Microwave Engineering Europe - October 2008 - (Page 36)

36 ANTENNA TESTING National Physical Laboratory test facility aids development of next-generation antennas Researchers at the National Physical Laboratory (NPL) are looking to help academia and industry develop the next generation of smaller, smarter and wearable antennas through its new testing facility at its base in Teddington, South West London. By Martin Alexander, Principal Research Scientist, Wireless Communications & EMC, National Physical Laboratory here is a massive growth in wireless communications devices including mobile phones, GPS, WiFi and WLAN. Connection methods like Bluetooth and UWB are commonly used over short distances. Alongside this, the demand for smaller devices that use less power has intensiﬁed. The result being that the entire unit is produced smaller and lighter. NPL has constructed a new facility for minimally invasive measurements of electrically small antennas and smart antennas for wireless communication platforms – in particular addressing the frequency range 400 MHz to 11 GHz. These require a unique type of measurement facility and antenna support that has a minimal effect on the antenna performance and to make the lowest uncertainty measurements. This is exactly what NPL can now provide industry and academia in the UK. Universities including Queen Mary’s London, Cambridge and Brunel are researching wideband and UWB communication, looking at very small units and consequently the use of very small antennas. In order for them to design such equipment they need to know the antennas’ coverage and efﬁciency to ascertain how much extra power they will need to make the communication system work. NPL already has a chamber that could do this for larger antennas but it has now designed one that can provide measurements for the new generation of handsets, the main difference being the nonmetallic antenna towers. The Wireless Communications project is part of the National Measurement System’s Electrical Programme, which is funded by the Department of Innovation, Universities and Skills. The project was formed in collaboration with focus groups formed of industrial and academic partners. Electrically small antennas inherently have wide-beam or near-cardioid radiation T patterns. Accurate measurement of gain and patterns of such antennas requires a special type of measurement facility — in particular an antenna support that has minimal effect on the antenna radiation. Small antennas, in which there will be no feed cable, need to be measured in isolation to get a true picture of ﬁnal performance. But if an external power supply or data cables are needed for the testing of the antennas, a hugely important factor is the effect these can have on the measured radiation pattern. The radiation caused by any common mode current on the cables is undesirable as it will interact with the intended antenna radiation and give a distorted measurement. In order to address this, the coaxial line (metallic) can be replaced by an optical ﬁbre (glass), which is not invasive to the ﬁelds being measured and will not support common mode currents. NPL’s new facility is a Small Antenna Radiated Testing (SMART) Range built in a screened room 7 m long, 6.2 m high and 6.2 m wide with a planned frequency range of 400 MHz to 11 GHz. The 0.45 m long pyramidal absorber is speciﬁed to have low reﬂectivity up to 110 GHz. The range contains an Orbit roll over azimuth positioner system, with software to acquire radiation patterns in amplitude and phase over a complete sphere. The source tower can be positioned to provide an antenna separation of up to 4 m. The new chamber is lined with TDK radio wave absorber material with very low reﬂectivity. This polyethylene based absorber is an improvement as is does not shed carbon powder like the standard polyurethane based absorber. The 0.45 m long pyramidal absorber is speciﬁed to have low reﬂectivity up to 110 GHz. The absorber is more rigid than standard absorber and this allows polythene blocks that are transparent to RF signals, to be placed anywhere that An early chamber at NPL. Courtesy of NPL © crown copyright 1959. is convenient on top of the ﬂoor absorber to give easy access to the antennas. NPL scientists found that these blocks also make ideal non-reﬂective antenna towers. The walls are lined with white polystyrene tiles that gives much better room illumination, requiring fewer lights, than for chambers lined with polyurethane absorber. NPL has also worked with Japan’s NECTokin Corporation to develop a wireless RF-tooptical link using an Electro-Optic transducer, the OEFS-PR-7G. NEC-Tokin optical department responsible for this has been devolved to Seikoh-Giken. This can receive RF signals up to 7 GHz and creates an RFto-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. Microwave Engineering Europe ● October 2008 ● www.mwee.com 036-038_MWEE.indd 36 6/10/08 17:25:51 http://www.mwee.com

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Microwave Engineering Europe - October 2008 Contents Comment News Cover Feature: AWR's AXIEM Software Brings 3D Planar Electromagnetic (EM) Simulation "Up Front" RFID: Augmented Reality: Beyond RFID and QR Codes for Mobile Phone Platforms Filters & Frequency Synthesis ZigBee Goes Green with Support for Smart Energy Simplify Mobile Data Applications and Services Test Enabling the State-of-the-Art in Automatic Test Equipment National Physical Laboratory Test Facility Aids Development of Next-Generation Antennas Selecting the Synthetic Test Environment for Transmit-Receive (T-R) Modules in a Phased Array Radar System Products Calendar

Microwave Engineering Europe - October 2008

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