 antenna (12.7) where V is the chamber volume; n is the free space impedance; A is the area of conducting surfaces (chamber walls, plate stirrers, etc.) with surface resistance P; O1 is the leakage cross-section of apertures and slots; and Oa is the absorption cross-section of absorbing objects. The total mode bandwidth, Af, for practical chambers can be very different, corresponding to Q-values between 30 and several thousands (Q = flAf). When measuring active terminals, it is important that If is larger than the modulation bandwidth; otherwise, measurement errors might appear. 12.2.2 Transmission Level and Calibration In practice, a measurement of radiation efficiency, radiated power, or receiver sensitivity is based on first determining the chamber transfer function given by Equation (12.5) by using a reference antenna with known radiation efficiency lrad2. Because we would like to have the same mode bandwidth, Af, during the calibration as well as during the actual measurement of the AUT, it is important that the AUT is present in the chamber with its port terminated in a matched load during the calibration. The actual measure- ment will then be done by terminating the reference antenna in a matched load and measuring the transmission to the AUT. The ratio between the average transfer func- tions for these two cases will be equal to the ratio between the radiation efficiency of the reference antenna and the AUT. The radiation efficiency lrad1 of the fixed antenna does not need to be known because it will be the same for both measurements. See specific sections that follow for detailed calibration and efficiency measurement procedures. It is also possible to calibrate the chamber without having the AUT in the chamber. For this case, the reference antenna should be removed when the AUT © 2009 by Taylor & Francis Group, LLC Long Term Evolution: 3GPP LTE Radio and Cellular Technology is measured. This is a simplified procedure with the advantage that we can use the same calibration when measuring several AUTs. In