A Multiplexer Design Methodology for Small Multifrequency Antennas

Integrating low power radio frequency (RF) amplifiers (hundreds of milliwatts to watts) with a small multifrequency antenna ( <tex-math notation="LaTeX">< λ /4 </tex-math>) can miniaturize small satellite communication systems. This requires impedance matching networks (MNs) with large impedance transformation ratios within a multiplexer to transform the low antenna resistance of a small antenna to the high optimum load line impedance of a low power RF amplifier. However, large impedance transformation ratios make MNs highly sensitive to unavoidable circuit variations, e.g., tolerances, causing mismatches that degrade performance beyond utility. This article presents a multiplexer design method that permits the predictable correction of such unavoidable impedance mismatches in the realized circuit. This is achieved by isolating each impedance transformation within the multiplexer configuration; this permits independent impedance match corrections that does not alter the remaining multiplexer performance. This condition is generated by introducing a new method to synthesize multiplexer branches by resonating ladder section impedance matching circuits. To validate our multiplexer design methodology, a triplexer circuit for a small tri-band 3U CubeSat antenna was designed, manufactured, and measured. The impedance match of each multiplexer branch was highly sensitive; Monte-Carlo simulation results show that resulting the anticipated variation in amplifier output power exceeds <tex-math notation="LaTeX">$6.8\,$ </tex-math>dB due to the variation in load line impedance. The impedance match of each multiplexer branch was successfully recovered to a realized impedance that yields less than 1 dB variation in transmit power from the target value, and the realized insertion loss was within 2 dB of the simulated insertion loss, confirming the practicability of this methodology.