Microwave Engineering Europe - January/February 2008 - (Page 24)

24 FAST SMALL ANTENNA TEST Very fast measurements of wireless devices with small antennas in reverberation chambers By Mats Andersson, CEO, Bluetest AB, www.bluetest.se The increasing importance of small antennas The market for products with small antennas is growing at a tremendous rate. The number of mobile phones sold is expected to exceed 1 billion during 2009 (Gartner). In each mobile phone there is a large number of small antennas for communication over several GSM and WCDMA bands, for Bluetooth and WLAN, and more mobile phones are also using integrated GPS, DVB-H, and so on. The number of products using wireless communication is growing from having mainly been mobile phones, WLAN routers and laptops to now also include cameras, wireless TV game controllers, mp3-players, RFID tags — to mention a few. One of the differences of small antennas compared to large antennas is that they are much harder to model accurately with software. This is mainly due to the fact that large antennas normally are used in an open environment without any neighbouring objects to disturb the antenna function. Small antennas on the other hand are usually integrated in a chassis containing materials that absorb radiation or in different ways disturb the antenna function. Furthermore, in a mobile phone there are a number of antennas that all affect each other. For small antennas the most important parameter is its antenna efﬁciency, i.e. the parameter that directly inﬂuences how much of the transmitter power is radiated into space, or how much of the radiation incident on the antenna reach the receiver. By optimizing the antenna function to as high antenna efﬁciency as possible it is possible to directly inﬂuence such important parameters as coverage, battery life time and bit rate in the up and down link. This optimization is very hard to do for small antennas with software but easy with measurements. Since most of the small antennas must have high efﬁciency over a number of frequency channels and sometimes over several frequency bands, there is a need for a large number of measurements both during development and evaluation of wireless products. The faster a company can optimize the antennas in its wireless products, the faster the products can be released on the market to increase the competitiveness of the company. The traditional way of measuring antennas in anechoic chambers, i.e. without any reﬂections, is very good for large antennas that normally are used in a Line-Of-Sight (LOS) environment, but is a slow and unsuitable way to measure small antennas that normally are used in an environment with a lot of reﬂections, i.e. indoors or in an urban environment. Such an environment is much easier to simulate in a reverberation chamber. It also has the advantage that it can be made much smaller and that the measurements are performed much faster than in an anechoic chamber. Another very large advantage that is attracting more and more interest for the technology is the possibility of making direct measurements of diversity gain and MIMO (Multiple Input Multiple Output) capacity for products with multiple antennas. The alternative ”drive tests” are complicated and often unreliable since you cannot drive through exactly the same environment more than once. Consequently, there is a large interest in using reverberation chambers among people working with terminals for new mobile broadband systems, as HSDPA, WiMAX and LTE (Long Term Evolution). What is a reverberation chamber? Reverberation chambers (or mode-stirred chambers) have been used since about thirty years to test how much electrical devices radiate, normally referred to as EMC (Electro Magnetic Compatibility) measurements to avoid interference to other electrical devices. The reverberation chamber is usually a metal-box, cavity, with different sizes in its three dimensions and with some type of mode stirrer mechanism. When one excites one or several antennas in the chamber at a suitable frequency a number of standing wave modes will be generated. By placing the Device Under Test (DUT) in the cavity one makes sure that all radiation generated stays in the cavity and by changing the boundary conditions for the modes in the cavity using a movable metal plate, often in the form of a propeller, it is possible to ensure that the radiated power can be detected regardless of in which direction it is sent out. Figure 1: Per-Simon Kilda’s vision 1999. Traditional reverberation chambers for EMC usually have an accuracy which is not better than 3 dB in standard deviation (STD). This is much too high uncertainty to measure antenna efﬁciency, radiated power or receiver sensitivity but quite sufﬁcient for EMC measurements. New idea = ”spin-off” company Bluetest At the end of the 1990´s Professor Per-Simon Kildal at the antenna group at Chalmers University of Technology had an idea on how to improve the accuracy and speed of reverberation chambers so they could be used to measure antenna efﬁciency, radiated power, receiver sensitivity of small antennas and wireless terminals with small antennas. The origins to the idea was the realization that the traditional way of measuring antennas in anechoic chambers was not at all suited for small antennas or wireless terminals with small antennas, for example mobile phones, since they are normally used in environments with a lot of reﬂections, i.e. indoors or in urban areas. At this time some companies had also started to become interested in terminals with Microwave Engineering Europe ● January/February 2008 ● www.mwee.com 024_025_026_028_029_MWEE.indd 24 24/01/08 15:15:43 http://www.bluetest.se http://www.mwee.com

Table of Contents Feed for the Digital Edition of Microwave Engineering Europe - January/February 2008

Microwave Engineering Europe - January/February 2008 Contents News Comment Radio: Raising the Bar for the Radio: Making 802.11n Work Cover Feature: The RF-System-In-Package Trend - Efficient Design with Advanced Design System 2008 Wireless Sensor Networks: The Zigbee PRO Feature Set: More of a Good Thing Very Fast Measurements of Wireless Devices with Small Antennas in Reverberation Chambers WiMAX Update 2008 Bridging the Gap from the CMOS DSP to the Antenna in OFDM Systems Products Calendar

Microwave Engineering Europe - January/February 2008

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