6 Antennas Most antennas are resonant structures NarrowbandSize is inversely proportional to frequency of operationTravelling wave antennas also importantWidebandSize dictates lowest frequency of operation1000 ft diameter; 50 MHz to 10 GHzchip 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? – radiation BAImagine 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 aerialRadiation of power in space can be controlled by carefully arranging the patterns of electron motionThis is the same as their sensitivity to received signals from different directions in space
16 Fundamental antenna parameters Radiation pattern; radiation power density; radiation intensityBeamwidth; directivity; sidelobe levelsEfficiency; gainPolarisationImpedanceBandwidthVector effective length and equivalent areaAntenna temperature
21 Radiation pattern definitions Isotropic antennaRadiates equally in all directions in space; physically unrealisableOmnidirectional antennaRadiates equally in all directions in one plane only; e.g. dipoles, monopoles, loops, etc.Directional antennaRadiates strongly in a given direction; has a principal or main lobe, the maximum of which point in the direction of the antenna’s boresideCan you guess what is meant by front-to-back ratio?
32 BandwidthMany properties vary with frequency and deteriorate in value from their optimum values:Pattern bandwidthDirectivity/gainSidelobe levelBeamwidthPolarisationBeam directionImpedance bandwidthInput impedanceRadiation efficiency
33 PolarisationAntenna polarisation refers to the orientation of the far-field radiated electric field vector from the antennaA vertical dipole radiates a vertical electric fieldA horizontal dipole radiates a horizontal electric fieldA 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
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 PolarisationThe radiation pattern performance of antennas is often specified in terms of its co-polar and cross-polar componentsDetailed 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” polarisationCross-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)RLXARrRgXgVgabIgRLThevenin equivalent circuit (suitable for electric radiators, e.g. monopole, dipole, etc.)aVaIaRrxRrXrxbXAIgGgBgGrGLBAabNorton equivalent circuit (suitable for magnetic radiators, e.g. loop, etc.)GrxBrxGrGLBAabIa
39 Impedance The antenna operation is characterised by an impedance ZA An equivalent radiation resistance, RrA loss (ohmic and dielectric) resistance, RLA reactance, XAWhen connected to a generator, usually via a transmission line, the usual transmission line and circuit theories applyRadiated powerMaximum 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
43 Effective aperture area Ae This is usually assumed to refer to the co-polar radiation pattern on the boreside of an antennaThe antenna effective aperture area is defined as a ratioPT is the power delivered to a matched load in WWi is the incident wave power density in Wm–2Ae is the antenna effective aperture area in m2For any passive antenna we can invoke the principle of reciprocity to show that
44 Antenna aperture efficiency For all aperture antennasThis allows us to introduce the concept of antenna aperture efficiencyFor aperture antennasFor 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