In this paper, we propose a convenient fixed-frequency beam steering method, using a single patch antenna controlled by only one electronically tunable component. The antenna is based on coupled-mode patch antenna (CMPA)  that is capable to scan the beam as the function of frequency. A ground-etched slot loaded with one varactor diode is tuned to be capacitive, resonant, or inductive. In order to test broader tuning range, two kinds of varactors with the ranges of 9.24 pF-1.77 pF and 2.67 pF-0.63 pF are implemented respectively. By analyzing how the loaded slot affects the cavity modes and fields, we demonstrate how the voltage bias tunes the frequency responses and steers beam of the antenna. Perturbed by the loaded slot, the frequency response of the antenna shifts from center frequency of 2.35 GHz with the bandwidth of 4.26% down to the band centered at 2.3 GHz with the bandwidth of 4.35%. The maximum scanning range is realized at around 2.29 GHz where the measured main beam continuously scans from -34° to +32° when the varactor with lower tuning range is used and biased. Meanwhile, the main beam of 2.35 GHz scans from +32° to +54° when the higher-range varactor is biased. The proposed single-element antenna is able to maintain high gain and efficiency that are comparable to a regular patch antenna with same size and substrate.
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