effective length
The voltage at the antenna terminals is determined from the incident field
The effective length is a vector
When taking the maximum value over θ,φ this becomes
For linear antennas
Source: C.A. Balanis©

43. Effective aperture area Ae
This is usually assumed to refer to the co-polar radiation pattern on the boreside of an antenna
The antenna effective aperture area is defined as a ratio
PT is the power delivered to a matched load in W
Wi is the incident wave power density in Wm–2
Ae is the antenna effective aperture area in m2
For any passive antenna we can invoke the principle of reciprocity to show that

44. Antenna aperture efficiency
For all aperture antennas
This allows us to introduce the concept of antenna aperture efficiency
For aperture antennas
For wire antennas where the physical aperture is taken to be the cross sectional area of the wire

45. Friis free-space transmission
From your propagation lectures, assuming matched antennas,
This expression is a statement of the principle of conservation of energy coupled with the notions of antenna gain and antenna effective aperture area

46. What next? Attempt the tutorial sheet on antennas
Next lectures on link budgets