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We propose a frequency-tunable waveguide antenna with a size-reduced aperture. Up to now, waveguide antennas with split-ring resonators have exhibited narrow-frequency bandwidths. To expand the operating frequency band, the resonant frequency of a split-ring resonator is tuned by adjusting the capacitance of the gap. A capacitor-loaded split-ring resonator is simulated and experimentally tested to validate its performance, and a varactor-loaded split-ring resonator is then investigated. The effective capacitance of the varactor-loaded split-ring resonator is changed by varying the external dc voltage. The tunable frequency range extends from 1.96 to 2.36 GHz (a fractional frequency range of 18.5%). The simulations and experiments yield good results for the reflection coefficient, radiation pattern, and gain. The proposed antenna retains the 70% aperture reduction of previously developed split-ring-resonator-loaded waveguide antennas.