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

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Presentation on theme: "Introduction to Antennas Dr Costas Constantinou School of Electronic, Electrical & Computer Engineering University of Birmingham W:"— Presentation transcript:

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, 3 rd Edition, Wiley- Interscience, 2005; ISBN:0-471-66782-X – Chapters 1 & 2 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 3

4 Antenna examples Wire antennas – Monopoles – Dipoles – Arrays 4

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

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 6 1000 ft diameter; 50 MHz to 10 GHz chip size = 2 x 1 mm 2 ; 60 GHz antenna

7 How does it work? – radiation 7

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11 How does it work? – radiation B A Sphere grows with time (i.e. delay increases with distance) 11

12 How does it work? – radiation 12

13 Source: MIT Open Courseware How does it work? – radiation 13

14 Source: MIT Open Courseware How does it work? – radiation 14

15 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 15

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 16

17 Radiation pattern 17 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 18 Polar pattern Linear pattern Source: C.A. Balanis©

19 Radiation pattern 19 Linear pattern Source: C.A. Balanis© 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

20 Omnidirectional antenna radiation pattern 20 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? 21

22 Field regions Reactive near-field – 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 ( E θ /H φ = 120π = 377 Ω ) – Power flow is real; no stored energy Field regions have no sharp boundaries 22 Source: C.A. Balanis©

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

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

25 Directivity 25

26 Directivity Isotropic antenna Current element L << λ 26

27 Directivity Half wave dipole L = λ/2 27

28 Beamwidth 28 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 Ω A is the main lobe half power beam solid angle Kraus approximation for non- symmetrical main lobes Tai & Pereira approximation for non symmetrical main lobes 29 Source: C.A. Balanis©

30 Antenna efficiency, η ant 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 30 Source: C.A. Balanis© Typical antenna efficiency values Dipole η ant ~ 98% Patch antenna η ant ~ 90% Mobile phone PIFA η ant ~ 50%

31 Antenna Gain 31 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 32

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 33

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 34 Source: C.A. Balanis©

35 Polarisation 35 Source: C.A. Balanis©

36 Polarisation Linearly polarised uniform plane wave ( E 0x and E 0y real) Circularly polarised uniform plane wave (+/- corresponding to positive/negative helicity) Elliptically polarised uniform plane wave (+/- corresponding to positive/negative helicity; E 0x and E 0y real) 36

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 3 rd 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 37

38 Impedance Transmitting operation Receiving operation 38 generator ( Z g = R g + jX g ) receiver ( Z rx ) Thevenin equivalent circuit (suitable for electric radiators, e.g. monopole, dipole, etc.) Norton equivalent circuit (suitable for magnetic radiators, e.g. loop, etc.) RLRL XAXA RrRr RgRg XgXg VgVg a b IgIg RLRL XAXA RrRr R rx X rx a b IaIa VaVa IgIg GgGg BgBg GrGr GLGL BABA a b G rx B rx GrGr GLGL BABA a b IaIa

39 Impedance The antenna operation is characterised by an impedance Z A – An equivalent radiation resistance, R r – A loss (ohmic and dielectric) resistance, R L – A reactance, X A 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 39

40 Impedance 40

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 41

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 42 Source: C.A. Balanis©

43 Effective aperture area A e 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 – P T is the power delivered to a matched load in W – W i is the incident wave power density in Wm –2 – A e is the antenna effective aperture area in m 2 For any passive antenna we can invoke the principle of reciprocity to show that 43

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 44

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 45

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

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