Millimeter-Wave Hybrid Mode Horn Geometries and Challenges in Realization

Abstract

Future wireless communication applications will use high-frequency in millimeter/sub-millimeter range for transmission and reception. The high-frequency offers several advantages like larger bandwidth with a high transmission rate and suppressed interference. However, the dimensions of the RF components become too complex to fabricate especially in the millimeter and sub-millimeter wave frequency range. Looking into this, there is a scope of research and development of high-frequency components for future wireless systems. A horn antenna is one of the crucial elements for long-distance wireless communication systems. Many different types of horn antennas are in use for a variety of practical applications. Amongst different horn antenna configurations, hybrid mode horns are preferred due to their advantages like symmetric beam patterns, low cross-polarization over a wide frequency band, reasonable gain, etc. Over the last few decades, researchers have explored a variety of hybrid mode horn antennas including the most common corrugated horn, dielectric horn, metamaterial horn, smooth-walled profile horn, etc. The corrugated horn is the most widely used hybrid mode horn. However, due to its complex inner structure, it is relatively challenging and costly to manufacture, especially in the millimeter and sub-millimeter wave frequency range. Apart from a conventional corrugated horn, there are several other hybrid-mode horn options available, including a hybrid corrugated horn (having both axial and radial corrugations) and smooth-walled spline profile horn antennas. All these horn antennas have their advantages and limitations. The present research work focuses on the design and development of different hybrid mode horn geometries at millimeter-wave frequencies with emphasis on the challenges in the realization of such horn antennas. At the beginning of the thesis, a comparison of various types of hybrid and multi-mode horn antennas is presented. A D-band radial corrugated horn antenna developed using a split block fabrication technique is described with the possible fabrication challenges. The said antenna was tested for the millimeter-wave plasma diagnostics system. The result analysis and all conclusions are discussed in the thesis at length. Further, we have proposed a wideband profiled horn antenna designed using the piecewise biarc Hermite polynomial interpolation and validated experimentally at 55 GHz. The proposed design proves S11 and directivity better than -22 dBi and 25.5 dBi across the entire band and only needs 3 node points if compared with the well-known smooth-walled spline profile horn antenna. The said design makes use of an increasing radius and hence does not present non-accessible regions from the aperture, allowing its fabrication with electro erosion techniques especially suitable for millimeter and submillimeter wavelengths. Additionally, we have proposed a concept of ‘the designing region of interest’ which may help the horn antenna designers to understand the effects on the performance of the horn due to profiling. A set of horn antennas have been developed using 3D printing technology. It has made it possible to fabricate the entire horn geometry in a single piece. The 3D printed, Q-band (33-50 GHz) hybrid corrugated horn and G-band (140-220 GHz) smooth-walled spline profile horn antennas were tested for return-loss and radiation performance. The results have been compared with the previously reported horn antennas fabricated using conventional techniques. All design details, fabrication, and results are described in the thesis. For terahertz wireless communication, there is a huge demand for wideband terahertz horn antennas. It is impossible to develop such horns using conventional fabrication techniques. We have proposed Y-band (330-500 GHz) axially corrugated horn antennas fabricated using silicon micromachining technology. On a single silicon wafer of 10 cm (525 microns), hundreds of tiny horns have been fabricated using the photolithography and deep reactive ion etching (DRIE) technique. To obtain multiple depths of the corrugations, silicon dioxide (SiO2) layers have been used as a hard mask to perform multiple photolithography processes.