Performance of a 2-layer radial hole effective media lens
The outer polyethylene layer of a 8λ0 radius 2-layer polyethylene/cross-linked polystyrene lens was implemented as a radial hole effective media that allows the entire lens to be milled from a single billet of cross-linked polystyrene, simplifying construction by avoiding layer fitting problems. Full wave simulations in a commercially available simulator showed that increasing the number of radial holes to 3000 gave peak directivity within 0.3dB of the original cross-linked polystyrene/polyethylene lens. Both Particle Swarm Optimization and icosahedron based methods were trialed for distributing the holes evenly over the sphere. The latter method produced symmetrical meshes allowing symmetry planes to be used to minimize computational burden, but was bound to the numbers of holes produced by successive triangular face subdivision iterations.
GBS/Milstar airborne RX and TX antenna
In 1999 an airborne Global Broadcast Service (GBS) and Milstar receive antenna was evaluated and successfully demonstrated in flight, atop an AFRL/IFG, C-135 SATCOM test aircraft. This Luneburg lens antenna received 20 GHz left hand circular polarization (LHCP) from a GBS terminal onboard the UFO-9 satellite and 20 GHz right hand circular polarization (RHCP) from a Milstar satellite. Phase II of this antenna adds a 44 GHz TX capability. This fully mechanically scanned antenna will still provide, /spl plusmn/80 degree coverage while rotating 360 degrees in azimuth. The hemispherical lens height was increased from 3 inches to 4 inches to increase gain and the lenses were redesigned with more shells to increase the efficiency for transmission at 44 GHz. A new feeder network was designed with 20 and 44 GHz horns to allow reception of 23.5 Mbps from the GBS terminal on the UFO satellites and to provide a medium data rate (MDR) transmit/receive (TX/RX) capability with the planned Milstar IV satellite. The 20/44 GHz antenna components, including Luneburg lens improvements and temperature test results are presented.
Using Fuzzy-Delphi technique to determine the concession period in BOT projects
BOT projects are affected by several risks and uncertainties. One of the most important issues in this type of contract is to determine the length of concession period considering these uncertainties. The previous studies have developed several models to determine the concession period. However, the uncertainties are not taking into account in most of the previous works. Moreover, none of the previous researches has the capability to aggregate the opinions of different experts being involved in a project regarding the values of the uncertain input parameters. In this research, a Fuzzy-Delphi technique is implemented to determine the length of concession period considering uncertainties. Using the proposed Fuzzy-Delphi technique, the values of different uncertain factors affecting a BOT project is determined considering opinions of a group of experts. The NPV value is calculated considering the resulted aggregated values of uncertain input parameters. Finally, the concession period is determined using fuzzy approach. A case study is conducted to evaluate the performance of the proposed methodology. It is shown that the concession period is determined more effectively using the proposed approach.