A Multiplexer Design Methodology for Small Multi-Frequency Antennas

Integrating low power RF amplifiers (hundreds of milliwatts to watts) with a small multi-frequency antenna (<λ/4) 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 paper 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 6.8 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.