16.810 IAP 2007 Engineering Design and Rapid Prototyping January 28, 2007 Version 1.3 Deliverable C CAD Model and Performance Analysis System: MIT Space Elevator Team Beamed Ribbon Climber Component or Subsystem: Microwave Beaming System Team Name: The Rather-Naïve-About-Optics Team Team Member 1: Ethan Huwe Team Member 2: Arka P. Dhar Team Member 3: Chris Mandy 1. Electromagnetic Wave Circuit This section deals with models of the trajectory of the microwaves from their source to the climber. Field modeling was performed using Comsol 3.3 acquired with a 14-day trial student license graciously provided by Comsol Multiphysics. Pre-Optimization decisions: As the number of constraints on the system were rather limited, the number of options for our design was large enough that we couldn’t address all possibilities within the time available. We chose for instance not to attempt to emit the electromagnetic radiation through helicoidal antennae simply because we didn’t have the necessary knowledge or the time to acquire it. Instead our system relies on magnetrons to emit electromagnetic radiation. We also chose not to transmit the radiation through coaxial cables as it would require two conversions, radiation to electricity out of the magnetron, and electricity to radiation on the emitting side of the beam, and we don’t have the knowledge to design this in a meaningfully efficient way. We have omitted from the CAD model of our design both the wave combiners and the polarizing scheme. This is mainly due to our unfamiliarity with Comsol and the large amount of work needed to add such intricate items with numerous boundaries. To achieve sufficiently powerful emitted radiation, 12 magnetrons would have to be assembled with beam combiners (WR-187 TE-1 4-port directional couplers) in two sets of 6, then connected to a WR-187 TE-1 single box quadrature branch line coupler, combining the two beams into one with a 90 degree phase difference, achieving polarized light. It is this beam which we have set as boundary condition for our CAD model in Comsol. Other possibilities for polarization include Fresnel or Moony rhombs 1and quarter-wave plate. Since quadrature branch line couplers seem to have poser losses on the order of 4%, we decided we wouldn’t be able to achieve such a value by designing our own optical crystal and opted for buying couplers. Optimization: As waveguides of a similar price can be obtained for any frequency and power range, and since achieving the required output (in our CAD model) resulted in similar costs in magnetrons, we’ve chosen to keep the specification of the one magnetron the MITSET team already has, and propagate 5.801GHz (wavelength of 5.17 cm) microwave radiation. The optimal standard waveguide size for this frequency is the WR-187 standard (‘G Band’) which has dimensions 1.872’’x0.872’’ (units always in inches for some reason) and a range of 3.93GHz to 5.85GHz. The microwave emitting horn was designed using Paul Wade’s mysteriously named Hdl_3b4 program, resulting in the following dimensions: - Axial length: 64.6mm - H-plane aperture: 135.9mm - E-plane aperture:100.6mm - Uncorrected Horn Gain: 15.8 dBi - H-Plane Phase Centre: 1.41λ - E-Plane Phase Centre: 1.31λ This horn had highest power flux, considering its interaction with the parabolic dish: decreasing the gain will result in a less efficient horn, increasing it will result in a larger horn that causes a larger amount of radiation to be reflected off it after being reemitted by the parabolic dish, and resulting in a diminished net overall efficiency. Modelling these in Comsol, we tweaked the input power, H-bend lengths in the wave guides to achieve the required power output of 127W/m2 (to power 2m-diameter rectennae). It was also found that adding a curved dielectric tubing behind the horn to guide the waves upwards increases the output power in a radius of 15 wavelengths around the horn by 30%. The shape of the curve does not significantly affect this value, so long as it is convex. The final output efficiency is 8% according to Comsol. This means that our system emits a power flux of 130W/m2 outside the parabolic dish. This value estimate is certainly not final (see section 3). Our initial back of the envelope calculation based on the link equation: With C/P the Received to Emitted Power flux ratio, Ll the line loss, La the transmission power loss, Gp the parabolic antenna gain, Gh the horn antenna gain, ηp and ηh their respective efficiency, D the rectenna diameter and H the horn antenna footprint on the parabolic antenna; yields an efficiency of 14%. 2. Structural Support a. Waveguides and Horn 2At first we decided to have a variable height adjustable horn holding stand, so that if required we can adjust the height of the horn to achieve better microwave collimation at the rectenna plates on the climber. But then analyzing the situation, we came to the conclusion, that such a feature is not that important as we can achieve the best result only when the horn is hold at the focal point. The focal point of the parabolic dish was found to be around 40in above and 60in from the end of the antenna. It was challenging to make the stand stable, because whatever structure we use, one end always bent forward because of the unstable weight distribution. (Ref: CAD modeling of the stand) Our remedy this problem was that to increase the height of the stand, so that even after it getting bent the horn will be hold at the focal point. Finally the dimensions that we got were Height (45in) and Length (63in). It proves to be pretty stable. More work can be done to improve the structural strength so that it can withstand wind produced vibrations etc. 3. Design notes As the system is quite complex, we would like to add a few notes concerning further modeling of the system and testing, for the benefit of the MITSET team. Time constraints prevented the team from completing the following 4 steps we feel are absolutely necessary prior to purchasing any parts and testing: • Thermal modeling and heat dissipation analysis. The software Comsol seems to provide appropriate tools for such an analysis, but a 3D model of the electromagnetic radiation path is necessary for accurate modeling to be performed. This is mainly due to the nature of the waveguides and horn which lack symmetry in the z-direction: the horn is “thicker than wide” in the z direction