Qinetiq is developing thin, lightweight materials to absorb potentially dangerous radar reflections from cars and airport buildings.
Conventional absorption devices must have a thickness of around a quarter of the size of the wavelength they are absorbing. But Qinetiq is working with Exeter University to develop metallic films that can absorb signals at wavelengths far greater than their own thickness.
To absorb a signal with a 40mm wavelength, a conventional device would have to be 10mm thick. But Dr. Alastair Hibbins, physics researcher at Exeter, said: ‘We are developing a device many times thinner than the radiation it is absorbing – so for a 40mm wavelength radar, the device could be just 0.5mm.’
The team is using a technique developed by the company’s spin-off Qinetiq Metal Printing (QMP) division to deposit layers of metal, creating a structure consisting of two metal layers sandwiching a layer of dielectric material, such as an epoxy resin. The top layer of metal contains an array of extremely tiny holes, which allow the radiation to be channelled into the device, where it is absorbed by the resin.
This creates an absorber that is thin, lightweight and flexible, allowing it to be used as an applique to cover vehicles and buildings, said Dr. Chris Lawrence, smart materials technology manager at Qinetiq.
The material could be particularly useful in automotive applications, where radar is often used for systems such as adaptive cruise control, to measure the distance between the car and the vehicle in front.
‘Various systems use 77GHz frequencies to monitor how close you are to the car ahead,’ said Lawrence, ‘but if everyone has a source at the front of their car, you’re going to have to be careful about stray reflections.’
If the car is in line with the vehicle in front, the radar system is likely to pick up a clean signal. But if the car is moving between lanes, or the system uses a rotating beam to sweep the area, signals could be reflected off the corner of the vehicle in front into the adjacent lane, causing confusion.
‘There are ways of encoding signals so you don’t get one type of signal talking to another type, but it is a lot easier if you can simply ensure that it never happens. We could put thin absorbing material cheaply on to all of those areas that you don’t want to get a reflection off, to ensure you don’t get a confusing signal,’ Lawrence said.
The technology could also be used at airports, where large buildings around the perimeter, such as hotels and cargo storage bases, can cause difficulties with radar systems.
Airport radar systems send out a signal to communicate with incoming aircraft, which then send a return signal, at a slightly different frequency, in the general direction of the airport antenna. ‘As it only sends the signal in the antenna’s general direction, you can get one signal going straight to the antenna, and other signals bouncing off the buildings, which then may or may not strike the antenna,’ said Lawrence. ‘So you can think there are several aircraft coming in when there is only one.’
Radar systems are divided into 360 single degree segments, and this problem is currently dealt with by ignoring segments where spurious signals are expected to appear. But in adopting this technique many airports have already used up a number of radar segments and it could be dangerous to go much further down this route.
Another problem is that existing radar-absorbing building claddings tend to be bulky, said Lawrence. ‘we could use a cheap, lightweight technology to prevent stray reflections bouncing off all the different items in and around an airport.’
The QMP deposition technique allows metals such as cobalt and copper to be printed directly on to virtually any water-resistant material, including synthetic paper, polyester and glass, and can also be used to produce low-cost radio frequency tags.