Millimeter-scale close-range displacement measurement is widely used in fields such as precision manufacturing, medical monitoring, and aerospace. Owing to its non-contact operation, strong immunity to interference, and moderate spatial resolution, microwave sensing has become an effective technique for such tasks. Conventional loop probe antennas, however, suffer from multi-mode resonances, and achieving a desirable compromise between antenna size and performance remains challenging. To tackle these issues, this paper proposes a Four-Element sector truncated structure based on mode suppression, together with a nested-configuration transceiver antenna. This design effectively suppresses multi-frequency resonances, enabling efficient radiation at a single target frequency of 24.01 GHz while enhancing receive-signal purity and anti-interference capability. Simulations demonstrate that, while maintaining high gain, the antenna size is reduced by 65.385% typical loop probe antennas reported in the literature; the operational bandwidth is below 0.1%; the reflection coefficient at the target frequency reaches -64.001 dB; and the center-frequency gain attains 11.028 dBi. Experimental measurements closely align with the simulations: the fabricated prototype exhibits a reflection coefficient of -42.807 dB at 24.14 GHz, an absolute bandwidth of 0.027 GHz, and the relative bandwidth of 0.004% is far below the narrowband requirement of 1%. Therefore, the developed antenna is not only compact and structurally simple, but also exhibits robust performance, meeting the requirements of millimeter-scale close-range microwave displacement measurement.