The inverse source problem of near-field antenna measurements is often formulated in terms of unknown equivalent electric and magnetic surface current densities. Exploiting just the constraints for the radiated field exterior to a closed Huygens surface results in ambiguous equivalent sources due to the excitation of non-radiating currents during the solution process. Therefore, enforcing the zero-field or Love side condition for the suppression of non-radiating currents based on a left-hand side preconditioner in the form of a Calderón projector is proposed. Transformation results based on realistic measurements demonstrate the effectiveness of this approach.

We present a low-cost and light-weight measurement equipment based on a software-defined radio (SDR) and a radio-frequency over fiber (RFoF) connection. The main purpose of the hardware are near-field antenna measurements, but possible use-cases also include imaging and other inverse-source scenarios. The two features “low-cost” and “light-weight” of the hardware are important for the use on an unmanned aerial vehicle: Crashes should not financially ruin the operator and payload is of course a central issue for any flying object. Regarding the RF properties of the hardware, the RFoF connection has the great benefit of extremely low loss as compared to coaxial cables, while the SDR offers a great deal of flexibility concerning measurement frequency, bandwidth, and signal filtering. One transmit channel of the SDR is employed to provide a coherent signal to an antenna and two receive channels can capture two components of the field, e.g., two linearly independent polarizations. As an important part of the RF circuitry, a drift compensation is implemented as a countermeasure against changing temperature conditions.

A near-field antenna measurement system is presented that consists of components that are rather unusual compared to conventional antenna measurement setups. Instead of a vector network analyzer (VNA), a dual-channel wideband software defined radio (SDR) is used to measure the signals at the ports of a dual-polarized probe antenna. Instead of an exact multi-axis positioner for the antenna under test (AUT) or the probe antenna, a multicopter moves the probe along a predefined trajectory on a surface around the AUT. Instead of using expensive laser interferometry equipment, the position and orientation of the probe antenna are determined by a 6-D tracking system that was originally developed for virtual reality (VR) applications. Still, the first measurement results show the usability of the low-cost system for antenna measurements in the frequency range of mobile communication services.

For antenna measurements, the device under test (DUT) is usually transported to an anechoic chamber equipped with a scanner system. To measure on-site, unmanned aerial vehicles (UAVs) are a flexible solution as they can precisely maneuver a probe antenna over a surface in the DUT’s near-field region. In this work, the designed multi-rotor UAV enables electromagnetically optimal placement of the probe antenna and effective gravity center re-balacing by payload position adjustment. The on-board software-defined radio (SDR) serves as a dual-channel receiver for the dual-polarized probe and as a signal source. For phase-coherent measurements, the source signal is transmitted to the DUT via an optical fiber tethering the UAV to the ground control station. Power supply cables allow unlimited flight times. A laser-based tracking system originating from virtual reality (VR) applications measures the probe position and orientation. The developed operator software guides the UAV along a trajectory and records the irregularly distributed near-field samples. An inverse equivalent sources solver (IESS) with full probe correction computes equivalent sources for antenna diagnostics and the DUT far field. The deployed components make the system very cost-effective. Still, verification measurements demonstrate its usability and the accuracy of the results for frequencies up to several GHz.

The accuracy of near-field antenna measurements combined with near-field far-field transformations (NFFFT) is deteriorated if objects are located in close vicinity to the antenna under test (AUT). We compare two equivalent sources based modeling techniques to suppress the influence of structures in the vicinity of the AUT. Simulation results for perfect electrically conducting objects showcase the capabilities and limitations of both approaches. The presented methods can easily extend existing NFFFTs in order to remove the influence of mounting structures or parts of the positioning system in anechoic near-field measurement facilities.

Reducing near-field measurement times is an important challenge for future antenna measurement systems. We propose to incorporate knowledge about material parameters of the antenna measurement environment within the simulation model. To do so, a method-of-moments code with surface discretization is implemented as a side constraint to the near-field far-field transformation problem performed with the fast irregular antenna field transformation algorithm. Transformation and source reconstruction results of synthetic measurement data demonstrate the effectiveness of the proposed method.