Given the relatively light commercial usage of these bands on Earth itself, there is sparse availability of amplifiers in these bands and no commercially offered, space-qualified parts. Therefore, custom solutions will need to be in consideration for sub-contracting in Phases II and III.
There are a few choices to consider for power amplifiers that have an impact on the rest of the satellite communication link design. The choices will be investigated with potential subcontractor partners during the Phase I timeline.
Solid-State
With GaAs and InP technology, the power output of high electron mobility (HEMT) power amplifiers is limited from +10 to +20 dBm (10 to 100 mW). However, watt-level GaN HEMT amplifiers at 94 GHz have been demonstration with up to 20% Power-added efficiency, and half-watt commercial versions are even available now. Previously, it was impractical to combine up to +40 dBm levels (10W) at V- and W-band frequencies due to the physical size and losses associated with combining 100's of amplifiers, but now only 10's of amplifiers are required for microstrip or waveguide combining.
Figure 1: Radial Waveguide-combined Solid-State Power Amplifier Module
The benefits of solid state are low voltage levels (10-20V), small size, and wider bandwidth with respect to tube amplifiers. The disadvantages are lower power levels and lower efficiencies. In the future, phased arrays and spatial combining approaches will bring additional benefits to the solid-state solution at mm-wave frequencies.
Travelling Wave Tube
Travelling wave tubes have been the mainstay of satellite communication systems from L-band to Ka-band for decades. Under vacuum, an electron beam is generated from a heated cathode, helix TWTs couple the RF in a helical geometry to slow down the RF propagation to the speed of the electron beam. The electron beam bunches and modulates the RF resulting in gain. Coupled cavity TWTs eliminate the helix and provide a similar slow wave effect with a series of radially arranged, coupled, resonant cavities. The benefits are wide bandwidth due to the slow wave structure as opposed to the narrowband operation of other electron gun type amplifiers such as Klystrons. The major downside to these amplifiers are the 10's of kVs required to bias the cathode to form the electron beam.
Figure 2: Thales TWT Module
A few vendors dominate the space-qualified TWT market: Thales (Europe), L3Com (US) and CPI (US). While solutions from these vendors exist in these frequency ranges, most have been designed for niche applications where pulsed peak output power needs of various climate science missions were more important than the broadband, average power requirements of an experiment such as this. The single exception is the reported coupled cavity TWT with 75W of output power at 59-64 GHz by Wilson et al. Although this TWT was operating at a 0.5% pulsed duty cycle, CW operation TWTs with at least 10W of output power in the V- and W-band can be expected given 20 years of TWT design improvements since this paper was published.
Further investigation will be required in Phase I to see which are the preferred technology choices for the power amplifier. It is likely that the power amplifier will be one of the more expensive components of the system due to the relative lack of commercial options.
References
Solid-State
Brown et al., W-band GaN power amplifier MMICs, IEEE IMS 2011
Micovic et al., 92-96 GHz GaN power amplifiers, IEEE IMS 2012
Schellenberg et al., W-Band, 5W Solid-State Power Amplifier/Combiner, IEEE IMS 2010
HRL W-band GaN PAs
DeLisio et al., Quasi-optical and spatial power combining, IEEE MTT 2002
Khan, P. et al., A Ka-Band Wide-Bandgap Solid-State Power Amplifier: Architecture Performance Estimates, IPN Progress Report 2005
TWTs
CPI TWTs
L-3Com TWTs
Thales TWTs
Wilson, J.D. et al, A high-efficiency 59- to 64-GHz TWT for intersatellite communications, IEEE IEDM 1991