Measurement uncertainties in automotive antenna measurements

For the validation of an open area test site usually the normalized site attenuation (NSA) is considered. However, the model for the calculation of the NSA in the various standards is a point dipole. While this may be a correct assumption for biconical antennas or reference dipoles with a well-defined phase center, it is wrong for the antennas used in an automotive antenna test setup. Measurements of automotive antennas for radio and TV reception generally are done on a dedicated test range consisting of a turntable that can bear the weight of the vehicle and a test antenna that is placed in a considerable distance in order to ensure far field conditions. For broadband measurements often log-periodic antennas are used. In these antennas the phase center is not known exactly for all frequencies. The phase center of the automotive antenna is not known either. This uncertainty in the position of the phase centers in combination with the ground reflection occurring between object under test and test antenna results in a measurement error that cannot be neglected. In this work we show how this error can easily be calculated for all test site geometries and give an exemplary calculation for the test site used at our company.