An electronically controlled polarization generator at Ka-band

As part of a target simulator, a linearly polarized signal was required with a variable tilt angle that could be controlled electronically and changed at a 1 kHz rate. The signal simulates the effect of rapid polarization changes that a missile might encounter in real time during flight. Tilt angles can be varied by adjusting the amplitude of the vertical and horizontal inputs to an orthomode transducer. To produce good cross-polarization, independent phase adjustments are also required. However, microwave components available in the 33.4 - 36 GHz operating range were inadequate to achieve the desired performance. A novel approach was developed to downconvert the input signal to a lower frequency range and use vector modulators available in the lower band to produce the appropriate phase and amplitude changes in each path, then upconvert back to the desired operating frequency to drive the orthomode transducer. A device was built and tested using this approach. A calibration and measurement procedure was developed to determine the vector modulator input settings that produced the most accurate tilt angles and best cross-polarization performance. By iteratively measuring cross-polarization and tilt angle, then adjusting the vector modulator controls, a tilt angle accuracy of +/-1 degree was achieved with a cross-polarization of -25 dB, exceeding the required performance. By implementing the architecture described, both the phase and amplitude of the horizontal and vertical signals to the orthomode transducer can be controlled. In addition to linearly polarized signals, other types of polarization signals can also be generated, including left-hand and right-hand circular, as well as the general case of elliptical polarization.