High-Reliability Waveguide Vacuum/Pressure Window
- Friday, 01 February 2013
This design is suitable for commercial, military, and space applications requiring a helium-leaktight vacuum pressure window.
The NASA Deep Space Network (DSN) uses commercial waveguide windows on the output waveguide of Ka-band (32 GHz) low-noise amplifiers. Mechanical failure of these windows resulted in an unacceptable loss in tracking time.
Waveguide Window assembly.">To address this issue, a new Ka-band WR-28 wav eguide w indow has been designed, fabricated, and tested. The window uses a slab of low-loss, low-dielectric constant foam that is bonded into a ½-wave-thick waveguide/flange. The foam is a commercially available, rigid, closed-cell polymethacrylimide. It has excellent elec trical properties with a dielectric constant of 1.04, and a loss tangent of 0.01. It is relatively strong with a tensile strength of 1 MPa. The material is virtually impermeable to helium. The finished window exhibits a leak rate of less than 3 × 10–3 cm3/s with helium. The material is also chemically resistant and can be cleaned with acetone.
The window is constructed by fabricating a window body by brazing a short length of WR-28 copper waveguide into a standard recta ngu lar fla nge, a nd machining the resulting part to a thickness of 4.6 mm. The foam is machined to a rectangular shape with a dimension of 7.06×3.53 mm. The foam is bonded into the body with a two-part epoxy. After curing, the excess glue and foam are knife-trimmed by hand. The finished window has a loss of less than 0.08 dB (2%) and a return loss of greater than 25 dB at 32 GHz. This meets the requirements for the DSN application. The window is usable for most applications over the entire 26-to-40-GHz waveguide band. The window return loss can be tuned to a required frequency b y varying the thickness of the window slightly.
Most standard waveguide windows use a thin membrane of material bonded into a recess in a waveguide flange, or sandwiched between two flanges with a polymer seal. Designs using the recessed window are prone to mechanical failure over time due to constraints on the dimensions of the recess that allow the bond to fail. Designs using the sandwich method are often permeable to helium, which prohibits the use of helium leak detection.
At the time of this reporting, 40 windows have been produced. Twelve are in operation with a combined operating time of over 30,000 hours without a failure.
This work was done by Michael J. Britclif fe, Theodore R. Hanson, Ezra M. Long, and Steven Montanez of Caltech for NASA’s Jet Propulsion Laboratory. NPO-48372
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