The antenna of the satellite must be pointed accurately toward the earth station antenna with which the satellite is in communication using a high-gain reflector antenna system. In order to achieve this accurate pointing, satellite antenna commonly employ tracking systems to provide signals indicative of the pointing errors in elevation and azimuth relative to the antenna beam of the earth station antenna. In tracking system three signals are used for the pointing accuracy of the satellite antenna. These three tracking signals are the azimuth difference signal, elevation difference signal, and the sum signal. The phase and amplitude characteristics of these three signals are utilized in a conventional manner to generate elevation angle error and azimuth angle error signals to control the pointing direction of the satellite antenna.
For circularly polarized beacon only one higher order mode for difference pattern is required for tracking information. Higher order mode may be TM01, TE21 or TE01.In this configuration TM01 mode is used for difference pattern while dominant TE11 mode gives sum pattern information. Coupler consists a two-arm turnstile junction. E-plane folded hybrid junction is tuned for optimum performance at 60 GHz. The dual ports of hybrid respond to TM01 mode by exciting only the H-port. Similarly, the hybrid responds to the TE11H mode by exciting only the E-port. Thus the two modes are separated. The TM01 mode signal is available at H-port of the E-plane folded hybrid junction while the TE11 mode, available at the E-port of E-plane folded hybrid junction, responding only the sum signal.
Layout of Mode coupler
Circular Waveguide of 4.2 mm diameter supports 2 modes TE11 and TM01 mode at 60 GHz. WR15 is used for folded hybrid junction. A taper section of 5 mm transform WR15 input section to circular Waveguide aperture.
With communication channel
Above design can be used only for tracking purpose and for one polarization communication. For dual polarized communication, a TE11 mode coupler is used that extract mode power of TE11 channel.