IN THIS era of telecommunications when most systems are being made wireless, the need for security has increased tremendously because various signals need to be protected and isolated from each other.
Meeting both these requirements simultaneously can present a host of challenges. Currently, mobile communication and Wireless Internet Local Area Network (WLAN) systems are being designed on the basis of a detailed analysis of Radio Frequency (RF) coverage and capacity requirements. Security and privacy issues can be addressed through good design but "eavesdropping" continues to be a real vulnerability.
A revolutionary technology, called Frequency Selective Surface (FSS), is increasingly being proposed and used as an answer to the deployment of secure wireless systems for indoor environments, taking advantage of innovative techniques in building design and the use of attenuating material.
Towards the end of the eighteenth century, the American physicist D. Rittenhouse discovered that some colours of a light spectrum were suppressed when a street lamp was observed through a silk handkerchief. This was perhaps the first "proof" that non-continuous surfaces can exhibit different transmission properties for different frequencies of incident wave. Hence, the surfaces were called Frequency Selective Surfaces (FSS).
Therefore, an FSS can be considered as a free space filter which could be used to pass certain frequencies and stop others. These filters are designed by fabricating some geometric metallic shapes on a dielectric material. The shapes could be a circle, cross, square, ring or a tripole, as shown.
These geometric metallic shapes on a dielectric material act like inductive and capacitive reactance to the incident plane waves and hence, behave as free space filters. The manufactured panels then could be mounted on the walls of a building or pasted on a window to get the desired band pass or band stop results.
The design of FSS is basically obtained by putting any of these shapes a half wavelength away from each other in a periodic manner. The wavelength is obtained by the frequency of operation. A very common example of FSS is the front screen of a microwave oven, used in our houses or workplaces. You must have noticed a periodic structure of hexagons or circles on the front glass.
This is, in fact, an FSS which stops the microwave frequency to come out of the oven, since it is harmful to human beings. How and why do geometric shapes make an FSS act as a band pass or band stop filter? Well, it is very simple.
As an example, shown here is a cross geometry and its equivalent circuit. It can easily be seen that when an incident wave impinges on this structure, a small current flows in the material (mostly made up of copper) making it an equivalent series LC circuit (L= Inductor, C= Capacitor). Inductance is in the arms of each cross and capacitance in between the adjacent edges due to currents produced in the structure as...