35   Figure 5: Deployment Band Breadboard Fixture 

Reflective Material   The reflective material for the antenna was a majo r design driver and yielded a novel design approach.  The low PIM requirement, required surface accuracy,  survival temperature, radiation environment, and  stowed packaging all factored into the material selection trade space. PIM is generated by loose or “dirty”  metal to metal contacts in the RF field. A low-PIM reflector required that any metal to metal contacts would need to be avoided if possible and closely analyzed where necessary.    There are many proven deployable reflector designs which use a woven metallic mesh material and achieve much tighter surface accuracies. In this type of design, the material must be managed very  carefully to maintain the tension of the mesh in or der to maintain low PIM. This is accomplished through  an intricate design adjusted through a labor intensive process to meet RF performance requirements. The material is also expensive and is not readily available for purchase. For TacSat-4, the cost and schedule ROMs showed that this design approach would not be feasible, nor was it required to meet the looser  UHF tolerances for surface accuracy.  The goal of the TacSat-4 antenna design was to take  advantage of the very loose RMS requirements in  order to lower the complexity, and in turn cost of the antenna. A gore approach was selected using several smaller reflector pieces that were joined together using capacitive coupling into a single reflector while avoiding any metal to metal contacts. The novel approach used Kapton ®-Copper flex circuit material.  Each gore is made of a sandwich material with Kapt on on the outside and a soft copper grid in the middle.  The copper thickness was selected so that it was three skin depths over the frequency range of the  antenna; so the gores acted as a RF reflector. It wa s important to minimize the amount of copper used in  the design in order to achieve the required mechani cal performance of the gore material as well as  minimize the mass of the reflector. The flex ci rcuit material has been used in space applications  previously but not in this manner. To gain confidence and qualify the material, several small material samples were thermally cycled in vacuum and exposed to expected radiation levels. Additionally, Kapton ®  material was fit on a scaled antenna model to evaluate  the behavior of the stowing and deploying process  necessary for the deployable reflector. As a result of  this testing, small through-holes were added to the  gore design with the intent to better allow out of plane bending and relieve stress concentrations when stowing the reflector. It was thoug ht that the holes would also hav e the secondary benefits of both  reducing the part mass and reducing air drag during depl oyments in air. The grid selection and hole size  were limited by RF requirements on the reflector. T he EDU gore design is shown in Figure 6. The part  size was driven by available raw material and manufactu rability of the part which in turn drove the number  of deployable ribs. Flex circuits are readily avail able in standard panel sizes, but not in the length and  NASA/CP-2010-216272