Antenna Miniaturization Using Slow Wave Enhancement Factor from Loaded Transmission Line Models

Miniaturization of slow wave antennas exploiting the slow wave enhancement factor is presented. The printed antennas are periodically loaded with shunt capacitors to slow down the guided wave in the structures. In this paper, the loaded unit cell of the equivalent transmission line model is utilized to extract the slow wave enhancement factor, the ratio of the loaded to the unloaded propagation constants of the wave in the antennas. From this model, the slow wave enhancement factor of a loaded antenna agrees very well with the miniaturization factor, and therefore load parameters in the circuit model can be readily obtained when a specific size reduction is attempted. This claim was substantiated by demonstrating two small radiators, a high-frequency (HF) slot-loop antenna and a planar inverted F antenna (PIFA), to achieve the desired size reductions. Experimental results show that both of the antennas demonstrate greater than ten-times size reduction from their unloaded counterparts at the expense of the degraded gains and impedance bandwidths. Specifically, the loaded slot loop presents the predicted gain and measured bandwidth on the order of -34.9 dBi and 0.38% for VSWR ≤ 2, respectively. Therefore, a matching network derived from filter design techniques is proposed to increase the antenna bandwidth so that a measured fractional bandwidth of 1.78% is achieved. The slot loop combined with the impedance matching circuit occupies a footprint size of 0.031λ₀ × 0.017 λ₀ at the operating frequency. On the other hand, the measured radiation gain and bandwidth of the loaded PIFA are reduced to -22.6 dBi and 0.15% for VSWR ≤ 2, respectively, with a footprint of 0.013 λ₀ × 0.018 λ₀ at the operating frequency.