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L.Garde Deployable Antennas have been designed, fabricated, tested, and flight demonstrated. These unique structures offer high packaging efficiency and performance. L.Garde antennas offer the potential to package very large apertures in very small volumes. L.Garde has demonstrated numerous examples of these antennas and their capabilities throughout its long history.
The inflatable antenna experiment or IAE was a NASA-JPL funded experiment project. The main objective of the experiment was to validate by on-orbit experiment, the deployment of a very large inflatable space structure. The structure consisted of a 14 meter diameter inflatable parabolic dish antenna with an inflatable torus perimeter support. Three inflatable struts, each about 33 meters long position the dish antenna away from the Spartan 207 spacecraft. The antenna surface is made of half-mil thick aluminized Mylar with a clear canopy forming a lenticular-shaped inflatable structure. The IAE was launched from the Space Shuttle STS-77 in May 20, 1996. The on-orbit experiment was at space shuttle altitude of about 200 kilometers. The experiment started with the Canadarm placing the Spartan 207-IAE experiment package off-board the space shuttle. The shuttle then moved a safe distance away but station-kept with the experiment package. After a few minutes, the command was given to initiate deployment: the IAE canister opened followed by the inflation of the three struts, torus, and the lenticular dish structure.
The same technology used on the Synthetic Aperture Radar (SAR) is applicable to a different antenna approach for the same purpose – a slotted waveguide using membranes. Initial system studies showed that the weight of these systems is similar, with the distributed SAR weighing about 1.2 kg/m2, and the slotted wave guide weighing about 1.6 kg/m2. However the slotted waveguide can package in a smaller volume, have a greater sensitivity, and for radars, allow higher power. The deployable waveguide program was funded by NASA Langley. The configuration investigated was a slotted waveguide array antenna (L-band) constructed of a metalized thin membrane material. The concept lends itself to compact packaging for launch and can be tensioned on an inflatable, rigidizable support structure which would deploy on orbit. One major advantage of the planar configuration is that there is no need for fabricating a doubly curved surface (such as for a reflector antenna) from membrane material.
The 3m diameter LDP inflatable antenna was the first low film stress antenna L.Garde built. It had an F/D of unity and had a surface accuracy of about 1.4 mm rms. This was higher than anticipated and was found to ge due to an error in the gore-cutting process. Gain and pattern measurements were carried out in an anechoic chamber using a near-field scanning technique at the Lockheed-Martin Company laboratory in Denver in 1996. Frequencies in the L, S, C, and X-bands were used. In order to expedite the measurement process over this frequency range, a wideband, circularly polarized feed was used, having low gain and a very broad beam compared to the 56° angle subtended by the reflector at the focal point. Consequently, feed spillover and ohmic loss in the feed line were both high. However these effects may be ignored since, in the near-field method, the actual measured aperture field is integrated over the whole aperture to obtain the patterns and then the on-axis directivity. For convenience, this is simply called the gain. The measured gain followed the general trend of the Ruze curve and performed well up to the X-band.
The IRD was a DoD program to develop large aperture, high precision, low mass, low stowed volume inflatable reflectors and rigidizable structures. The aperture diameter of the parabolic antenna designed and built was 7m with an F/D of unity. Its surface accuracy was measured to be 1.3 mm rms. The antenna film material used was 0.5 mil thick aluminized Mylar. This antenna reflector was one of the first low film stress reflectors (100 psi film stress) built by L.Garde. The lower the material film stress, the lower the mass of the support structure. Rigidization of the torus and struts was achieved with the use of a water-based gel material. Other rigidization methods that could be used are UV rigidization and Sub-Tg rigidization. In the Sub-Tg, the material is allowed to passively cool below its glass transition temperature ensuring a rigid support structure.
JPL funded this program to develop a 1.5m x 3.3m rigidizable structure triple-plane synthetic aperture radar. The as-built SAR had an areal density of 2 kg/m2 with a plane separation tolerance of better than 0.75 mm between layers with flatness of ±0.28 mm rms. The three panel membranes were tensioned with edge cords at the “catenary” edges resulting in an isotensoid stress state in the membrane. JPL provided the antenna RF design and the etched thin membranes. The SAR operated in the L-band.
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