A Compact Dynamic Antenna for Physical Layer Wireless Security

We propose a novel omnidirectional antenna design incorporating directional modulation for secure narrow planar information transmission. The proposed antenna features a compact size and stable omnidirectional radiation performance by employing two tightly spaced, printed meander line monopole antennas, acting as a single radiating element. To achieve a narrow information secure region, the proposed antenna is fed by differential power excitation of two ports with real-time dynamic switching. This leads to phase pattern modulation only along the electrical polarization, resulting in directionally confined information recoverable region in the E-plane, while maintaining highly constant or static omnidirectional H-plane pattern, inducing a $360^\circ$ information recoverable region. The dynamic antenna is designed and fabricated on a single layer of Rogers RO4350B which provides a miniaturized planar size of $0.36 \times 0.5 , \lambda_0^2$ at 2.7 GHz and easy integration. To validate the wireless communication performance, the fabricated antenna is directly fed with a 10 dB power ratio by a radio frequency (RF) switching system and evaluated for 16-QAM and 256-QAM transmission in a high signal-to-noise ratio (SNR) environment. Experimental results demonstrate that for 16-QAM transmission, a narrow E-plane information beam (IB) of approximately $34^\circ$ and omnidirectional H-plane IB are obtained, and a narrower E-plane IB is achieved around $15^\circ$ for 256-QAM. These results confirm that the proposed antenna offers a simple yet effective approach to enhance planar physical information security with a compact dynamic antenna system.