電磁最佳化實驗室EM Optimization LabYen-Sheng Research Group, Department of Electronic Engineering

A miniaturized dual-band circularly-polarized (CP) antenna that operates on 915-MHz and 2.45-GHz radiofrequency identification (RFID) bands is proposed for handheld readers. The dimensions of the proposed antenna are 85.0 × 85.0 × 1.6 mm³, whereas broad impedance bandwidths are obtained (871‒1297 MHz and 2.16‒2.80 GHz). The antenna consists of two pairs of cross dipoles fed by concentric ring-delay lines. The first pair is thin strips terminated with an arrowhead-shaped trace, providing CP resonance at 915 MHz. The second pair is composed of rectangular patches, creating CP resonance at 2.45 GHz. The distinct feature of the proposed antenna is twofold. First, the double cross dipoles are arranged at a 45° angle. This leads to a simple and compact configuration, avoiding the use of complex feeding networks or power dividers. Second, the frequency of minimum axial ratio (AR) in each band is tunable by controlling the ratio between the lengths of the arrowhead-shaped trace and the length of the rectangular patches. Accordingly, the proposed antenna achieves 3-dB AR bandwidths of 882‒936 MHz and 2.35‒2.58 GHz. Meanwhile, the radiation pattern exhibits a half-power beamwidth of about 110°, facilitating the detection of tags abroad with arbitrary orientations.