NASA’s Jet Propulsion Laboratory, Pasadena, California
A window for a high-sensitivity microwave receiving system allows microwave radiation to pass through to a cryogenically cooled microwave feed system in a vacuum chamber, while keeping ambient air out of the chamber and helping to keep the interior of the chamber cold. The microwave feed system comprises a feed horn and a low-noise amplifier, both of which are required to be cooled to a temperature of 15 K during operation. The window is designed to exhibit very little microwave attenuation in two frequency bands: 8 to 9 GHz and 30 to 40 GHz.
The window is 15 cm in diameter. It includes three layers (see figure):
This Low-Loss Microwave Window is made from commercially available materials and is relatively inexpensive. The outer layer is made of a poly(tetrafluoro- ethylene) film 0.025 mm thick. This layer serves primarily to reflect and absorb solar ultraviolet radiation to prolong the life of the underlying main window layer, which is made of a polyimide that becomes weakened when exposed to ultraviolet. The poly(tetrafluoro- ethylene) layer also protects the main window layer against abrasion. Moreover, the inherent hydrophobicity of poly(tetrafluoroethylene) helps to prevent the highly undesirable accumulation of water on the outer surface.
The polyimide main window layer is 0.08 mm thick. This layer provides the vacuum seal for the window.
A 20-mm-thick layer of ethylene/ propylene copolymer foam underlies the main polyimide window layer. This foam layer acts partly as a thermal insulator: it limits radiational heating of the microwave feed horn and, concomitantly, limits radiational cooling of the window. This layer has high compressive strength and provides some mechanical support for the main window layer, reducing the strength required of the main window layer.
The ethylene/propylene copolymer foam layer is attached to an aluminum window ring by means of epoxy. The outer poly(tetrafluoroethylene) film and the main polyimide window layer are sandwiched together and pressed against the window ring by use of a bolted clamp ring.
The window has been found to introduce a microwave loss of only about 0.4 percent. The contribution of the window to the noise temperature of the microwave feed system has been found to be less than 1 K at 32 GHz and 0.2 K at 8.4 GHz.
This work was done by Michael Britcliffe and Manuel Franco of Caltech for NASA’s Jet Propulsion Laboratory. NPO-40846.
This Brief includes a Technical Support Package (TSP).
Two-Band, Low-Loss Microwave Window
(reference NPO-40846) is currently available for download from the TSP library.
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Overview
The document outlines the development of a Two-Band, Low-Loss Microwave Window designed for use in low-noise, cryogenically cooled microwave feed systems, specifically for NASA's Large Array Task. This innovative window, with a diameter of 15 cm, is engineered to operate efficiently in dual frequency bands of 8 to 9 GHz and 30 to 40 GHz, catering to the needs of advanced communication systems.
A key feature of the window is its construction, which includes a thin (0.08 mm) layer of Kapton film, a commercially available polyimide material known for its vacuum sealing properties. To enhance the durability of the Kapton against UV radiation and mechanical stress, a 0.25 mm layer of Teflon radome material is sandwiched on top. This Teflon layer not only protects the Kapton but also prevents moisture accumulation, which can lead to increased microwave loss.
The window is backed by a 20 mm thickness of Propazote PPA-30 foam, which serves as a thermal insulator, minimizing radiation cooling from the feed horn operating at 15K. This insulation is crucial for preventing moisture condensation on the window, which could significantly degrade performance. The foam also supports the Kapton film, reducing the mechanical load on it.
The completed window exhibits very low microwave loss, approximately 0.4 percent, contributing less than 1K at 32 GHz and 0.2K at 8.4 GHz. This performance is critical for low-noise systems, where even a 1 percent loss is considered unacceptable. The window has been tested in laboratory conditions, successfully enduring over 100 pressure cycles from atmospheric pressure to 10^-5 Torr.
Designed for outdoor service on a 12-meter antenna, the window has 30 times more area exposed to atmospheric pressure than existing systems in the NASA Deep Space Network. The life goal for this window is set at two years or longer, making it a robust solution for future space communication needs.
Overall, this document highlights the technical specifications, materials used, and performance metrics of the Two-Band, Low-Loss Microwave Window, showcasing its significance in enhancing microwave communication systems for space exploration.
