Experimental evaluation of a beamforming array calibration system

We have developed an automatic calibration system for arbitrary shaped array. A signal generator drove a horn antenna to provide a signal source and a spectrum analyzer was connected to the output of an 8 - channel beamformer to collect data. The entire interface was automated using LabVIEW, which included a program to upload the weights to the beamformer, control system for the signal generator, spectrum analyzer and a rotator. Our calibration method involved connecting each of the eight channels on the beamformer to an eight to one relay that would select one channel to pass through to the output of the relay. Then we connected one port of a vector network analyzer (VNA) to the output of the 1 to 8 relay and the other port of the VNA to the output of the beamformer. The algorithm for calibration would have the VNA apply a weight to one of the channels at a time, and the VNA would read the output. A LabVIEW program would switch through all eight channels storing the applied weights and the outputted weights. Then it would compare all eight weights and find the channel with the largest error and correct that channel. The program would loop until the total error was within a user set threshold. After calibration we ran beamforming and nullsteering tests in the anechoic chamber at various angles to test the accuracy of the beam or null point angle, the side lobe level and the depth of null. The test results show that the calibrated array has a much improved beam pattern comparing with the one without calibration and it is very close to the simulated pattern. We improved the general accepted approaches by introducing an automatic system with a properly defined calibration procedure, thus freeing people from tedious, time consuming calibration process.