Reflector antennas are widely used in telecommunication applications, radars, and radio astronomy. Most high-gain antennas are reflector antennas. Reflector antennas are secondary radiators, which redirect the radiation of the primary source, the feed. The feed is usually a small horn antenna. Also feed arrays can be used. Reflector antenna has usually one or two reflectors.
In general, the reflector can be of any shape but most reflector antennas are based on a rotated conic section : plane, hyperboloid, paraboloid, ellipsoid, or sphere. Properties of rotated conic sections are discussed in Section 4.1.1. Also shaped reflectors are usually based on these basic shapes and can be described as (nearly) planar, hyperbolical, etc.
In Section 4.1.2, collimating reflector antennas are presented. Collimating reflector antennas are based on a parabolic reflector. Diverging-beam antennas based on hyperboloids and/or ellipsoids are presented in Section 4.1.3.
4.1.1 Rotated conic sections
Many reflector antennas are based on rotated conic sections because of their geometrical properties. An illustration of conic sections is in Figure 4.1. A line, a hyperbola, a parabola, an ellipse, and a circle are special cases of a general conic section.
Paraboloid, ellipsoid, and hyperboloid have special focusing properties. Focusing properties reflectors based on these rotated conic sections are illustrated in Figure 4.2.
Rays starting from a focal point (one of the focal points) are also drawn in Figure 4.2.
Rays reflected from a paraboloid are parallel, i.e., collimated.
Rays starting from one focal point of an ellipsoid are reflected to the other focal point.
Rays starting from one focal point of a hyperboloid are reflected so that they appear to come from the other focal point.
Figure 4.1: A conic section is an intersection of a plane and one or two napes of a cone.
Figure 4.2: Focusing properties of rotated conic sections; a) paraboloid, b) ellipsoid, and c) hyperboloid .
All rotated conic sections can be expressed with the following equation :
) 1 ) (
r e , (4.1)
where r is distance from a focal point to the surface in direction θ, e is eccentricity, and f is the focal length (or radius). For a sphere e = 0, an ellipsoid e < 1, a paraboloid e = 1, a hyperboloid e > 1 and for a plane |e| .
4.1.2 Collimating reflector antennas
Collimating reflector antennas are usually based on a paraboloid reflector. A paraboloid reflector antenna is the easiest and cheapest type of antenna to get a high directivity, for example in communication applications.
A paraboloid collimates the radiation coming from a focal point, i.e., transforms a spherical wave to a plane wave, as illustrated in Figure 4.2 a). The paraboloid can be fed directly from the focal point or a subreflector antenna can be used whose focal point coincides with the focal point of the paraboloid. In a Cassegrain antenna a hyperboloid subreflector is used. If an ellipsoid subreflector is used then it is called a Gregorian antenna. The Cassegrain geometry is more common because the structure is more compact.
A single paraboloid reflector, Cassegrain, or Gregorian antenna can be either centre fed or offset antenna. With offset structure the aperture blockage effect of the feed or
subreflector and its supports can be avoided. Aperture blockage causes lowered aperture efficiency and increased side-lobe level. Figure 4.3 shows a Cassegrain antenna fed from the vertex of the paraboloid and an offset Cassegrain antenna.
Figure 4.3: Centre fed and offset Cassegrain antennas.
The offset structure causes higher cross-polarisation than the symmetrical centre fed geometry. For example, the cross-polarization level is typically -20 dB to -25 dB for a single offset reflector . The cross-polarization caused by the offset structure can be minimized with so called compensated design that is based on the Mizugutch condition . The Mizugutch condition is also called “the basic design equation” for offset dual reflector antennas and its derivation is given e.g. in . The Mizugutch condition is based on choosing correctly the subreflector eccentricity and the angles between subreflector and main reflector.
The Mizugutch condition to cancel the cross-polarisation component of an offset paraboloidal reflector antenna is :
2 cos 1
tan 1 2
where is the angle between the feed axis and axis of the subreflector, β is the angle between the axis of the subreflector and that of the paraboloidal main reflector, and e is the eccentricity of the subreflector (ellipsoid e < 1 or hyperboloid e > 1) . As an example, a Gregorian geometry is illustrated in Figure 4.4.
4.1.3 Diverging-beam reflector antennas
The basic diverging-beam reflector antennas are based on using ellipsoid and/or hyperboloid reflectors. Ellipsoid and hyperboloid reflectors, due to their optical focusing properties, can be used to relocate the focal point of the antenna system.
Ellipsoids/hyperboloids do not collimate the radiation to one direction and therefore they alone cannot be used for high gain antenna. Dual reflector ellipsoid/hyperboloid geometry is mainly usable for initial condition for a shaped-beam reflector antenna.
The Mizugutch condition for hyperboloids and ellipsoids is derived in :
sin tan 1
m s s
e e e
where the subscripts m and s stand for the main and the subreflector, respectively, and e’s are the eccentricities of the surfaces, is the tilted angle of the subreflector axis with respect to the axis of main reflector and β is the angle between the axis of subreflector and the axis of feed . As an example, ellipsoid-hyperboloid geometry is illustrated in Figure 4.5.
Paraboloidal main reflector
Elliptic subreflector Ffeed
Axis of the subreflector Axis of the main reflector Axis of the feed
Figure 4.4: Geometry of a Gregorian type offset reflector antenna. The focal point of the main reflector is F and the focal points of the subreflector are F and Ffeed.
The dual reflector feed systems, in Chapter 6, are based on the dual offset hyperboloid- hyperboloid geometry. The reflector surfaces are shaped surfaces, not hyperboloids. For both sub- and main reflectors one focal point is behind the reflector surface, therefore the geometry is similar to the hyperboloid-hyperboloid geometry.