Shrinking antennas

3 min read

Research underway at Sheffield University aims to produce antennas for low frequency wireless communications so small they could be incorporated in vehicle surfaces or clothing.

Applications for low frequency communications at 100MHz and above include military applications, space missions, and emergency services, but the antennas can be relatively large and prone to breaking. They are especially vulnerable on the battlefield and in space.

Project leader Prof Richard Langley, head of Sheffield’s Communications Group, said: ’A conventional antenna is typically between a quarter and half a wavelength long [a car radio antenna is around 75cm long, a quarter of a wavelength of a 100MHz FM station]. We aim to reduce that to anything from a 50th to a 20th of a wavelength.’

In addition to examining new methods of reducing the size of antennas, the project will also look at using metamaterials to create high impedance surfaces (HIS). ’This allows us to put antennas very close to them and make them compact, and in doing so, that makes the antenna what we call “platform tolerant”,’ said Langley. ’In other words, we can put an antenna on a human body and shield it, or on to a vehicle or aircraft.’

The team is also aiming to shrink these electronic band gap (EBG) surfaces, and is in the process of patenting technology which uses these techniques.

Another advantage of band gap materials is that they shield the antenna and associated radiation from the body. These special surfaces consist of a piece of metal — or if they’re worn, a piece of woven fabric that behaves like metal — with a patterned metallic surface one or two millimetres above it.

The combined action of those two layers stops waves creeping over the surface and going behind it. If an antenna is mounted on it, it is forced to radiate away from this surface, which stops it going backwards into the body or the vehicle on which it is mounted.

This could lead to antennas being incorporated into clothing. ’This would not only be for the security forces, but for all sorts of communication applications,’ said Langley. ’As well as ubiquitous computing, where you essentially carry computers with you, it will most likely be used for telemedicine applications or athlete monitoring.’

Langley foresees that following a hospital stay, a patient could be fitted with monitors to oversee indicators such as heartbeat or blood sugar levels. These could then be relayed wirelessly via miniature worn antennas from the patient’s home to the hospital where he or she would be monitored remotely, freeing up beds and saving money. Athletic training and performance could be monitored in a similar way.

The project will also incorporate passive and active lumped circuit element components into the antennas and HIS surfaces to reduce their size. ’If you look at many antennas, you can break them down into circuit components, so you can model them as an inductor and a capacitor, for example,’ explained Langley.

’What we plan to do is enhance communications performance with lumped components, which are little blocks of electronics which will increase their value by a factor of 10. By doing that, we should be able to shrink the size of the antenna.’

Industrial partners on the project are Harada Industries Europe and BAE Systems Advanced Technology Centre.

Harada makes car antenna systems. As roof-mounted whip antennas are used less, they are often mounted on glass. ’The problem is they’re metallising the glass,’ said Langley, ’so we need something like a band gap structure to mount the antenna on the car, and for it to be as small as possible.’

Similarly, many products produced by BAE Systems for military customers often use low-frequency applications.

’The police and emergency services also use 400MHz tetraband where the antennas are large, and they’d like to reduce the size,’ added Langley. ’The other benefit is if you make things small you can hide them, and from a military and security point of view that’s useful.’

The project runs until 2011, by which time the researchers aim to have provided some demonstrators to prove the technology and see how small the antennas and EBGs can go, while being effective.

Langley warns there is a proviso. ’The laws of physics tell you that the efficiency of how the antenna works is reduced significantly, as is the bandwidth. We’re beginning to show that’s not the case with this type of approach, that you can actually overcome the physical limits by adopting these techniques. But we still need to make them efficient, otherwise they’re useless.’

Berenice Baker