Figure 1. A Channel Having Asymmetric Steps is cut into the lower block.An identical channel is cut into the upper block. Then with the help ofalignment pins, the blocks are assembled so that the two channels mergeinto one channel that makes a transition between two orthogonal orientationsof a WR-3 waveguide.
A split-block waveguide circuit that rotates polarization by 90° has been designed with WR-3 input and output waveguides, which are rectangular waveguides used for a nominal frequency range of 220 to 325 GHz. Heretofore, twisted rectangular waveguides equipped with flanges at the input and output have been the standard means of rotating the polarizations of guided microwave signals. However, the fabrication and assembly of such components become difficult at high frequency due to decreasing wavelength, such that twisted rectangular waveguides become impractical at frequencies above a few hundred gigahertz. Conventional twisted rectangular waveguides are also not amenable to integration into highly miniaturized subassemblies of advanced millimeter- and submillimeter- wave detector arrays now undergoing development. In contrast, the present polarization-rotating waveguide can readily be incorporated into complex integrated waveguide circuits such as miniaturized detector arrays fabricated by either conventional end milling of metal blocks or by deep reactive ion etching of silicon blocks. Moreover, the present splitblock design can be scaled up in frequency to at least 5 THz.

Figure 2. The Return Loss and Transmission of a prototype of a polarization rotatingwaveguide like that of Figure 1 was measured over the nominal frequencyband of WR-3 waveguide.
The main step in fabricating a splitblock polarizationrotating waveguide of the present design is to cut channels having special asymmetrically shaped steps into mating upper and lower blocks (see Figure 1). The dimensions of the steps are chosen to be consistent with the WR-3 waveguide cross section, which is 0.864 by 0.432 mm. The channels are characterized by varying widths with constant depths of 0.432, 0.324, and 0.216 mm and by relatively large corner radii to facilitate fabrication. The steps effect both a geometric transition and the corresponding impedance- matched electromagnetic-polarization transition between (1) a WR-3 rectangular waveguide oriented with the electric field vector normal to the block mating surfaces and (2) a corresponding WR-3 waveguide oriented with its electric field vector parallel to the mating surfaces of the blocks.

A prototype has been built and tested. Figure 2 presents test results indicative of good performance over nearly the entire WR-3 waveguide frequency band.

This work was done by John Ward and Goutam Chattopadhyay of Caltech for NASA's Jet Propulsion Laboratory.

Topics:
Assembling Electronic equipment Fabrication Machining processes Manufacturing processes Milling Powertrains Semiconductors & ICs Transmissions Waveguides
This Brief includes a Technical Support Package (TSP).
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Split-Block Waveguide Polarization Twist for 220 to 325 GHz

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NASA Tech Briefs Magazine

This article first appeared in the March, 2008 issue of NASA Tech Briefs Magazine (Vol. 32 No. 3).

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Overview

The document discusses a novel technology developed by NASA's Jet Propulsion Laboratory (JPL) for a Split-Block Waveguide Polarization Twist, specifically designed for frequencies ranging from 220 to 325 GHz, with scalability to terahertz frequencies (up to at least 5 THz). The traditional approach to creating dual-polarization heterodyne receivers involves connecting separate components with waveguide flanges, which presents challenges such as circuit losses, fabrication tolerances, and bulkiness, making it unsuitable for integration into large-format detector arrays. Additionally, optical techniques used in this context are often bulky and fragile.

The proposed solution involves the design of specially shaped asymmetric channels that connect E and H plane split rectangular waveguides. This innovative design utilizes simple channel geometries with only three fixed depths, which simplifies the fabrication process and makes it compatible with silicon deep reactive ion etching (DRIE) micromachining. The technology aims to overcome the limitations of traditional twisted rectangular waveguides, which become impractical at frequencies above a few hundred gigahertz.

The document highlights that the new polarization twist design has been predicted to achieve broad-band operation with an input return loss better than -21 dB across the entire waveguide band. Experimental measurements have confirmed this performance, demonstrating a return loss well below 20 dB, indicating effective signal integrity.

The novelty of this technology lies in its ability to integrate waveguide polarization twists directly into standard split-block waveguide circuits using common fabrication methods, such as end-milling or silicon DRIE. This integration not only enhances the performance of waveguide circuits but also allows for scalability to extremely high frequencies, which is crucial for advancing applications in aerospace and other fields.

The document serves as a technical support package under NASA's Commercial Technology Program, aiming to disseminate aerospace-related developments with broader technological, scientific, or commercial applications. For further inquiries or assistance, contact information for JPL's Innovative Technology Assets Management is provided, emphasizing the collaborative nature of this research and its potential impact on future technologies.