ween the transmitting and receiving antennas in the chamber; because we would like the chamber to provide an isotropic reference environment with Rayleigh fad- ing, it should be as low as possible. The direct coupling is given by the free space transmission formula, also known as Friis transmission formula (see, for example, Kildal [19]): (12.4) where r is the distance between the antennas, a is the wavelength, and G and G, are the realized gains of the two antennas, respectively, in the direction of the opposite antenna. From Equation (12.4) it can be understood that the main lobes of the antennas should not point toward each other. If the direct coupling, Sd, is much smaller than the chamber contribution, S to the total transmission, S21, the chamber provides a Rayleigh fading environment. Effective methods to reduce the direct coupling include platform stirring, polarization stirring, and using fixed antennas that have nulls in the direction of the antenna under test (AUT). The transmission between two antennas in free space is governed by the Friis transmission formula (12.4). The corresponding formula for transmission between two antennas in a reverberation chamber is described by Hill's transmis- sion formula [20], which is valid under the assumptions that the direct coupling is negligible and that the chamber is large with many modes excited. We choose to refer to Hill's formula as the chamber transfer function and to present it in the following form: G chamber - (12.5) where fis the frequency; C is the speed of light; V is the chamber volume; lrad1 and lrad2 are the total radiation efficiencies (i.e., including mismatch) of the two antennas; and If is the average mode bandwidth. © 2009 by Taylor & Francis Group, LLC Measuring Performance of 3GPP LTE Terminals The mode bandwidth Af consists of four additive contributions due to losses in the walls, leakage through apertures and slots, antennas in the chamber, and absorbing objects in the chamber: antenna object (12.6) all lossy walls slots antennas objects