High-Performance Smooth-Walled Horn Antennas for THz Frequency Range: Design and Evaluation

Traditionally, corrugated conical horn antennas have been the main choice for use in astronomical receivers in the range of millimeter and submillimeter waves. They present low cross-polar level and high coupling efficiency into the fundamental Gaussian mode. However, this type of antenna is difficult to manufacture, inevitably increasing its price and extending the production process. In this article, we present two kinds of feed horn antennas, aimed for use in a frequency range equivalent to atacama large millimeter/submillimeter array (ALMA) Band 6 (211-275 GHz),which can be fabricated in a much simpler way with the conventional machining tools. Specifically, we present the design and performance comparison of smooth-walled spline-profile horns in two geometries, diagonal, and conical. Optimization of the designs has been made by means of an algorithm that allowed us to obtain models whose electrical and mechanical characteristics make them competitive when compared with corrugated horns. In particular, they are 40% shorter than the conventional corrugated horns suited for this band, representing an advantage given the stringent space constraints of most astronomical receivers. We also demonstrate that they can be coupled efficiently to an astronomical-grade optical system, using ALMA Band-6 receiver as an example. Furthermore, we have constructed the diagonal horn and characterized it thoroughly. Experimental results of the radiation pattern at room temperature show a good cross-polar performance with levels below -20 dB and Gaussicity above 96%. Our calculations show a good antenna-efficiency level with losses less than 1%. All these properties demonstrate the feasibility of this type of horns to become the main option at the time of choosing a feed system for cutting-edge astronomical applications.

I. Introduction

Corrugated conical horn antennas have been, traditionally, the main choice when it comes to developing instruments for astronomy applications in the range of millimeter and submillimeter waves (including part of the terahertz (THz) frequency spectrum) [1]. They have been selected due to their excellent characteristics. In particular, their coupling to the fundamental Gaussian mode (in short Gaussicity) is around and a cross-polar level lower than  dB [2]. Nonetheless, this sort of antennas are difficult to manufacture, inevitably extending the production process, and therefore, increasing their price. There are other options to corrugated horns, e.g., conical and pyramidal horns. However, they lack symmetry in the radiation pattern, and additionally, pyramidal horns present astigmatism [3].