NPL's SMART range

The development of smaller, smarter and even wearable antennas has received a boost thanks to the completion of a new facility at the

(NPL).

Wireless communication through mobile phones, GPS, WiFi and WLAN is becoming increasingly in demand, while devices such as laptops commonly use Bluetooth and ultra wideband (UWB) over short distances. However, with this has come demand for smaller devices with lower power consumption, allowing the entire unit to be made smaller and lighter.

The new facility will allow minimally invasive measurements of electrically small antennas and smart antennas for wireless communication platforms, allowing their performance to be accurately measured and compared during development and testing.

'Universities including Queen Mary University of London (QMUL), Cambridge and Brunel are researching wideband and UWB communication,' said Dr Phil Miller from NPL's Time Quantum and Electromagnetics team. 'They are looking at very small units and consequently the use of very small antennas.

'But so they can design such equipment they need to know the antennas' coverage and efficiency to ascertain how much extra power they will need to make it all work. We previously had a chamber that could do this for larger antennas but we have now designed one that can provide measurements for the new generation of handsets. QMUL is also looking at body-mounted antennas for military use.'

NPL's Wireless Communications project is part of the National Measurement Service (NMS) Electrical Programme. It arose as a result of focus groups comprising people within industry and academia, and was developed in partnership with them.

Electrically small antennas inherently have wide-beam or near-cardioid radiation patterns. However, accurate measurement of gain and patterns of such antennas requires a special facility — in particular an antenna support that has minimal effect on antenna radiation.

NPL's facility has a Small Antenna Radiated Testing (SMART) Range built in a screened room 7m long, 6.2m high and 6.2m wide with a planned frequency range of 400MHz to 11GHz. The range contains a rollover azimuth positioner system, with software to acquire radiation patterns. The reflectivity of the antenna supports needs to be as low as possible to make the lowest uncertainty measurements. Meanwhile, a source tower can be positioned to provide a range of between 0.5m and 4m.

Small antennas, in which there will be no feed cable, need to be measured in isolation to get a true picture of final performance. If an external power supply or data cables are needed for testing, these may affect the measured radiation pattern and give a false report. The undesirable radiation caused by any common mode current will interact with the intended antenna radiation and give a distorted measurement.

NPL has therefore developed a wireless RF-to-optical link using an Electro-Optic transducer, the OEFS-PR-7G — the result of a collaboration between NPL and Japan's NEC-Tokin Corporation. This can receive RF signals up to 7GHz and creates an RF-to-optical transducer link that allows the RF cable to be replaced by a minimally perturbing optical fibre that will barely interfere with the signal of the device being measured.

The antennas using the facility are expected to operate primarily over the frequency range 400MHz to 11GHz.

The chamber also uses a TDK radio wave absorber material with very low reflection to cover its walls. 'Normally, foam with carbon is used,' said Miller. 'Unfortunately the carbon leaks from this, creating a coal dust-style problem. The TDK doesn't do this so we now have a very clean environment, too. There is no other facility like this in the UK, especially one using RF-to-optical transducers.'

Research at NPL's previous antenna facility helped with the development of O2's 3G network. The company used NPL's measurement chamber to ensure it chose the right equipment before investing in a multi-million pound infrastructure upgrade.

To support the increased data transmission needs of 3G and meet the terms of its licence, O2 had to install a new network of antennas, capable of handling larger data volumes and higher network transmission speeds. By moving from older fixed-tilt antennas to new variable tilt models, the reliability and access to the network could be improved.

But to do this, the company had to choose between various types of antennas whose specifications were provided by their makers rather than an independent body, making their likely performance hard to judge. NPL therefore provided testing services to verify the manufacturers claims and compare the devices on offer, confirming their gain and directivity.