Successful operation of in-flight internet and entertainment services requires accurate polarization alignment of the satellite and receive antennas for linearly polarized signals. Typically, the polarization alignment of the satellite and transceiver is accomplished by an electromechanical device that either rotates the antenna feed or other spatially operated devices that rotate the received signal vector. As will be seen, it is possible to provide the same rotational adjustment by means of a solid-state approach, rotating a received signal vector such that the result provides the original signals transmitted with cross-contamination from the other polarization removed.
Figure 1 shows a block diagram of the vector rotator circuit, based on a low-frequency operational-amplifier (op-amp) circuit described in an earlier article.1 Whereas the low-frequency op-amp version had the advantage of being frequency-invariant over the usable range of op amps, it was limited in frequency by those same devices. That basic low-frequency version is shown in Figure 2.
The proposed hybrid circuit performs the same vector rotation, but is better suited for microwave and millimeter-wave frequencies. The op amps in the original circuit perform the function of summing and subtraction of the scaled in-phase (I) and quadrature (Q) input signals. In the hybrid circuit, the inverting and adding functions are provided by 180-deg. hybrids, and the purely summation function is provided by 0-deg. power combiners. The op-amp circuit used resistors to set the scale factor of the input I and Q vectors, whereas in the hybrid circuit, attenuators set the scale factors.
The theoretical operation of the circuit is based on the vector representation of the received signals represented in eq. 1:
SR = [HTcos(ϕ) – VT sin(ϕ)]xˆH + [HTsin(ϕ) +VTcos(ϕ)]ŷV (1)