Abstract: Base on the principles for TE₂₁-mode auto-tracking, the number of the coupling holes and coupling coefficient with the performance of the coupler are studied in details. Then the design principles and methods of the TE₂₁-mode coupler with multiple holes are presented, and accordingly a compact space-borne Ka-band circularly polarized TE₂₁-mode coupler for wideband is designed. The test data validates the design procedures.