energy and transitions it from the sub-reflector to the waveguide. The waveguide transports RF energy out  of the antenna in electro-magnetic wave form.    Multiple types of secondary  reflectors  were considered including Gregorian, Displaced Axis, Cassegrain,  and “Hat” style feeds. One of the key challenges was finding a feed that did not have to be placed far from  the vertex of the parabolic reflector. If the feed was located far away from the parabola, it would be difficult  to stow it in the short 15-cm height. While the “hat” style feed provided the most RF gain, they also had to  be deployed  the furthest.  The  Cassegrain  secondary  reflector,  while  not providing  the best  gain  performance, would actually fit within the stowed volume as its geometry allowed the secondary reflector  to be deployed below the focal point. Therefore, the Cassegrain secondary reflector was selected.   As already  noted  the horn  takes  the RF energy  from  the sub-reflector  and  concentrates  it into  the  waveguide. While the horn is a very technical complex piece of RF design (discussed at length in [4]), from  a mechanical perspective, it is a highly toleranced conical shape.   The waveguide presented a greater mechanical challenge. The waveguide must be connected to the fixed  base of the antenna and also to the horn, which starts near the base of the canister, but then deploys to  the top of the canister. While “flexible” waveguides exist, they are actually mostly rigid and would not work.  Therefore, the only solution was to allow the horn to telescope around a fixed waveguide. This was a new  RF innovation demanding tight mechanical tolerances. It was also risky, as a number of RF engineers did  not think it would work. However, early prototyping of the concept with non-deploying hardware proved the  concept would work form an RF point of view.   Detailed Mechanical Design of KaPDA  Design of the Ribs  The design of the ribs is crucial for defining the antenna’s parabolic shape, and therefore was the first place  to start. Th e prior RF analysis indicated that 30 ribs were required to avoid significan t losses, due to the  “flat facets” which occur in between the ribs . To fit a 0.5-meter antenn a in a 15-cm-tall canister , the ribs had  to be folded in half when stowed . Therefore , each rib would have 2 hinges . While designs were also  investigated to fold the ribs three times, i t was determined this would result in an overly complex deployment  sequence.   The next step was to determine how to enable the ribs to achieve a surface accuracy of 0.4-mm RMS. This  was first accomplished by making the ribs deep, increasing the area moment of inertia, so they would be  stiff against  the tension  of the mesh.  To maximize  the amount  of depth  where  it is most  needed  (where  bending  moments  are the highest),  the rib was  deepest  at the base.  It can be observed  that the tip rib  steadily gets less deep the further it is from the center of the antenna as less moment is applied to it. This  design provides an approximately equal distribution of bending stress across the entire length and results  in less material where it is not required.   The second key features to achieve accurate deployment were the hinges. The depth of the ribs was carried  into the hinges to minimize the effect manufacturing tolerances could have on the hinges. The hinges had  a hard stop located 12.7 mm on the opposite side of the hinge pin, compared to the approximately 3 mm  which  the Aeneas  antenna  had  between  its hinge  pin and  the position  setting  set-screw.  This  one  architectural change increased the deployment accuracy of the hinges by at least 4 times. Further, using  flat hard  stops  instead  of setscrews  prevented  the position  from  changing  with  each  deployment  as set  screws can move. The tips of the setscrews would dig into the aluminum hinge on deployment, whereas a  flat hard stop prevents the deformation with better distribution of the load.   Rib Fabrication Process  As the hinges did not use setscrews, this also mean t that the hinges were not adjustable. Therefore , it was  very importan t that the ribs and hinges were placed relative t o each other with a high degree of accuracy.  While this could be achieved with extremely tight manufacturing tolerances, this was deemed too  © 2018 All Rights Reserved. Government sponsorship acknowledged. 

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