A Novel Quad-Band Electrically Small Antenna With Low Q

Multi-band Electrically Small (ES) antennas are promising candidates for wireless sensing nodes and wearable devices, where multiple protocols are required along with a compact size. However, designing ES antennas for multi-band operation is challenging. While various methods for multi-band ES antenna designs have been explored, none have successfully achieved fully ES performance (ka < 1 for all bands) for a quad-band design, as the increasing number of bands often leads to significantly reduced or even vanished bandwidths. Herein, a novel method to design a quad-band fully ES antenna is presented. The proposed method is based on the even and odd modes of a split-ring antenna and uses radio frequency trap loading to achieve dual-band operation for each mode. The proposed antenna is ES for all bands. At each band, the radiating structure uses almost the whole available volume, so a good bandwidth is obtained for all four bands. The proposed antenna is fabricated, and the performance at each band is measured in a proper setup designed for multi-band ES antennas. The measured results validate the proposed method, with Q values only roughly 3–8 times Chu’s limit among four bands, which is considered good for ES antennas, especially given that the proposed antenna is cylindrical rather than spherical. As far as the authors know, the proposed antenna is the first quad-band fully ES antenna.