Summer 2009 Issue

Antennas

Frequency Selective Surface

By Kent Britain, WA5VJB

 

Photo A. L-Band frequency selective surface. (Tnx to W6RSF)
 

Wow . . . 25 years ago you could get into serious trouble with security for even saying the phrase “frequency selective surface” (FSS) outside of a secure area.

The idea is simple—to make a sheet of metal become transparent at some radio wavelengths and reflective at other radio wavelengths. The idea has been used in optics for centuries, a sheet of glass that blocks some wavelengths but passes other wavelengths—blue, red, whatever color you want. I would like to thank Jerome Glaser, W6RSF, of Glaser Associates for photo A of the L-Band FSS.

Back then, 25 years ago, the development of frequency selective surfaces was a very important factor in the design of stealth aircraft. An antenna makes a great radar target. A 1/4-wave whip will reflect radar on all of its 1/4-wave multiples. However, if you put that antenna under a surface that only lets one frequency pass, then your antenna is no longer a microwave radar target. The surface of the aircraft looks like a smooth surface to the microwave radio waves.

Frequency selective surfaces also have ham and commercial uses. In figure 1 we have one of the simpler FSSes. It’s just an array of crossed dipoles tuned for the frequency of interest. In this example we will tune the “+” for 12 GHz, the Ku satellite band.

In figure 2 we have an example of a dual-band dish using a FSS. The prime feed at the focus of the dish is tuned to the 4 GHz C-Band. The 12-GHz Ku-Band frequency selective surface is transparent at 4 GHz and the dish works as a prime focus dish at 4 GHz. However, at 12 GHz the signal sees the FSS as a Cassegrainian sub reflector. Thus, for Ku-Band the dish is really a Cassegrainian feed dish. Now both 4-GHz and 12-GHz systems can work without interference or blockage.

Let’s take it a step further and add a 6-GHz FSS as shown in figure 3. Again the 4-GHz and the 12-GHz signals do not see this 6-GHz surface and the 6-GHz signals think they have the dish to themselves. This can often result in a more efficient dish design than using a multi-band dish feed and gives the antenna designer a broad range of design options.

Now let’s take a piece of good old chicken wire, or as the feed stores like to call it, poultry netting. Chicken wire makes about a 3-GHz high-pass filter. Signals below 2 GHz reflect back from the wire; signals above 4 GHz pass through the holes. Therefore, you can think of chicken wire as a simple high-pass filter.

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