A NEW REFLECTOR ANTENNA DESIGN PROVIDING TWO DIFFERENT PATTERNS

Mesut KARTAL1 İsmail GÜNGÖR2 Bora DÖKEN3

1
Department of Electronics and Communications Engineering, Istanbul Technical University, Maslak 34469, Istanbul,
Turkey
2
Department of Electronics and Communications Engineering, Istanbul Technical University, Maslak 34469, Istanbul,
Turkey
3
Vocational School, Istanbul Technical University, Maslak 34469, Istanbul, Turkey,
1 2 3
e-mail: kartalme@itu.edu.tr e-mail: ismailgungor@itu.edu.tr e-mail: dokenb@itu.edu.tr

Abstract
Antennas with cosecant squared pattern are designed for air-surveillance radar systems. These permit an adapted
distribution of the radiation in the beam and causing a more ideal space scanning. In the practice a cosecant squared
pattern can be achieved by a deformation of a parabolic reflector. Moreover, it can be achieved by using multiple feed
antennas with normal parabolic reflector surfaces. A fan beam antenna which is a directional antenna producing a main
beam having a narrow beamwidth in one dimension and a wider beamwidth in the other dimension can also be used for
similar purposes. This pattern can be achieved by a truncated parabolic reflector or a circular parabolic reflector.
Frequency selective surfaces (FSSs) have been used in the reflector antenna designs in recent years. A FSS is an array
of periodically arranged patches or apertures, showing a particular filtering behavior. Its selectivity in frequency is
obtained by the design and allows the transmission of signals in a certain frequency range only. This work is proposed
to design a reflector antenna that provides both fan beam and cosecant squared pattern by using a FSS. For the
simulation and optimization of the antenna, high frequency simulation software (HFSS) and SUPERNEC antenna
simulation programs are used.

Keywords: Cosecant squared pattern, fan beam antenna, beamwidth, frequency selective surface, reflector
antenna, simulation, optimization, HFSS, SUPERNEC.

1.Introduction
Reflector antennas are probably the most widely used antennas for high-frequency and high-gain applications in
radio astronomy, radar, microwave and millimeter wave communications and satellite tracking and communications [1].
Although reflector antennas can take various geometrical configurations, the most popular shape is the paraboloid
because of its excellent ability to produce a high gain with low side lobes and good cross-polarization characteristics in
the radiation pattern [1]. Because of its narrow beamwidth, radiating patterns of reflector antennas is defined as “pencil
beam”. A reflector antenna consists of one reflector surface and a feed antenna. Reflector designs can be planar, have a
parabolic arc, or incorporate a design that uses neither of the aforementioned designs. Feed structures can be placed in
front of the reflector or be fed from the back, depending on the reflector technology being implemented [2]. While
reflector antennas have high gain and are very directive making them well suited for satellite - earth stations also
reflector antennas with different patterns such as cosecant squared are widely used in radar systems. In practice a
cosecant squared pattern can be achieved by a deformation of a parabolic reflector (Figure 1.1-a). Moreover, it can be
achieved by multiple feed antennas with normal parabolic reflector surfaces. Also another important reflector antenna
type for radars is fan beam antennas [3]. They produce a main beam having a narrow beamwidth in one dimension and
a wider beamwidth in the other dimension. This pattern can be achieved by a truncated parabolic reflector or a circular
parabolic reflector (Figure 1.1-b).

(a) (b)

Figure 1.1 (a) Deformation of parabolic reflector to obtain a cosecant squared pattern, (b) A Fan beam antenna

978-1-4244-6051-9/11/$26.00 ©2011 IEEE

 In recent years, frequency selective surfaces (FSSs) have been used in reflector antenna designs as reflector
surfaces (Figure 1.2). An example of that is Marconi and Franklin's reflector. This reflector is very much similar to the
most famous FSS design, an array of half-wave dipoles. A large reflector antenna is constructed by using wire-grids [4].
Also, FSSs have been considered in design of multi-frequency reflector antennas for data communication links. FSS
structures are periodic arrays of special elements printed on a substrate [4]. In general, the FSS structures can be
categorized into two major groups: patch-type elements and aperture-type elements. FSSs have different behaviors as
low pass, high pass, band pass or band stop filter characteristics [5]. The waves are transmitted or reflected according to
the properties of the incident waves and the structure of FSS. Because of their frequency selective behavior, FSS have
found many filtering applications in microwave and millimeter-wave engineering. Some examples are radome design,
polarizers, beam splitters and reflector antennas [6].

Figure 1.2 A parabolic reflector antenna that has a grid reflector surface

In this work, FSS structures are used for a new reflector antenna design. For this purpose FSS structures are
inserted to a fan beam reflector antenna to obtain a cosecant squared antenna form. Thus, the new designed reflector
antenna can provide both a fan beam pattern in a frequency band and a cosecant squared pattern in another frequency
band.

2.Frequency Selective Surface (FSS)

FSSs are periodic structures which have different behaviors as low pass, high pass, band pass or band stop filter
characteristics. They have been widely used in broadband communications, radar systems, and antenna technology in a
long time [5]. Generally, FSSs can be categorized into two major groups: patch – type elements and aperture – type
elements. For patch – type elements, conducting materials are used in frequency selective surfaces. The patches are
placed on a dielectric material. FSS structures provide different electromagnetic filter behaviors by different design
styles. For example, a square patch array performs as a low pass filter while a conducting grid performs as a high pass
filter (Figure 2.1) [7].

Figure 2.1 Electromagnetic filter behaviors of FSS structures that are differently designed

For analyzing the electromagnetic behavior of frequency selective surface, finite element method, finite
difference method or method of moments are commonly used [8]. Despite their accuracy in analysis, these techniques
require time consuming simulations and do not allow the designer to have a good insight into the physics behind the
structures. Equivalent circuit representations are useful for quickly predicting the performance of frequency selective
surface and allow performing a very simple model able to describe every kind of shape after a full-wave computer
simulation. These equivalent circuit models also provide useful physical insight into the performance of the FSS. The
equivalent circuit model can also be employed in the design of the multilayered frequency selective surfaces [9].
 3. Antennas with Cosecant Squared Pattern
Antennas with cosecant squared pattern are special designed for air-surveillance radar sets. These permit an
adapted distribution of the radiation in the beam and causing a more ideal space scanning. There are a couple of
variation possibilities, to get a cosecant squared pattern in practice as deformation of a parabolic reflector or a stacked
beam by more horns feeding a parabolic reflector.

In this work, one feed antenna is used and some modifications are made on the parabolic reflector. First of all, a
normal reflector antenna that has a pencil beam pattern is represented in figure 3.1. For the simulation, SUPERNEC
antenna simulation program is used.

Figure 3.1 A parabolic reflector antenna with pencil beam pattern

In figure 3.2, a reflector antenna with cosecant squared pattern that has been traditionally designed is
represented. For the design, a part of reflector surface is cut and the cut side is bowed to feed antenna. For the
simulation, SUPERNEC antenna simulation program is used.

Figure 3.2 An antenna with cosecant squared pattern

4.Design Procedure of a Reflector Antenna that Provides both Fan Beam and
Cosecant Squared Pattern in Different Frequency Bands
First of all, a fan beam antenna is constructed (Figure 4.1). Then a FSS structure that has same geometric form
with a part of parabola is inserted to the fan beam antenna to obtain a cosecant squared antenna form such as figure 3.2.
Thus in a frequency band, the FSS structures reflect electromagnetic waves and as a result, a cosecant squared pattern
can be obtained. However in another frequency band, the FSS structures transmit electromagnetic waves and thus a fan
beam pattern can be obtained. For this design is required two feed antennas. If we want to use one feed antenna,
different two polarizations can be used in a feed antenna. However for getting a good cosecant squared pattern with one
feed antenna, a FSS structure is used in the feed antenna. The FSS structure is inserted to inside of the horn antenna’s
radial flange with an angle. The FSS structure has same geometric form with a side of the horn antenna’s radial flange (Figure 4.2).

Figure 4.1 A fan beam antenna Figure 4.2 A horn antenna with a FSS structure
 In figure 4.3, fan beam patterns are represented for a frequency band. In figure 4.4, cosecant squared pattern is
represented for another frequency band by using the same antenna.

(a) (b)
Figure 4.3 (a) Sight of the pattern in Theta: 90 and Phi: 90, (b) Sight of the pattern in Theta: 90 and Phi: 180

Figure 4.4 Cosecant squared pattern of the antenna

5. Conclusion
In this paper, a novel design for reflector antennas has been introduced. Frequency selective surfaces have been
just used as reflector surfaces for reflector antennas in many years. However in the design, frequency selective surfaces
are used for a different purpose on the antennas. In this work, it is proposed that two different important radiating
patterns are obtained with an antenna in two different frequency bands by using frequency selective surfaces. In future,
we will try to obtain the two patterns in a same frequency band by using different polarizations.

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