In-Situ Measurement of Transmitter Antenna Input Current Using a Software-Defined Radio

In phased-array transmitters, array calibration is often used to correct for the magnitude and phase changes of voltage waves from the signal sources to the antennas in the different array elements. Inserting a reconfigurable impedance tuner between the power amplifier and the antenna in each element allows the range to be maximized upon changes in operating frequency or scan angle. However, tuning the impedance causes an undesirable real-time change in the magnitude and phase of the element transmission parameters. Proper assessment of this real-time change is needed to use equalization and impedance tuning to maintain the array pattern. Specifically, the antenna input currents must be monitored, and the antenna input currents can be used with the known antenna patterns to calculate the array transmission pattern. Measurement assessment of the antenna input current for impedance-tuning operations is demonstrated using a dual-directional coupler with a software-defined radio voltage measurement input. This is a practically implemented version of the approach that shows a path forward to system implementation of this “on the fly” transmitter calibration approach for arrays containing reconfigurable circuits, or for use in adjusting traditional array calibrations during operation.