Many existing large ground reflector antennas have been designed as Cassegrain systems-i.e., paraboloid/hyperboloid combinations. Other large ground antennas are simply paraboloid designs. Upgrading the gain of these systems to a gain comparable to that obtainable with a dual shaped reflector antenna system has been an important and costly objective of many such ground stations. A potentially economic method for such an antenna upgrade is presented herein. It involves a redesign of only the subreflector portion of a Cassegrain antenna or the introduction of a subreflector feed system for a parabaloid. A pair of offset subreflectors are synthesized which will give a controllable high gain amplitude distribution in the aperture of the large paraboloid. The synthesis method that is used is based on an approximate formulation for an offset dual shaped high gain antenna that was first presented by Galindo-Israel and Mittra in 1977. In that approximate formulation, the geometrical optics (GO) energy was scattered from a subreflector and then from a second large reflector which reflected a uniform phase distribution. In the present offset dual shaped subreflector (DSS) antenna, the second reflection is from a smaller (sub) reflector and it scatters a spherical wave that feeds a hyperboloid or feeds a large paraboloid directly. Excellent results are shown for the approximate synthesis of the DSS.

Synthesis of offset dual shaped subreflector antennas for control of Cassegrain aperture distributions

In this paper, the performance of Fresnel lens antennas is investigated in detail. The performance is evaluated in terms of aperture efficiency and stable radiation pattern over a wide frequency band. This paper proposes an efficient technique for enhancing aperture efficiency by smoothly compensating for the spherical phase front arriving upon the lens surface. The feeder has been optimized to fit an axially symmetric cos n -like radiation pattern. For this, an accurate technological process has been used to manufacture a lens that allows for smooth compensation of the phase shift. The lens is compared with a classic Fresnel lens having the same physical dimensions and feeding system. The improvement is validated by measurement, which has revealed a maximum measured gain of 38.9 dBi corresponding to a maximum measured aperture efficiency of 59% with a −2 dB radiation frequency bandwidth of 29.25 GHz around 90 GHz. This amounts to an aperture efficiency enhancement of 60% and −2 dB radiation frequency bandwidth enhancement of 72% as compared with those of the classic Fresnel lens.

Fresnel Lens at Millimeter-Wave: Enhancement of Efficiency and Radiation Frequency Bandwidth

An efficient direct numerical synthesis method for dual offset reflector antennas which is directly based on the physical optics procedure for both reflectors (PO-PO-method) and where the reflector surfaces are advantageously characterized in the spatial domain by a 2D Fourier-transformation is described. The method uses an evolution-type optimization algorithm to shape both reflectors simultaneously so as to generate the desired far field with prescribed criteria. The efficiency of the design method is demonstrated for two computer-optimized dual-offset antenna designs for space applications, a very compact optimum shaped Gregorian Earth station antenna with high offset angle (70 degrees ) and small subreflector size (13 lambda ) and a shaped Cassegrain atmospheric sensor antenna for 200 GHz. The theory is verified by available measured results. >

Direct PO optimized dual-offset reflector antennas for small Earth stations and for millimeter wave atmospheric sensors

An efficient direct numerical synthesis method for dual offset reflector antennas is described which is directly based on the physical optics procedure for both reflectors (PO- PO-method) and where the reflector surfaces are advanta- geously characterized in the spatial domain by a two-dimen- sional Fourier-transformation. The method involves an evolu- tion strategy optimization algorithm in order to shape immediately both reflectors, so as to generate the desired far field with prescribed criteria. The efficiency of the design method is demonstrated for two computer-optimized dual off- set antenna design examples for space applications. The first example, a very compact optimum shaped Gregorian dual off- set earth station antenna with high offset angle (70') and small subreflector size (13 1L) achieves a very low sidelobe attenuation level within the envelope of only 23-25 log 8 dBi, and low cross- polarization attenuation (38 dB). The second example, a shaped dual offset Cassegrain atmospheric sensor antenna for 200 GHz, demonstrates that beam scanning by more than 10 half-power beamwidths is possible with nearly constant half-power beam- width by simply displacing linearly the subreflector. The con- sequent advantage is the very low mass to be moved. The theory is verified by available measured results.

for Small Earth Stations and for Millimeter Wave Atmospheric Sensors

In the paper by V. Galindo-Israel and R. Mittra it is claimed that a dual shaped subreflector (DSS) feed can be used to upgrade the efficiency of classical Cassegrain antennas. In this comment it is shown that upgrading is not possible unless the diameter of the DSS feed is extremely large. In fact, if the Cassegrain antenna at Goldstone, CA, used as an example in the above paper, is fed by the DSS feed, the efficiency in X -band will be about the same as or less than if it is fed directly by an optimized scalar horn. The low efficiency is caused by diffraction. Instead, the efficiency can be upgraded by using a scalar horn with shaped lens.

Comments on "Synthesis of offset dual shaped subreflector antennas for control of Cassegrain aperture distributions"

A design method giving high efficiency and low sidelobes is discussed for large aperture offset reflector antennas. A new reflector shaping technique using the subreflector and the beam waveguide reflector with the parabolic main reflector is proposed to simplify the main reflector manufacturing process. The effectiveness of the technique is confirmed by the model experiments. One problem with this reflector shaping technique is that the subreflector edge level cannot be controlled independently of the main reflector edge level. By investigating the relation between the gain reduction and the subreflector edge level, which affects the wide-angle sidelobe levels, the realizable characteristics of antennas are studied. In order to decrease the subreflector edge level without reducing the aperture efficiency, a technique using an extended reflector is also proposed. Its effectiveness is shown by theoretical and experimental investigations.

S. Sato

Papers

High efficiency and low sidelobe design for a large aperture offset reflector antenna