Non-invasive Antenna Pattern Characterization Using Time-domain and Frequency-domain Measurement Techniques

We investigated the performance limitations due to time-domain multi-sine stimuli with difierent Peak-to-Average-Power Ratios (PAPRs) on non-invasive radiation pattern mea- surements of a broadband antenna over 500MHz to 4500MHz and compare these results with frequency domain quasi-CW measurements using a vector network analyser. Both measurements are made in the same environment, an anechoic chamber using an opto-electric fleld sensor system to minimize distortion of the antenna pattern. The PAPR of the time domain stimulus waveform directly afiects the system dynamic range and the uncertainty of the results. The wireless communications landscape is changing as a result of deregulation of additional RF spectrum resources and development of multiple-input-multiple-output (MIMO) systems, driving the need for over-the-air (OTA) testing of complete systems. In an anechoic chamber at the UK National Physical Laboratory (NPL), we have replaced the standard metal coaxial feed by an optical flbre system that has minimal efiect on the antenna radiation-pattern (1). This system is particularly useful for measuring antenna systems where the impact of the RF feed is signiflcant. The well-established closed-loop, swept-frequency CW testing, using a vector network analyser (VNA), i.e., frequency domain (FD) testing is ideal for passive antenna testing but is not compatible with highly modulated signals in an active system and this will be especially true for MIMO, where multiple simultaneous data-channels are present. We have previously investigated scalar OTA testing of the antenna system radiation pattern where the signal is often modulated, bursty and intermittent (2) but as the phase information is lost these techniques cannot be used for MIMO systems. In time domain (TD) antenna characterizations (3) the frequency content is embedded in the source and received waveforms. Provided the TD measurement system is linear, equivalent results are expected for either TD or FD stimulus. In this paper we will explore the impact of waveform Peak-to-Average-Power-Ratio (PAPR), and noise on time-domain antenna measurement dynamic range using a broadband antenna and compare the results with FD results. 2. TIME-DOMAIN WAVEFORMS The time-domain waveforms must comprise su-cient frequency components to characterize the antenna and these components are all simultaneously present in the waveform.