MSF 2-degree satellite feed extension is proposed along with expected antenna performance. To solve the two polarization obstacle, it becomes necessary to go to either square or circular waveguide. The lowest frequency mode is called the fundamental mode. As is often the case, several higher-order modes often turn up over a frequency band along with the fundamental waveguide mode. In general, larger waveguide dimensions promote lower frequencies for higher-order mode propagation.
Square waveguide is difficult to build and design feed structures for parabolic dishes.
Circular waveguide, on the other hand, lends itself naturally to feed design, providing optimum performance and supporting continuous polarization rotation adjustment. Circular waveguide, however, has a much reduced frequency range for mode free operation in the fundamental TE11 mode. Also, any change in waveguide diameter acts as a higher-order mode converter. Indeed, signal energy is transferred out of the fundamental mode into the higher-order modes. The most important higher-order mode is the TM01.