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.

The sensitivity of the arc‐induced long period fiber grating (LPFG) sensor was improved utilizing the double‐pass configuration. The sensitivity in terms of transmission attenuation of the resonance wavelength for the double‐pass LPFG configuration was better compared to the single‐pass configuration when it was used to monitor the avidin–biotin interaction over a period of 60 min. © 2014 Wiley Periodicals, Inc. Microwave Opt Technol Lett 56:2766–2769, 2014

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 0.20 o. 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, SLL dB 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 SLL dB. In other words, the Gregorian configuration performs relatively better than its Cassegrainian counterpart.

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Analysis of Terahertz waves comes in three main forms, physical optics, geometrical optics, and Gaussian optics. Physical optics has the highest accuracy but it is time consuming when it is applied in the design of large radio telescopes. Also, it is only capable of computing radiation characteristics. Geometrical optics, on the other hand, reduces computational time significantly. But it does not give accurate results when designing telescopes which are to operate at Terahertz frequencies. Gaussian optics is a good trade-off between these two methods and it is a popular approach used in the design of large radio telescopes — particularly those which operate near/in the Terahertz band. Since it accounts for the effects of diffraction, this method produces reasonably accurate results. This chapter describes Gaussian optics, with emphasis given on its application in the design of radio telescopes.