Parabolic Reflector Antenna Dr Jaikaran Singh

Contents General Properties Parabolic Reflector Antenna Reflector Type Cylindrical Parabolic Reflector Paraboloid of Revolution Pattern of large circular aperture with uniform Illumination Cassegrain Reflector Antenna Feeding Method 3/16/2019Antenna and Wave Propagation2

General Properties 3/16/2019Antenna and Wave Propagation3

General Properties 1.Focus : Any distance from P to F 2.Vertex : Distance from F to center 3.Directrix : Equal distance between PF & PQ 3/16/2019Antenna and Wave Propagation4

General Properties AA’ is plane normal to axis PS=QS-PQ & PF=PQ PF+PS=PF+QS-PQ=QS According equation, all waves from isotropic source at focus that are reflected from parabola arrive at AA’ with equal phase. BB’ is called aperture plane 3/16/2019Antenna and Wave Propagation5

Parabolic Reflector Antenna Parabolic Reflectors are Microwave antennas. Frequency Range : Above 1 MHz Application : Radio and wireless, Telescope The gain of the paraboloid is a function of aperture ratio (D/λ). The Effective Radiated Power (ERP) of an antenna is the multiplication of the input power fed to the antenna and its power gain. Fed with Usually horn antenna and Cassegrain feed. The reflected wave forms a colllimated wave front, out of the parabolic shape. f/D Ratio : important parameter of parabolic reflector. Values varies: 0.25 to /16/2019Antenna and Wave Propagation6

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Direct radiation from source can be eliminated by means of primary source. 3/16/2019Antenna and Wave Propagation8

For getting uniform aperture field distribution, angel θ1 must be small by increasing length L by keeping D constant. 3/16/2019Antenna and Wave Propagation9

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Principle of Operation Definition : parabola is - Locus of a point, which moves in such a way that its distance from the fixed point (called focus) plus its distance from a straight line (called directrix) is constant. According Figure The point F is the focus (feed is given) and V is the vertex. The line joining F and V is the axis of symmetry. PQ are the reflected rays where L represents the line directrix on which the reflected points lie (to say that they are being collinear). Hence, as per the above definition, the distance between F and L lie constant with respect to the waves being focussed. 3/16/2019Antenna and Wave Propagation11

Properties of Parabola All the waves originating from focus, reflects back to the parabolic axis. Hence, all the waves reaching the aperture are in phase. As the waves are in phase, the beam of radiation along the parabolic axis will be strong and concentrated. The gain of the paraboloid is a function of aperture ratio (D/λ). The Effective Radiated Power (ERP) of an antenna is the multiplication of the input power fed to the antenna and its power gain. wave guide horn antenna and Cassegrain feed is used as a feed radiator 3/16/2019Antenna and Wave Propagation12

Advantages Reduction of minor lobes Wastage of power is reduced Equivalent focal length is achieved Feed can be placed in any location, according to our convenience Adjustment of beam (narrowing or widening) is done by adjusting the reflecting surfaces 3/16/2019Antenna and Wave Propagation13

Disadvantage Some of the power that gets reflected from the parabolic reflector is obstructed. This becomes a problem with small dimension paraboloid. 3/16/2019Antenna and Wave Propagation14

Applications The cassegrain feed parabolic reflector is mainly used in satellite communications. Also used in wireless telecommunication systems. 3/16/2019Antenna and Wave Propagation15

Reflectors Types Cylindrical Parabola Parabolic Reflector Truncated Paraboloidal Short Cylinderical with plates (Pill box Antenna) Cheese Antenna 3/16/2019Antenna and Wave Propagation16

Cylindrical Parabola It is generated by moving the parabolic contour parallel to itself It provides mouth and has a line instead of a point as the focus. 3/16/2019Antenna and Wave Propagation17

Parabolic Reflector It is fed by a point source, a waveguide horn. It generates pencil beam. It has 3D curved surface generated by rotating a parabola about its own axis. 3/16/2019Antenna and Wave Propagation18

Truncated Paraboloidal It represents a cylinder that are asymmetrical section cut from a parabolic surface. These also generated fan beams in azimuth or elevation depending on the location of asymmetry 3/16/2019Antenna and Wave Propagation19

Short Cylindrical with plates (Pill box Antenna) It represents a cylinder that is short in axial direction and provided with conducting end plate. It is also called pill box 3/16/2019Antenna and Wave Propagation20

Cheese Antenna It is a combination of a pillbox and a parabolic cylinder. 3/16/2019Antenna and Wave Propagation21

Paraboloidal Reflector Its is mostly used reflector It generate pencil beam required in tracking radars and point to point communication Its having many names like parabolic dish, dish reflector, microwave dish, parabolic reflector or simply dish antenna 3/16/2019Antenna and Wave Propagation22

Spherical Reflector Wider scanning angle compare to parabolic reflector Simple spherical reflector does not produce equiphase radiation pattern and pattern generally poor Reflector with large radius 3/16/2019Antenna and Wave Propagation23

Cassegrain Feed Casse grain is another type of feed given to the reflector antenna. In this type, the feed is located at the vertex of the paraboloid, unlike in the parabolic reflector. A convex shaped reflector, which acts as a hyperboloid is placed opposite to the feed of the antenna. It is also known as secondary hyperboloid reflector or sub-reflector. It is placed such that its one of the foci coincides with the focus of the paraboloid. Thus, the wave gets reflected twice. 3/16/2019Antenna and Wave Propagation24

Working of a Cassegrain Antenna 3/16/2019Antenna and Wave Propagation25

When the antenna acts as a transmitting antenna, the energy from the feed radiates through a horn antenna onto the hyperboloid concave reflector, which again reflects back on to the parabolic reflector. The signal gets reflected into the space from there. Hence, wastage of power is controlled and the directivity gets improved. When the same antenna is used for reception, the electromagnetic waves strike the reflector, gets reflected on to the concave hyperboloid and from there, it reaches to the feed. A wave guide horn antenna presents there to receive this signal and sends to the receiver circuitry for amplification. Take a look at the following image. It shows a paraboloid reflector with cassegrain feed. 3/16/2019Antenna and Wave Propagation26

Cassegrain Antenna : Features Widely used in telescope Permit reduction in axial dimension of antenna Permit greater flexibility in design of feed system and eliminate the need of long transmission line Larger the sub reflector nearer it will be to the main reflector and shorter will be axial dimension of antenna assembly Larger or smaller the sub reflector-larger the aperture blocking accordingly Losses in Transmission line can be degrade the receiver sensitivity. To reduce the aperture blocking, the sub reflector can be made of horizontal grating of wires. Such a sub reflector is called trans reflector 3/16/2019Antenna and Wave Propagation27

Cassegrain Antenna : Features Trans reflector passes the vertically polarized wave with negligible attenuation and reflect horizontal polarized wave radiated by the feed. The main reflector is often coated with polrization twisters also called twist reflector. These are equivalent to quarter wave plate which produce a 90 degree rotation of plane of polrization. The Horizontal polrized waves reflected by twist reflector at the surface of the main dish, are the transformed into vertically polrized wave and pass through a sub reflector with negligible attenuation 3/16/2019Antenna and Wave Propagation28

Cylindrical Parabolic Reflector 3/16/2019Antenna and Wave Propagation29

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Paraboloid of Revolution 3/16/2019Antenna and Wave Propagation31

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Pattern of large circular aperture with uniform Illumination 3/16/2019Antenna and Wave Propagation33

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Comparison of radiation pattern of Circular aperture and square Aperture The pattern of square aperture 10 ʎ on a side and for a circular aperture 10 ʎ are compares In both cases the field is considered as uniform in magnitude and phase. The pattern is given as a function of ɸ in xy plane. The pattern in the xz plane are identical to those in the xy plane. Although the beam width for circular aperture is greater than square aperture, the side lobes in circular aperture are smaller. Similar effect can be produced in square aperture by tapering the illumination. 3/16/2019Antenna and Wave Propagation36

Feeding Methods Dipoles Dipoles with Parasitic Reflector Open Ended Waveguide better phase characteristics Waveguide Horn Rear Feed Front feed Offset feed 3/16/2019Antenna and Wave Propagation37

Dipoles with Parasitic Reflector Parasitic reflectors may be other type Another dipole A plane Half Cylinder A hemisphere Disadvantage Radiates along length only Poor polarization Part of radiation is perpendicular to primary pattern which causes cross polarization and hence reduction of gain. 3/16/2019Antenna and Wave Propagation38

Open Ended Waveguide More energy directed in forward direction Circular paraboloidal fed by circular waveguide in TE11 mode TE10 mode of rectangular waveguide does not give circular symmetry of radiation pattern so current distribution is different Rectangular waveguide feed is good for generating fan beam. 3/16/2019Antenna and Wave Propagation39

Waveguide Horn More Directivity Act as point source with large reflector For uniform radiation pattern across parabolic aperture only small angular portion of the pattern should be used. The ratio f/D should be large for uniform illumination A part of radiated energy is loss so efficiency reduced By increasing f/D ratio, the losses can be reduced 3/16/2019Antenna and Wave Propagation40

Rear Feed Transmission line is not in center and this result in an asymmetrical pattern In case of transmission line is in center as in case of dual aperture feed waveguide is in the center of dish and energy is made to bend 180 degree at the end of waveguide by a properly designed reflecting plate It forms a compact system Minimum length of transmission line is required resulting in less line loss. 3/16/2019Antenna and Wave Propagation41

Front Feed Obstruct aperture Impedance mismatch in feed results Reflection from dish cause standing waves in transmission line which again cause impedance mismatch and degrade performance By using impedance mismatching and apex matching plates mismatch can be reduced results in lower gain 3/16/2019Antenna and Wave Propagation42

Offset Feed Only half of the parabola is used Normally hog horn is employed in place on conventional pyramidal horn No aperture blocking No impedance blocking No Impedance mismatch Seriously affects performance More difficult to scan 3/16/2019Antenna and Wave Propagation43

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