A QMSIW Cavity-Backed Self-Diplexing Antenna With Tunable Resonant Frequency Using CSRR Slot

A compact self-diplexing antenna is presented here using substrate-integrated waveguide (SIW) technology. The antenna operates at 2.45 GHz for the ISM band and 3.5 GHz for the WiMAX band. Firstly, the antenna's size is reduced by around 75% by employing the quarter-mode SIW cavity, and then further size reduction is achieved by inserting the circular complementary split-ring resonator (CSRR) slot on each QMSIW cavity. The antenna simultaneously radiates at two distinct resonant frequencies when it is excited by a microstrip feedline at the individual port, offering isolation better than 33 dB in both frequency bands. Notably, the proposed antenna's uniqueness lies in its tunability, and the lower frequency band can be tuned in a range of 2.45–2.75 GHz, while the upper-frequency band from 3.5 to 3.8 GHz by rotating the CSRR slot at angles of 0°–180°. This rotation advantageously aids miniaturization and improves port decoupling. Moreover, both the operating frequencies can be tuned independently by varying the length of the cavity, radius, and angle of the CSRR slot. After experimental testing, the proposed design demonstrates exceptional alignment with the simulated results, offering a peak gain of 3.5 dBi in the lower-frequency band and 3.15 dBi in the upper-frequency band, with a total efficiency exceeding 68%.