The TE₂₁ tracking mode coupler is usually used in satellite auto tracking feed antennas. A mode coupler comprises a power dividing section and a mode converting section. The power divider splits the input signal into two parts, which are coupled to an overmoded circular waveguide through longitudinally slots. This paper presents the effects of shape and size of slots on sidewall coupling. A prototype x band mode converter with rounded-end shaped slots is built and tested. A back-to-back transmission measurement shows good agreement with simulation results. The optimum size and shape of side wall slots has 20%, 3dB transmission bandwidth.