The higher order modes in the feeds of the satellite monopulse tracking antennas

Corrugated horns have been widely applied as optimum feeds for reflector antennas due to the characteristics associated with their rotationally symmetric radiation field patterns which have very low side lobe and cross-polar levels. Most ground stations now use Cassegrain antennas with conical corrugated feed horns for satellite tracking and control as they are optimum systems which can support high gains, better efficiencies, and low cross-polar components. These systems utilize the hybrid HE₁₁ mode for the main signal and one or two of the following modes, TM₀₁, TE₀₁, (HE₂₁ or TE₂₁) for creation of the error signals that are used by the automatic tracking units to derive the angular tracking information. The antenna systems use both the TM₀₁ and TE₀₁ modes for monopulse tracking. Nearly all the antenna systems now extract the tracking information from the HE₂₁ or TE₂₁ modes in corrugated or in smooth waveguides respectively. The. tracking mode sensor extracts the higher-order-mode signal from a circular overmoded waveguide section following the corrugated antenna feed. It is explained how the tracking information can be extracted from the HE₂₁ or TE₂₁ signals.