Antenna II LN07_Reflector Antennas 1 /34 Reflector Antennas

Antenna II LN07_Reflector Antennas 2 /34 Reflector Antennas  Since discovery of EM propagation in 1888 by Hertz, reflector antennas has been introduced.  Many various geometrical shapes are introduced at World War II by radar applications.  Its progress was in radio astronomy, microwave communication, and satellite tracking.  Most popular shapes are plane, corner, and curved reflectors (parabolic) as shown:

Antenna II LN07_Reflector Antennas 3 /34 Corner Reflectors  Plane Reflector:  Its simple type is a plane reflector introduced to direct energy in a desired direction.  Polarization of source and its position relative to surface can be used to control radiating properties (pattern, impedance, directivity) of overall system.  Image theory has been used to analyze radiating characteristics of such a system.  Perturbations by dimensions finite can be accounted for by method of GTD.  Corner Reflector:  To better collimate energy, corner reflector is introduced.  Because of its simplicity in construction, it has many unique applications.  In a radar, it can be used for deception of enemy used in EW systems when its included angle is 90◦ as shown:  Because of this unique feature, military ships and aircraft (stealth vehicles such as B-2, F117, F22, F35) are designed with minimum sharp corners to reduce their detection by enemy radar.  In most practical applications, included angle is 90 o ; however other angles are sometimes used.  For reflectors with infinite sides, gain increases as included angle between planes decreases.  Corner reflectors are also widely used as receiving elements for home TV.

Antenna II LN07_Reflector Antennas 4 /34 Corner Reflectors

Antenna II LN07_Reflector Antennas 5 /34  Image theory to analyze field radiated by a source in presence of a corner reflector: Corner Reflectors

Antenna II LN07_Reflector Antennas 6 /34  90 o Corner Reflector:  Its radiation characteristics are most attractive. Corner Reflectors

Antenna II LN07_Reflector Antennas 7 /34  Referring to figures, total field of system can be derived by summing contributions from feed and its images:  In far-zone, normalized scalar field can be written as:  Where for: Corner Reflectors

Antenna II LN07_Reflector Antennas 8 /34 Corner Reflectors  It is evident that for small spacing, pattern consists of a single major lobe.  Whereas multiple lobes appear for larger spacing (s>0.7λ).  For s=λ pattern exhibits two lobes separated by a null along φ=0 ◦ axis.

Antenna II LN07_Reflector Antennas 9 /34  Other Corner Reflectors:  A similar procedure can be used to derive array factors and total fields for:  By using long filament wires as feeds, that azimuthal plane (θ=π/2) array factor is:  Multiple lobes begin to appear when: Corner Reflectors

Antenna II LN07_Reflector Antennas 10 /34 Parabolic Reflector  Parabolic Reflector:  Overall radiation characteristics can be improved if configuration is upgraded.  In geometrical optic, a beam of parallel rays is focused at a focal point.  By principle of reciprocity, a point source is placed at focal point produces a parallel beam.  Disadvantage of front-fed is that T-line from feed must usually be long.  Long lines may not be tolerable in many applications such as low-noise receiving systems.  A arrangement for feeding in focal point is Cassegrain feed:  A parabolic reflector can take two different forms as:  Two techniques that can be used to analyze:  Aperture distribution method.  Current distribution method.

Antenna II LN07_Reflector Antennas 11 /34 Reflector Antennas  Example:

Antenna II LN07_Reflector Antennas 12 /34 Two-dimensional configuration of a paraboloidal reflector Reflector Antennas  Surface Geometry:  Surface of a paraboloidal reflector is formed by rotating a parabola about its axis.  Design is based on optical techniques, and it does not take into account any deformations (diffractions) from edge of reflector.  Referring to figure and choosing a plane perpendicular to axis of reflector:  Since:  Terms of the rectangular coordinates:  A unit vector that is normal to local tangent at surface reflection point:  A gradient is taken to form a normal to surface:

Antenna II LN07_Reflector Antennas 13 /34 Reflector Antennas  A unit vector, normal to S:  To find angle between unit vector n and a vector directed from focus to reflection point:  In a similar manner, angle between unit vector n and z-axis:  Relating subtended angle θ 0 to f/d ratio:  Induced Current Density:  If surface can be approximated by an infinite plane, by the method of image:  It is valid when curvature of reflecting object is large compared to a wavelength. Using: Another form is: H i and H r are defined at surface of conductor Physical-optics approximation

Antenna II LN07_Reflector Antennas 14 /34 Reflector Antennas  If reflecting surface is in far-field of source generating incident waves:  Two techniques that can be used to analyze:  Aperture distribution method.  Current distribution method.  Aperture Distribution Method:  Using GTD (ray Tracing), field reflected by surface of paraboloid is first found over a plane.  Plane is normal to axis of reflector as aperture plane:  Equivalent sources are then formed over this plane.  Usually it is assumed that equivalent sources are zero outside projected area.  These equivalent sources are then used to compute radiated fields.

Antenna II LN07_Reflector Antennas 15 /34 Reflector Antennas  Aperture Distribution Method (cont.):  Let us assume a y-polarized source with:  Incident field, with a direction perpendicular to radial distance: Where:

Antenna II LN07_Reflector Antennas 16 /34 Reflector Antennas  Aperture Distribution Method (cont.):  It can be shown:  To find aperture field E ap at plane through focal point, first E r is found:  An equivalent is formed at aperture plane: Where:

Antenna II LN07_Reflector Antennas 17 /34 Reflector Antennas  Aperture Distribution Method (cont.):  To demonstrate utility of technique, a 35GHz reflector, with an f/d=0.82, d=24.99cm.  Its feed is a conical dual-mode horn.  Since feed is symmetry, patterns do not possess any cross-polarized components. Principal E- or H-plane pattern of a symmetrical front-fed paraboloidal reflector Field point locations of constant amplitude contours in the aperture plane of a symmetrical front-fed paraboloidal reflector

Antenna II LN07_Reflector Antennas 18 /34 Reflector Antennas  Aperture Distribution Method (cont.):  Cross-Polarization:  Polarization components of the paraboloidal reflector is shown as:  The y-component is designated as the principal polarization.  The x-component as the cross-polarization.  It is evident that symmetrical cross- polarized components are 180 o out of phase.  Direction of induced current determines far- field polarization of antenna.  By combination crossed electric and magnetic dipole feed, far-field radiation is free of cross-polarization.  Because of its ideal characteristics, it is usually referred to as a Huygens’ source. Electric and magnetic dipole fields combined to form a Huygens’ source with ideal feed polarization for reflector Co-polarization and cross-polarization components

Antenna II LN07_Reflector Antennas 19 /34 Reflector Antennas

Antenna II LN07_Reflector Antennas 20 /34 Reflector Antennas  Aperture efficiency:  Spillover efficiency:  fraction of total power that is radiated by feed and collimated by reflecting surface.  Taper efficiency:  Uniformity of amplitude distribution of feed pattern over surface of reflector.  Phase efficiency:  Phase uniformity of field over aperture plane.  Polarization efficiency:  Polarization uniformity of field over aperture plane.  Blockage efficiency.  Random error efficiency.  Total efficiency:

Antenna II LN07_Reflector Antennas 21 /34 Gains of some worldwide large reflector antennas Reflector Antennas  Gain:  Factors that reduces antenna gain are:  Asymmetrical patterns.  Phase center error.  Cross-polarized field components.  Blockage.  Random surface error.

Antenna II LN07_Reflector Antennas 22 /34  Example :  One of world's largest solar parabolic dishes at Ben-Gurion National Solar Energy Center in Israel. Reflector Antennas

Antenna II LN07_Reflector Antennas 23 /34  Resolution: Arcsec. Reflector Antennas

Antenna II LN07_Reflector Antennas 24 /34 Reflector Antennas  The Very Large Array (VLA) at Socorro, New Mexico, United States.

Antenna II LN07_Reflector Antennas 25 /34 Reflector Antennas  Another Very Large Array (VLA):  On 12 May 2012, another Atacama Large Millimeter Array (ALMA) antenna was carried.  Atacama is a desert in northern of Chile.

Antenna II LN07_Reflector Antennas 26 /34  Very Large Array (VLA) dish details: Reflector Antennas

Antenna II LN07_Reflector Antennas 27 /34  Green Bank Telescope which stands near Green Bank, West Virginia. Reflector Antennas

Antenna II LN07_Reflector Antennas 28 /34 Cassegrain Antennas  Cassegrain Reflectors:  Cassegrain improves performance of large ground-based microwave reflector antennas for satellite tracking and communication.  Cassegrain is attractive for applications that require gains of 40dB or greater.  Both single- and double-reflector were designed to convert a spherical wave at source into a plane wave.  Magnification factor is ratio of main reflector diameter to sub reflector diameter.

Antenna II LN07_Reflector Antennas 29 /34 Cassegrain Antennas  Geometrical optics for reshaping and synthesis of reflectors of a Cassegrain system.

Antenna II LN07_Reflector Antennas 30 /34  By technique of equivalent parabola, main dish and sub-reflector are replaced by an equivalent focusing surface at a certain distance from real focal point. Cassegrain Antennas

Antenna II LN07_Reflector Antennas 31 /34  Cassegrain Forms:  In addition to classical Cassegrain forms, there are other configurations: Cassegrain Antennas

Antenna II LN07_Reflector Antennas 32 /34  Gregorian Forms: Cassegrain Antennas

Antenna II LN07_Reflector Antennas 33 /34  Spherical Reflector:  A paraboloid reflector is an ideal collimating device.  But in many applications, it is poor in angular scanning.  Although scanning can be accomplished by:  Mechanical rotation of entire structure.  Displacement of feed alone.  But it is used by large mechanical moment.  By contrast, spherical reflector can make an ideal wide-angle scanner.  This is obtained from its perfectly symmetrical geometrical configuration.  But spherical reflector are poor inherent collimating properties.  For example: a point source is placed at focus of sphere, does not produce plane waves. Spherical Reflector  This departure of reflected wave front from a plane wave is known as spherical aberration.  It depends on diameter and focal length of sphere.  By reciprocity, plane waves incident on a spherical reflector do not converge at focal point.  However focusing of wave (at various angles) is performed by translating and orientating feed.  A spherical reflector has capability of focusing plane waves incident at various angles.  It is performed by translating and orientating feed and by illuminating different parts of geometry.  For three rays, focusing characteristics of a typical spherical reflector is illustrated in:  A caustic is a point, a line, or a surface through which all the rays in a bundle pass and where the intensity is infinite.

Antenna II LN07_Reflector Antennas 34 /34  Example:  Arecibo Observatory is a radio telescope near city of Arecibo in Puerto Rico.  Its spherical reflector has a 305m dish in diameter.  Surface is made of almost 40,000 perforated aluminum panels.  Its focal point is 150m above reflector.  Attached devices to antennas are very sensitive and highly complex radio receivers.  These devices operate immersed in a liquid helium, to maintain a very low receiver temperature.  At such cold temperatures electron noise in receivers is very small.  Therefore, incoming radio signals, which are very weak, are amplified.  Arecibo system operates at frequencies from 50MHz (λ=6m) up to 10GHz (λ=3cm).  1MW planetary radar transmitter located in a special room inside dome. Spherical Reflector