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ElectronicsWeekly.com (August 7, 2009) – Queen’s University Belfast has developed the first quasi-optical frequency selective surface (FSS) that can simultaneously filter both horizontally and vertically polarised sub-millimetre wave signals.

“This surface works for vertical and horizontal polarisation. Previously they could only be one or the other,” lead engineer Dr Raymond Dickey told Electronics Weekly.

The devices operate in the 250-360GHz range and are intended for radiometers in weather satellites.

Developed at the University’s Institute of Electronics, Communications and Information Technology (ECIT), one of the key challenges was that the devices have to be strong enough to withstand space launch as well as selective and low-loss.
The answer was to make the surfaces extremely thin - 10µm.

“They are free-standing, made out of silicon-on-insulator wafers,” said Dickey.

Using deep reactive ion etching 5,000 slots are micromachined into a 30mm diameter active area through the 10µm thick top silicon layer, then the carrier wafer and buried oxide is removed.

Sequential layers of titanium, copper, silver and gold give the mesh suitable electrical properties.

The final device has two if these membranes mounted almost touching (see photo). “Interference between the screens cancels signals at 350GHz,” said Dickey.

The patterns are a series of C-slots (with shorts at the right hand side) and rectangular slots (half visible at the bottom of the photo.

The outer one wavelength C slots work on vertical signals, said Dickie, while the inner half-wavelength slots work on horizontal signals.

In use, the surface passes frequencies that enable the associated radiometer to detect ozone.

“Attenuation is under 0.6dB between 316 and 325GHz, and isolation is greater than 30db between 345 and 358GHz,” said Dickey.

Rejected energy is reflected, enabling it to be passed on to other instruments.

“Beam splitters operating at higher frequency ranges are also being developed,” said the University. “These include one designed to operate at 664GHz which will be the highest frequency dual polarisation FSS ever developed.”

The work is being funded by the UK Centre for Earth Observation Instrumentation, the European Space Agency and EADS Astrium.

by Steve Bush

Adapted from ElectronicsWeekly.com