Offset Cassegrain Research Articles

Analysis of Offset Antennas in Radio Telescopes

We present an analysis on the performance of two popular dual offset antennas design, i.e. the offset Cassegrain and Gregorian reflector antennas. In our study, we have adopted the design parameters for the Cassegrain configuration used in the Atacama Large Milimeter Array (ALMA) project. Modifications on the original parameters are made so as to meet the design requirement of the offset configurations. To reduce spillover loss, we have adjusted the angle between the axis of the primary reflector and that of the sub-reflector to 0.20o. The results obtained from the physical optics computation show that the amplitude at the main lobe of the Gregorian configuration is approximately 74.02 dB, while that of the Cassegrain configuration is approximately 74 dB. The maximum (relative) side lobe level, SLLdB for the Cassegrain and Gregorian configurations are found as -3.67 dB and -3.69 dB respectively. Although the magnitude of the main lobe for both configurations is comparable, the Gregorian antenna gives relatively lower SLLdB. In other words, the Gregorian configuration performs relatively better than its Cassegrainian counterpart.

Analysis of Reflector Antennas in Radio Telescopes

We present an analysis on the performance of the Cassegrain and Gregorian on-axis, off-axis and offset antennas. In our study, we have adopted the design parameters for the Cassegrain configuration used in the Atacama Large Millimeter Array (ALMA) project. Modifications on the original parameters are made so as to meet the design requirement for the off-axis and offset configurations. To reduce spillover loss in the offset antennas, we have adjusted the angle between the axis of the primary reflector and that of the sub-reflector, so that the feed horn is placed right next to the edge of the primary reflector. This is to allow the offset antennas to receive the highest power at the feed horn. The results obtained from the physical optics simulation show that the radiation characteristics of both Cassegrain and Gregorian antennas are similar. The offset designs exhibit the best performance, followed by the on-axis, and, finally, the off-axis designs. Our analysis also shows that the performance of both offset Cassegrain and Gregorian antennas are comparable to each other.

Development of a Transportable Telescope for Galactic Survey at 500 GHz in Antarctica

We have developed a transportable 30-cm submillimeter-wave telescope to operate at the Dome Fuji station in the Antarctic plateau. Transportability is an important requirement in the design; the telescope can be divided into several subsystems by hands. The maximum weight of the subsystems is restricted to be below 60 kg, so that the telescope can be assembled without a lifting machine. A small 4 K mechanical cryocooler is used for cooling down a SIS mixer. Total power consumption was designed to be less than 2.5 kW. The optical system was designed to satisfy the frequency independent matching condition at the subreflector and the feed horn of the SIS mixer, so we could accommodate a higher frequency receiver without changing mirrors. A quasi-optical filter was employed for the single sideband operation in observations of the CO (J = 4-3) line at 461.04 GHz and the [CI] (3P1-3P0) line at 492.16 GHz. It was equipped with a 1 GHz width spectrometer that covers a velocity width of 600 km/s with a velocity resolution of 0.04 km/s at 461 GHz. We carried out test observations at a 4400-m altitude site in northern Chile during winters of 2010 and 2011. The typical system noise temperature, including atmospheric loss, was 3000 K (SSB) at 461 GHz, that is mainly limited by atmospheric opacity. The beam size of the 30-cm offset Cassegrain antenna at 0.65 mm of wavelength was measured to be 9'.4±0'.4 by cross scanning of the sun. This angular resolution of the 30-cm telescope is same as those of the Columbia-CfA-U. Chile CO (J = 1-0) surveys. We estimated the main beam efficiency to be 87±5% by observing the new moon. We succeeded in mapping Orion Molecular Cloud A and M17 SW in CO (J = 4-3) followed by test observations toward Orion KL in both CO (J = 4-3) and [CI] (3P1-3P0) .

Measurement of the Offset-Cassegrain Antenna of JEM/SMILES Using a Near-Field Phase-Retrieval Method in the 640-GHz Band

This communication describes the results of the measurements made for the flight model of the offset Cassegrain antenna of superconducting submillimeter-wave limb-emission sounder (SMILES) aboard the International Space Station. We have employed a near-field phase retrieval method in which the aperture phase distribution is estimated only from the amplitude distribution measurements over two near-field planes. The far-field patterns estimated from the estimated near-field patterns were compared with theoretical calculations based on physical optics in which the surface errors measured for the main and sub reflectors were taken into account. As a result of the comparison, the far-field patterns estimated from the phase retrieval method were found to be in very good agreement with the physical-optics calculations to the sidelobe levels as low as -55 dB. We have also found that patterns of machined flaws on the surface of the main reflector were clearly identified in the retrieved near-field phase pattern. This demonstrates that the phase retrieval is an effective method to evaluate aperture antennas in the submillimeter-wave region, where accurate phase measurement is rather difficult.

A Novel Beam Squint Compensation Technique for Circularly Polarized Conic-Section Reflector Antennas

Beam squint generally exists in offset reflector antennas with circularly polarized feeds. It is manifested by a small beam shift of the radiation pattern in the plane perpendicular to the principal offset plane, which can significantly affect the beam pointing accuracy. In this paper a practical and widely applicable compensation technique for the beam squint is proposed. Simulation results show that a small lateral feed displacement in the perpendicular plane can effectively minimize or eliminate the linear phase shift caused by the depolarization effect, thus compensating for the beam squint effect. This is in practice very useful for offset reflector antennas where the previously suggested method based on feed tilting may not be proper. A simple formula is derived to quickly estimate the optimal feed displacement for both right- and left-hand circularly polarized feeds. Three representative examples: a single offset parabolic reflector, a suboptimal offset Cassegrain reflector, and an axially symmetric Cassegrain reflector with an off-focus feed, are presented to validate the proposed method. Satisfactory results are achieved for all three examples.

EBG Focal Feed Improvements for Ka-Band Multibeam Space Applications

This letter presents the enhancement of an electromagnetic band-gap (EBG) antenna that is used to feed a reflector antenna for a space multibeam application in Ka band. One of the main difficulties in a typical focal feed cluster design is to obtain high illumination performances with neighbored and close radiating elements. The new structure presented in this letter tackles this difficulty by using a two-level EBG antenna as a multibeam focal feed. This last one is designed to illuminate a side-fed offset cassegrain antenna (SFOCA) working in Ka band and dedicated to a multibeam telecommunication coverage over Europe. A prototype has been measured, and the reflector antenna performances have been evaluated in order to show the good results obtained with this new multibeam focal feed.

EBG Focal Feed Design Optimization

Recent works have shown that the electromagnetic bandgap (EBG) antenna can be used as a reflector focal feed. The design of such feeds needs to take into account several parameters to obtain both high illumination performances and low reflector antenna side-lobe level. Among these EBG antenna parameters, the quality factor, the excitation probe used and the phase center location constitutes key elements to obtain high performances. This article presents useful design curves to conceive an efficient EBG focal feed. These curves have been used to realize a 30-GHz single-feed EBG device illuminating a side-fed offset Cassegrain antenna (SFOCA). Results show good performances in terms of aperture efficiency and radiation patterns of the reflector antenna compared to the Potter horn previously used with the same reflector.

Measurement and Evaluation of Submillimeter-Wave Antenna Quasioptical Feed System by a Phase-Retrieval Method in the 640-GHz Band

A phase-retrieval method is applied to the quasioptical feed system of the offset Cassegrain antenna of the Superconducting Submillimeter-Wave Limb-Emission Sounder (JEM/SMILES) to be aboard the International Space Station for evaluating the beam alignment by estimating the phase pattern from the beam amplitude pattern measurements. As the result, the application of the phase retrieval method is demonstrated to be effective for measuring and evaluating the quasioptical antenna feed system. It is also demonstrated that the far-field radiation pattern of the antenna main reflector can be estimated from the phase-retrieved beam pattern of the feed system.

Enhanced EBG Resonator Antenna as Feed of a Reflector Antenna in the Ka Band

Recent studies have shown that a multibeam reflector antenna could be illuminated by a multifeed electromagnetic bandgap (EBG) structure, in order to achieve a high gain multispot coverage with the simple feed by beam concept and only one aperture. This letter deals with the design of a metallic EBG antenna in the Ka band, feeding a side-fed offset cassegrain antenna (SFOCA) for a European multispot coverage. This reflector presents a high focal-length-to-diameter ratio limiting the defocusing effects for multibeam applications. It, therefore, requires focal feeds with a high directivity and a good radiation pattern quality. The well-known drawbacks of an EBG antenna are the narrow radiation bandwidth for high directivities and also the high sidelobes level reducing the reflector antenna efficiency. Consequently, the work presented in this letter consisted in improving the EBG antenna performances by using a more efficient feed. The replacement of the usual microstrip patch by a horn allowed to double the radiation bandwidth while decreasing the sidelobes level (-15 dB) of a 24-dB EBG antenna. A metallic prototype excited by a single horn has been manufactured at 30 GHz, and the measurements agree with the simulation. This device with one feed allows good SFOCA performances, similar to those obtained with a conventional focal feed like a Potter horn.

Effect of random surface errors on radiation characteristics of the side-fed offset Cassegrain antenna

In this paper the average power pattern of the side-fed offset Cassegrain (SFOC) dual reflector antenna is analyzed, and the effect of the random surface error on radiation characteristics of the antenna is introduced. Here, the random surface error is defined as the error of the standard reflector in its normal direction and the errors in a small zone of the reflector are considered as equal. We also assume that the phase error on the aperture led by the random surface error obeys a Gaussian distribution with zero mean, under which the expression of the average power pattern is deduced. Finally, the data related to the radiation characteristics of the antenna are calculated and the corresponding curves are presented. The obtained results can be used for the user to determine the manufacturing accuracy of the reflector of the SFOC antennas.

Designing classical offset cassiegrain or gregorian dual-reflector antennas from combinations of prescribed geometric parameters part 2: feed-horn blockage conditions

This paper is a short appendix to [C. Garnet, June 2002], where a classical offset Cassegrain of Gregorian dual-reflector antennas were designed from combinations of prescribed geometric parameters. However, in [C. Garnet, June 2002], the model assumed a point-source feed, and did not take into account the physical dimensions of the feed horn needed to illuminate the subreflector. Now, as in [C. Garnet, April 1998], we consider a feed horn. The feed horn is therefore simulated as a cone, which is a fair description in most cases, and is sufficient in the initial design stage of an antenna.

Designing classical offset Cassegrain or Gregorian dual-reflector antennas from combinations of prescribed geometric parameters

This paper proposes a simple procedure for the design of classical offset Cassegrain or Gregorian dual-reflector antennas from combinations of prescribed geometric parameters. This procedure has already been applied to classical Cassegrain and Gregorian antennas, to classical displaced-axis Cassegrain and Gregorian antennas, and to classical offset Dragonian antennas. The antenna systems can be fully characterized by 21 parameters, of which only five need to be provided by the antenna designer, as the remaining 16 parameters can be derived in closed form using the procedure described here. In this paper, we assume that the main reflector has a circular aperture, while the subreflector has an elliptical aperture All the antenna geometries presented satisfy the Mizugutch condition (1976), which is the geometric-optics condition for zero cross-polarized radiation. This procedure is very close to the one used for offset Dragonian systems, but all the relevant information is repeated here for completeness.

SABOR: a fast analysis tool for horn and reflector antennas

SABOR (software de analisis de bocinas y reflectores) is an antenna-analysis approach for the computation of horn- and reflector-antenna performance. The program covers and expands topics already developed with other software. The program can analyze almost any kind of horn (E and H pyramidal horns, circular horns, corrugated horns). The current configuration of SABOR can also analyze centered and offset parabolic reflectors, dual-offset Cassegrain and Gregorian reflectors, and centered Cassegrain systems. For feeding reflectors, the program allows use of the classical cos/sup q/ pattern and any of the horn mentioned above. Usage of real horns improves the accuracy of computed efficiencies and the simulation of the cross-polar radiation.

A method for designing high-power offset Cassegrain antennas

Starting from the geometrical parameters of the feed and the desired gain of the Cassegrain antenna, a method to determine the design starting parameters for a high-power offset Cassegrain antennas is discussed. Then, by numerically solving the equations governing the antenna geometry, a set of design curves are obtained, with which the structure of the antenna can be determined by trading off the electrical and geometrical system requirements. The method presented can be applied to both high or low gain feeds for constructing a high power microwave electromagnetic environment, where complex electronic systems can be tested. For linearly polarized feeds with high gain, the structure parameters of antenna with zero cross polarization can be obtained, which is equal to the design results of Brown and Prata (see IEEE Trans. Antennas Propagat., vol.42, no.8, p.1145-53, 1994).

A design procedure for classical offset dual reflector antennas with circular apertures

A geometrical optics procedure for designing electrically optimized classical offset dual reflector antennas with circular apertures is presented. Equations are derived that allow the size and spacing of the main and subreflectors of the antenna system, along with the feed horn subintended angle, to be used as input variables of the design procedure. The procedure, together with these equations, yields an optimized design, starting from general system requirements. The procedure is demonstrated by designing both an offset Cassegrain and an offset Gregorian antenna, and is validated by analyzing their radiation patterns using physical optics surface current integration on both the main and subreflectors. >

The ACTS multibeam antenna

The Advanced Communication Technology Satellite (ACTS), to be launched in 1993, is introduced briefly, and then the Ka-band multibeam antenna, consisting of electrically similar 30 GHz receive and 20 GHz transmit offset Cassegrain systems utilizing orthogonal linear polarizations, is described. Dual polarization is achieved by using one feed assembly for each polarization in conjunction with nested front and back subreflectors, the gridded front subreflector acting as a window for one polarization and a reflector for the other. The antennas produce spot beams with approximately 0.3 degrees beam width and gains of approximately 50 dBi. High surface accuracy and high edge taper produce low sidelobe levels and high cross-polarization isolation. A brief description is given of several Ka-band components fabricated for ACTS. These include multiflare antenna feedhorns, beamforming networks utilizing latching ferrite waveguide switches, a 30 GHz HEMT low-noise amplifier, and a 20 GHz TWT power amplifier.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Scan limitations of shaped dual-offset reflector antennas for multiple satellite access

The performance of offset Cassegrain antennas, shaped to provide wide scan angles, is outlined for use in earth stations which are capable of accessing several satellites simultaneously. The effect on the beam efficiency of scanning over a rectangular-shaped coverage region located about the geostationary arc is described. Limitations on scan performance are discussed as is the effect of satellite tracking by feed movement.

Derivation and application of the equivalent paraboloid for classical offset Cassegrain and Gregorian antennas

The equivalent paraboloid is derived for classical offset Cassegrain and Gregorian antennas. The important practical case of systems with circular exit apertures is discussed in detail, and a condition for a symmetric equivalent paraboloid is derived. For such systems, diffraction effects are investigated using tapered and scanned feeds to illuminate the equivalent paraboloid and the two-reflector system. >

Subreflector shaping to improve multiple-beam performance of a Cassegrain antenna

A method of designing multiple-beam antennas based on shaping the subreflector of an offset Cassegrain antenna is described. It is applied to a compact system having a diameter of 300λ that is required to produce beams up to 4-6°either side of boresight. The performance is shown to be only slightly less than that obtainable by also shaping the main reflector, indicating that efficient multiple-satellite-access antennas can be designed without specially shaped main reflectors.

Beam scanning characteristics of front fed offset cassegrain–type multibeam antenna

AbstractThis paper shows theoretically and experimentally that the front‐fed offset Cassegrain (FFOC) antenna is suited as a multi‐beam antenna to be borne on an international satellite. First, the problems are identified for conventional reflector configurations. Then the beam deflection characteristics are shown numerically in the case where an FFOC reflector surface is used as a multibeam antenna. The performance of the test antenna is confirmed experimentally. From the measured results of a test antenna of 120 wavelengths, it is found that the reduction of the gain in the beam deflection within a circle of the visible radius of 10° is less than 1.8 dB which agrees well with the computed results. The cross‐polarization level is less than‐34 dB. Because the cross‐polarization component generated by the primary radiator is −40 dB, the cross polarization generated by the reflector system is quite small.