Comparison of Antenna Patterns from Active and Passive Measurements

Patterns are the most challenging antenna parameter to estimate. The pattern detail is critical for high gain satellite systems and also essential for evaluating low gain diversity antennas for MIMO systems. For small devices with integrated antennas, such as mobile phones and Internet of Things (IoT) devices, antenna pattern measurement is usually done by attaching a coaxial test cable directly to the antenna port (often through a matching network), bypassing the rest of the system making the measurement passive. The test cable must be carefully routed to best suppress the cable's structural scattering contribution. With present measurement systems, it is possible to estimate a pattern using active, over-the-air (OTA) tests. In this paper we compare the results from passive and active tests. Then, by computing the correlation between the measured patterns, we demonstrate the effect of the test cable. This study also demonstrates the potential of antenna pattern estimation from active testing, which is regarded as necessary at higher microwave and mm-wave frequencies where the test fixture can significantly modify the radiation characteristics making the passive measurement inaccurate and in some cases essentially useless.