Transmitter Design Considerations for a Small Satellite Multi-Frequency Antenna

This paper presents a transmitter design strategy for integrating several high efficiency Class-E RF amplifiers for small satellite subsystems operating at different frequencies to a single antenna. An electrically small monopole/dipole type antenna is preferred for small satellites. However, integrating Class-E RF amplifiers to the low antenna resistance and high capacitive reactance of this class of antenna presents two challenges. First, matching networks with large impedance transformation ratios that are impractically sensitive to circuit and antenna port variations are necessary to achieve the high optimum load resistance of conventional low power Class-E design solutions. Second, a low antenna resistance results in comparatively significant real circuit and components resistive losses which means that the Class-E amplifier efficiency becomes a strong function of the dissipative losses as well as the Class-E waveforms. We propose a transmitter design that is based on a novel Class-E amplifier design strategy, and a multiplexer design technique that builds impedance matching into each multiplexer branch that can be adjusted without altering the multiplexers filtering characteristics. We reduce the impedance transformation ratio required by introducing a Class-E amplifier design strategy that tailors the optimum fundamental load impedance of the amplifier to the antenna impedance by varying three Class-E design parameters which control the transistor voltage and current waveforms. To realize high power added efficiency, we propose utilizing the adjustable impedance matching within each multiplexer branch to predictably and independently optimize each Class-E amplifiers load impedance for real circuit and components in the multiplexer configuration without causing an unwanted deviation in the performance of the remaining Class-E amplifiers multiplexed to the common antenna port. This is achieved because the independence of each impedance match to the multiplexers filtering characteristics ensures that optimizing each amplifiers load impedance does not alter how each amplifier loads the common antenna port. To demonstrate the transmitter design strategy, an example transmitter design operating at 144.2 MHz, 432.6 MHz, and 1297.8MHz was synthesized and simulated for a tri-band 3U CubeSat antenna which has been designed for Ionospheric remote sensing. The simulation results show that the transmitter design strategy can achieve a power added efficiency and power gain of greater than 54.9% and 13.1 dB respectively for an antenna exhibiting a resistance as low as 1.07 Omega.