The electromagnetic field in any test chamber is a combination of the direct signal and the reflected signals from the walls of the chamber. These reflected signals cause errors in all measurements and are typically the largest contributor of uncertainty in the measurement.
Since the reflected signals phase in and out with the direct signal at different positions in the chamber, they cause a ripple in the electromagnetic field in the test zone (quiet zone). The magnitude of this ripple is an indication of the purity of the electromagnetic field.
Evaluating the ripple in chambers designed to measure essentially omni-directional antennas is not an easy task. The traditional methods of free space VSWR or field probes are not appropriate because the test apparatus introduces too many errors.
The CTIA has developed a method that uses dipole and loop antennas to evaluate the ripple in a chamber. The dipoles and loops used in this procedure must have pattern asymmetry of less than +/-0.1dB.
Right: A Howland VA100-13-D dipole antenna set up for ripple measurements in a Model 3500 Wireless Test Lab The dipole is mounted horizontally on a rigid, low-reflectivity RF cable assembly. The foam test fixture on the Phi axis column has multiple mounting positions for all of the test positions specified in the CTIA test procedure.