The antenna is composed of two main deployable structural components that help it achieve the large packing factor necessary to fit within the small volume of the CubeSat. The primary component of the antenna array is a tension stiffened truss, which is preloaded using a large tape spring. The truss bays are formed from solid discs connected by thin Kevlar thread. The Kevlar threads are set up in a hexapod configuration, and are fully tensioned and preloaded from the force of the tape spring, which runs through the center of the truss. The truss gets its overall stiffness from the properties and configuration of these Kevlar wires.
There are 21 elements on the antenna corresponding to the full desired frequency range of 30 to 300 MHz. The assembly, including cable spooler and motor, fits within a 2-U CubeSat. This is achieved by keeping the dipole discs thin, and by stacking them back-to-back. Additionally, the tape spring boom rolls up on itself to provide excellent stowed volume.
When stowed, the dipoles are sandwiched inside the fixed tube of the CubeSat, and cannot deploy until they are forced outward by the tape spring. To ensure the hexapod thread does not get tangled or frayed during antenna stowing and deployment, a scheme was devised where each individual wire is wrapped in a figure-eight configuration around two posts. The wire is laid on top of itself in subsequent wraps around the posts, with little chance of forming a tangle or knot. When the discs are pulled apart, there is minimal force required to dislodge the wire from the posts. This thread management scheme is effective; however, it does have drawbacks in that it is very tedious and requires several sets of hands working simultaneously. The posts also serve a dual purpose and provide a torsional locking feature between adjacent discs.
This work was done by Mark W. Thomson, Vine M. Bach, Phillip E. Walkemeyer, Daniel L. Kahn, Andrew Romero-Wolf, and Samuel C. Bradford of Caltech for NASA’s Jet Propulsion Laboratory. NPO-49107