Microwave Engineering Europe - January/February 2008 - (Page 26)

26 FAST SMALL ANTENNA TEST Figure 2: Sketch of Bluetest’s standard chamber. The network analyzer is connected via switch to the three ﬁxed orthogonal monopole antennas, and to a dipole in the chamber. The dipole is mounted in a holder on a rotating platform. Antenna efﬁciency When the reference measurement is done it is possible to take an antenna where the efﬁciency is unknown and measure it. This is done in a similar manner as described above and the present measured power from the antenna (i.e. Antenna Under Test) we call PAUT . It is now possible to calculate the antenna efﬁciency of the unknown antenna with the following formula: erad = PAUT/Pref This measurement takes one minute in the High Performance chamber. The main difference between Bluetest´s chambers and traditional EMC chambers is that with the same size of the cavity it is possible to generate many more independent samples. To manage this, the Bluetest chambers use several independent mode stirrer techniques, that are illustrated in Figure 2. The mode stirring techniques are: 1) Mechanical mode stirring with two orthogonal metal plates: by moving the plates along the full height and depth of the cavity a large number of independent ﬁeld distributions are obtained; 2) Platform stirring: by letting the the DUT move in a circle in the chamber many more independent samples are measured than if the DUT is in a ﬁxed position; 3) Polarization stirring: by using the three ﬁxed orthogonal monopole antennas and by measuring power on all three antennas we increase the number of independent samples by a factor of three; 4) Frequency stirring: by averaging over frequency it is possible to obtain even higher measurement accuracy, however the frequency resolution is decreased. In general one can say that the larger the chamber with respect to wavelength the higher the accuracy. Bluetest recommends that the standard chamber be used from 850 MHz and that the High Performance chamber is used from 700 MHz. To be able to measure with good accuracy at 400 MHz one needs a chamber of an approximate size of 2.0 m x 2.5 m x 3.0 m. If you have a sufﬁcient number of independent modes it is possible to show that the DUT will experience isotropic incidence, i.e. the performance of the antenna or mobile phone is measured over all directions. This property is used for measurements of antenna efﬁciency, total radiated power (TRP) and total isotropic sensitivity (TIS). If you look at the measured power between the DUT and the wall antennas it is Rayleigh distributed, i.e. the statistical distribution you obtain when a large number of plane independent waves interfere with each other. This statistical fading is similar to the fading one ﬁnd smany times in real environments indoors and in city centres. We can use this property for fast receiver sensitivity measurements and to estimate diversity gain and MIMO capacity. Real measurements Reference measurement Always start with a reference measurement using an antenna with a known radiation efﬁciency. It can be known with the help of calculations or measured in another measurement chamber. This procedure is similar to the use of a standard gain horn in anechoic chambers. By ﬁrst measuring the reference antenna we get an estimate of the total losses in the chamber. It is therefore a necessary requirement that objects that can contribute to losses are not moved from or into the chamber between the reference measurement and the measurement of antenna efﬁciency, radiated power, etc. The reference antenna is positioned at least 0.5 wavelengths from the walls or mode stirrers in the chamber and at least 0.7 wavelengths from absorbing materials, for example a head phantom. A network analyzer is used and the average received power from each of the three ﬁxed antennas to the reference antenna is measured during continuous mode stirring. In the High Performance chamber it takes one minute to measure a value with less than 0.5-dB standard deviation. Since we know the efﬁciency of the reference antenna and the received power from the reference measurement, we can normalize the received power to what it would have been if the reference antenna had an efﬁciency of 100 percent. We call this power Pref. Total radiated power The Total Radiated Power (TRP) is the power that is is transmitted, from for example a mobile phone, integrated over all directions. This power is affected by the output power of the ampliﬁer, mismatch between ampliﬁer and antenna, the antenna efﬁciency, objects in the vicinity to the antenna that contribute to losses, for example mobile phone chassis, hand, head, etc. To measure the total radiated power in a Bluetest chamber, the DUT, for example a mobile phone, is positioned on the turntable at least 0.5 wave lengths from any wall and at least 0.7 wavelengths from any absorbing material. A base station simulator is connected to the three ﬁxed antennas. It is now possible to initiate communication between terminal and base station simulator or vice versa. If a mobile phone is measured, a special SIM card is normally used and after a call has been set up, the base station simulator orders the terminal to send at full power. The power between the mobile phone and ﬁxed antennas is then measured. Since the base station simulator measures the received power and we know how large the chamber losses are from the reference measurement it is easy to calculate the total radiated power. Just as in the case when you measure antenna efﬁciency it takes one minute with the High Performance chamber to reach a result with a standard deviation less than 0.5 dB. Total Isotropic Sensitivity The Total Isotropic Sensitivity (TIS) is the power that reaches the receiver through the antenna integrated over all directions. This sensitivity is affected by the sensitivity of the receiver, mismatch between receiver and antenna, the efﬁciency of the antenna, objects in the vicinity to the antenna that contribute to losses, for example mobile phone chassis, hand, head, etc. To measure the total isotropic sensitivity in a Bluetest chamber, the test object is positioned in Microwave Engineering Europe ● January/February 2008 ● www.mwee.com 024_025_026_028_029_MWEE.indd 26 24/01/08 15:16:04 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

For optimal viewing of this digital publication, please enable JavaScript and then refresh the page. If you would like to try to load the digital publication without using Flash Player detection, please click here.