Walls That Can Be Tuned

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 incident electromagnetic waves strike, as shown in the figure. At resonance frequency of LC circuit, the signal would either pass or stop according to the design analogy and the form of incident wave impinging on the FSS.

In short, an FSS is not more than a parallel or series LC circuit, to behave as a free space filter. Incorporating some active device(s) in between these elements can make the FSS active, adding more functionality and configurability in design.

An FSS could be used in many engineering applications such as:

-- Radio Frequency Identification (RFID) tags;

-- Collision avoidance;

-- Radar cross-section augmentation;

-- Robotic guided paths;

-- Electromagnetic interference protection;

-- Photonic band-gap structures;

-- Low probability of intercept systems (for instance stealth);

-- Waveguide or cavity control coupling;

-- and, wireless security, etc.

It is not necessary for a reader without a scientific background to understand the terminologies given above, because my focus is on the last application. This is because the use of wireless communication pertains to the masses. Therefore, the importance of FSS in the security of modern-day wireless technology will be discussed.

Cellular communication

Cellular phones cannot just be disruptive or annoying, but on occasions, they can give rise to security threats because they could be used to set off a device. But the annoying sound of a cellphone ringing in the cinema or theatre could be on the way out with the development of a high-tech frequency selective surface that blocks their signals.

Quiet zones could be created in hospitals, schools and airport lounges with the use of high-tech FSS. Also, the surface, made up of a metallic grid, could help foil terrorists. This type of security is mostly needed in airports and other high-risk areas frequented by personnel belonging to the defence forces and security agencies.

The beauty in the design and development of an FSS is that it would only block a particular kind of signals while other signals could be allowed to pass. In engineering terms, it would behave as a band stop filter to the frequency to be blocked and a band pass filter to the rest.

It must be noted that before the advent of commercial FSS, large aluminum sheets were incorporated in the walls of modern buildings to stop unwanted signals. A major problem with this technique was that besides blocking the desired frequency, it would stop other frequencies as well.

However, with the commercial manufacturing of frequency selective surfaces, it has now become possible to be selective -- either to stop or pass a particular kind of waves. Therefore, the aim of creating secure zones in buildings to avoid terrorist activities could be achieved through the development of either of the following:

-- A passive FSS, or;

-- An active FSS.

A passive FSS is the one in which some funny shapes are designed on a dielectric sheet for a particular frequency to be passed or stopped. Once this surface is fabricated, its properties cannot be altered. The other basic point is that they have to be relatively large enough to be pasted on