The use of the reflectors started even before the 2nd world war in the fields of defense and space communication. World’s largest radio telescope; Green Banks uses an offset reflector [20] having diameter 100-m. The basic theory on feed and reflector system are found in some classic text books [21–26]. Mainly, an offset reflector system is composed of three parts; reflector, feed and supporting stand for feed. The supporting-stand and feed do not make any blockage for the main lobe which is highly preferable for such type of systems. Conventionally, horn is considered as a primary feed due to its ability to control the reflector radiated field by the suitable excitation of feed pattern.
The schematic and focal plane aperture of an offset parabolic reflector is illustrated in Figure 1.1.
In practical application, different kind of aperture shapes like elliptical, circular, rectangular etc are used. Offset parabolic reflector having circular aperture is most widely used in practical application and has been considered for this thesis. The detail of configurations of offset reflector system including its all relevant axies are shown in Figure 1.1. Here, {xA, yA, zA}, {xF, yF, zF} and {xR, yR, zR} are the coordinate of the reflector, the feed and the reflector radiation field, respectively. F,D and H are the dimensions of reflector viz. focal length, diameter and offset height. The other parameters used to model of such system are tilt angle (α) of feed and half illuminated angle (θF). These can be
1.2 Basic Theory and Literature Survey
Figure 1.1: Offset reflector illuminated with feed.
represented as:
θF = tan−1 D+H
2F −tan−1 H
2F (1.1)
α= tan−1 D+H
2F + tan−1 H
2F (1.2)
As mentioned earlier, offset reflector is most preferable in practical applications for having features such as reduced aperture blockage, high antenna efficiency, low side lobe levels, and isolation between feed and reflector as compared to other narrow beam antennas. On the other hand, literature provides the evidence on the basis of measurement as well as based on the analytical study that offset reflector has a strong depolarization effect on the main lobe [1, 27]. Due to such depolarization effect, cross polarization appears in the main lobe when reflector is illuminated with linear polarized feed and on the other hand beam squinting occurs when the reflector is excited with circular polarized feed.
As mentioned in Section 1.1, such depolarization effect degrades the system performance and such effects are more pronounced when F/D ratio is small. To remove the depolarization effect, several techniques [1–4,28] have been proposed in literature as specified in earlier Section. Based on literature survey, focal plane matching technique is found to be one of the most attractive solution to eliminate the cross-polar power of offset reflector antenna. It may be noted that the linear polarized matched feed
1. Offset Parabolic Antenna system: a brief introduction
has wide applicability in the development of the frequency reuse systems. For such systems, achieving cross polar power level 30 dB below the reference power (co-polar power) is generally considered acceptable.
Figure 1.2: (a) focal plane field for x-polarized feed; (b) focal plane field for y-polarized feed.
Focal plane fields of an offset reflector are illustrated in Figure 1.2 for the excitation of linear x andypolarized feed, respectively. It can be observed that an unwanted field of complex value appears along with the desired field. The basic principle of the focal plane matching technique, is used to suppress such unwanted power, which satisfies the criteria that the tangential electric fields at feed’s aperture are complex conjugate of the focal plane fields of the offset reflector antenna (as specified for linear polarized feed in Figure 1.2). Also, it is found in literature [1, 29] that multi-mode horn and antenna array have the ability to meet such criteria. Rudge and Adatia in [1] have shown the use of appropriate higher order mode(s) along with fundamental mode to compensate the cross-polar power.
Also, several combination of modes are proposed for the matched feed design which are enlisted in Table 1.1.
Table 1.1: Waveguide modes for multi-mode matching
Feed structure plane of symmetry (x) plane of asymmetry (y) Smooth-walled cylinder TE111+ TM111+ TE121 TE211+ TM211+ TE221 Corrugated cylinder HE111+ HE121 HE211+ HE221
Smooth-walled rectangular TE01+ TE11/TM11 TE10+ TE20
As we know, generating the conjugate mode inside the horn using the junction(s) always affects the return loss. On the other hand, maintaining the conjugate phase of higher order mode(s) for wide band at the feed aperture is a quite difficult task because main operating mode and the conjugate mode always propagate with different propagation constants. Current trend in matched feed design is to achieve the wide-band conjugate matching and maintaining the return loss within the operating band, which is very challenging. Several matched feed structures are recently reported in literature [6–12]
1.2 Basic Theory and Literature Survey
Figure 1.3: Matched feed configurations: (a) using TE111+ TM111+ TE121 published in 2009 [6]; (b) operated with HE111+HE121published in 2009 [7]; (c) employing TE01+TE11appeared in 2009 [8] ; (d) using TE111+TE121 published in 2011 [9]; (e) operated with TE111+ TE121published in 2012 [10]; (f) employing TE111+ TE121/TE211+ TE221 appeared in 2013 [11]; (g) using TE111+ TE121published in 2015 [12];
based on the operating modes as specified in Table 1.1. Such motivating works are already introduced in Section 1.1. Most attractive and some good performing matched feed configurations reported in literature are shown in Figure 1.3.
Design of matched feeds using the available CAD tools is a challenging task. Basically, CAD tools are built based on any low frequency methods like FEM, MOM, FIT and FDTD employ volume or surface meshing. Generally, two kind of simulation models are used to analyze the feed and reflector model as (a) feed is kept within same radiation boundary to obtain the reflector pattern and (b) feed and reflector patterns are solved separately where evaluated far-field pattern of feed is used as a source for reflector. Usually, the reflector is at least several wavelengths in diameter. The number of meshes needed to solve scattering from such structures becomes huge and it demands very large amount of computing resources and time. Some recent CAD tools address this issue by providing a hybrid platform for computation. Generally, reflector pattern is calculated using PO while the reflector is illuminated by the feed pattern computed using the basic method used in a particular CAD tool.
1. Offset Parabolic Antenna system: a brief introduction
With PO, main lobe pattern can be computed quite accurately, although the predictions of the far side lobe pattern is not accurate [8]. Keeping in view the facts mentioned, in this thesis, PO has been used to compute the reflector pattern and a 2-D field solver is developed to keep the computation requirements reasonable. On the other side, to analyze the feed, authors are motivated to develop the feed model using 2-D solver for overall efficient analysis. A hybrid technique which is a combination of MM/2D-FEM/MOM, is decided based on the literature [16–19] to analyze the feed.