Abstract:
Open-ended ridged waveguide phased array antennas are of high interest for low Earth orbit satellite
communication applications due to their high efficiencies and power handling capacities. This paper analyzes
the scan limitations with respect to active matching, axial ratio (AR), and realized gain of circular and
hexagonal waveguide apertures with three and six ridges for a Ka-band satellite downlink application. Strong
degradation of the active reflection coefficient (ARC), the coupling coefficient to the terminated
(cross-polarized) port of a polarizer, the AR, and the realized gain are observed at high scan angles and
frequencies when using a large element periodicity near the grating lobe threshold. For the open-ended
hexagonal six-ridged waveguide, the scan limitations are investigated in detail for several element
periodicities. The degradations can be related to three conditions. First, the onset of higher-order Floquet
modes leads to single-mode scan blindness associated with one of the two orthogonal fundamental waveguide
modes. Second, a transmission zero of one of the two fundamental waveguide modes far outside the operational
band, occurring when the phase constants of the fundamental waveguide modes align with those of the
fundamental Floquet modes, is interpreted as surface wave propagation and results in an increased attenuation
of the affected fundamental waveguide mode at the maximum operational frequency. Third, the onset of
higher-order waveguide modes induces modal coupling between the fundamental and the first two higher-order
waveguide modes. Each of these conditions results in unequal contributions from the two fundamental waveguide
modes to the radiated field, thereby degrading the overall performance. These scan limitations can be
mitigated by using open-ended ridged waveguides with an electrically short evanescent end-section, allowing
for large element periodicities while meeting the AR requirements for Ka-band satellite communication
applications.
