HALF-WAVE PARABOLIC REFLECTOR ANTENNA OPTIMIZATION

Transcription

1 HALF-WAVE PARABOLIC REFLECTOR ANTENNA OPTIMIZATION Parker Singletary, Carson Smith Advisor: Dr. Gregory J. Mazzaro Department of Electrical & Computer Engineering The Citadel, The Military College of South Carolina 171 Moultrie St., Charleston, SC September 2015

2 PROJECT GOALS Design, in simulation, a UHF antenna producing maximum poweron-target, directly in front of the antenna, at a given distance Minimal power reflection into feeder line Optimize antenna parameters by using FEKO, a method-ofmoments-based electromagnetic field solver, to vary its physical dimensions 2

3 PROJECT INSPIRATION U.S. Military Active Denial System (ADS) 95 GHz directed energy beam used for non-lethal crowd dispersal Heats molecules in the top layers of target s skin Interested in the high directivity aspects of the parabolic reflector antenna and wanted to learn more about its applications 3

4 OUR APPROACH Budget, fabrication, and instrumentation constraints led us to choose a half-wave-dipole reflector antenna Chose a frequency of 480 MHz so that a physical model could be built and tested on the our school s campus Chose a 0.3-meter dipole to make impedance real at center frequency (l dipole =0.48λ) In simulation, vary the radius and depth of the reflector using the grid solving method in FEKO to yield high directivity and gain while minimizing VSWR Verify simulated design with open-air measurements 4 30 cm dipole used for signal reception

5 GAIN OPTIMIZATION RESULTS Varied the parabola radius between cm and depth between cm in 100 simulations Gain peaked when the half-wave dipole was located at the focal parameter location of the dipole Larger reflectors yielded higher gain values Gain corresponding to run number Antenna parabola radius corresponding to run number Antenna parabola depth corresponding to run number run = simulation iteration 5

6 VSWR OPTIMIZATION RESULTS VSWR was lowest when the dipole was located at the focus of the antenna Larger reflectors yielded lower VSWR values VSWR corresponding to run number Antenna parabola radius corresponding to run number Antenna parabola depth corresponding to run number 6

7 SELECTED MODEL Although simulation run 95 yielded the most optimal results, we selected run 75, so the implemented antenna would not be too large to handle during testing In size range, chose model with high gain (8 db) and reasonable VSWR (2.5) Determined parabola s equation for fabrication Depth = 23 cm Radius = 33 cm Y= x 2 7

8 IMPLEMENTED MODEL Frame 2x2 and 2x4 pine Reflector Aluminum roofing flashing Dipole Stripped 14 gauge residential wire Dipole support ½ inch PVC Feedline Coaxial cable with BNC connector Reflector supports were placed according to the parabola s equation 8

9 Power Received (dbm) RADIATION PATTERN AND BEAMWIDTH TEST Connected implemented antenna to function generator producing a 1 mw constant sinusoid at 480 MHz Utilized second dipole as a receiver connected to a spectrum analyzer for measurements Placed second dipole at a distance of 50 feet from transmitting antenna (~20λ) and measured the average power received along a 180 swath at 5 intervals Determined that the antenna has a dB beamwidth Received Power vs. Angle Received Power vs. Angle Azimuth (degrees) 3 db down 9

10 Power Received (dbm) BANDWIDTH TEST Test performed at 90 azimuth Transmission frequency varied between 400 MHz and 500 MHz in 5 MHz intervals Test results showed the center frequency was closer to 450 MHz, much lower than designed 3-dB bandwidth of 75 MHz Received Power vs. Frequency Frequency (MHz) Received Power vs. Frequency 3dB down 10

11 Gain (dbm) COMPARISON OF RESULTS Experimental gain pattern tracked theoretical gain pattern within 1dBm Performed curve fitting in Excel with a 4 th -order polynomial to smooth measured data, for a clearer comparison against simulated results Testing methods likely induced much of the error (e.g. multipath, including ground bounce between Tx and Rx) Tests were performed on a large field as an anechoic chamber was unavailable Theoretical vs. Experimental Gain Angle (Degrees) Theoretical Gain Experimental Gain Poly. (Experimental Gain) 11

12 CONCLUSIONS We were able to successfully design, simulate, and build an optimized half-wave dipole reflector antenna Simulation results showed clear tradeoffs between VSWR and gain when manipulating reflector geometry Larger parabola radii resulted in more desirable gain and VSWR while depth variation yielded more contrasting output parameters, so parabola depth drove the design The antenna s operating frequency enables it be used for a variety of applications at low RF power, potentially in a secure point-to-point RF link or an RF device jammer 12

13 ACKNOWLEDGEMENTS Department of Electrical & Computer Engineering at The Citadel Dr. Gregory J. Mazzaro, Advisor Altair, FEKO software provider Applied EM, Inc. (Hampton, VA) Dr. C. J. Reddy Thomas G. Campbell, Sr. 13 Singletary, Smith, Mazzaro