This example is typical of single offset reflectors, therefore single offset paraboloids illuminated by conventional feeds are limited by XPOL performance.
Dual offset reflector configurations can be designed for low geometrical optics XPOL when illuminated by a pure linearly polarized feed.[11,12] The just fully offset reflector is upgraded to a low cross-polarized, just fully offset Gregorian dual reflector antenna by adding a concave ellipsoidal sub-reflector to the original single reflector system.
XPOL is now -47.06 dB, more than 25 dB lower than the XPOL of the single configuration.
Since the goal is to consider practical antenna systems, feed antennas with relatively high XPOL must be considered, and the influence of feed XPOL on the total system XPOL must be investigated.
Equation 1 is similar to the relation of two parallel admittances. The result in Equation 1 shows that either the feed or the reflector XPOL can dominate the system XPOL.
Enforcement of this constraint leads to deterioration of system XPOL in the described configuration.
After rotation, the previous design conditions[11,12] for low XPOL are no longer satisfied.
The proposed configuration presents XPOL = -35.04 dB, thus meeting the desired specification for low XPOL.
It was shown that the XPOL of dual offset Gregorian antennas designed for low XPOL generally is limited by the feed XPOL.