Simulations Key to Non-Lethal Microwave Weapon

Dr. Robert A. Koslover - SARA, Inc., Cyprus, CA

Scientific Applications and Research Associates (SARA), Inc. of Cyprus California is using COMSOL Multiphysics in a funded project to develop an antenna that supports directed transmissions of high-power microwaves (hpms). With power levels up to 100 megawatts, the hpm signals are so potent-enough to power a small town-they actually degrade electronic circuits in their path. Even co-axial cables cannot withstand the large volumes of power.

A device that aims hpms at a target

SARA develops cutting-edge, multidisciplinary technologies in areas with high scientific and/or market potential and transforms these new technologies into successful products and services for clients in business, industry, and government. The folks at SARA already know a device that aims HPMs at a target is an important contribution — a powerful but non-lethal weapon that blows out the defense, vehicle, and communications systems of a national enemies and fleeing criminals. The method is non-lethal because HPMs emit very low levels of radiation, well within the range assumed safe to humans by health experts. Since the co-axial cables that conventionally transmit microwaves cannot be used with HPMs, SARA scientists are using COMSOL Multiphyics to develop new pyramidal horn waveguides that will solve their problem.

Criteria for a good design

COMSOL Multiphyics simulations (Fig. 1) enabled Dr. Robert Koslover, a senior scientist at SARA, to design and fine tune the feed horn for the HPM antenna. A feed horn is a hornshaped medium for propagating electromagnetic waves, a "waveguide" that conveys electromagnetic signals between a transceiver and reflector. At a glance, the pyramidal feed horn from the HPM antenna (Fig 2) would be mistaken for a commercial L-band (1.1-1.7 GHz) microwave feed horn except for the thick dielectric window on the face of it.

The dielectric window came about because the strong electric fields associated with the HPMs actually break down air molecules and produce an electric arc. Having the feed horn enclosed by a window allows the designers either to remove air from within the horn or fill it with a gas that slows down electrons and eliminates the arc.

Another function of the window is to act as a conduit for the microwave energy; after all, the window is not present in other feed horn designs. It follows that the energy should pass through the window with minimal attenuation and distortion. Dr. Koslover explains that the best way to determine which window geometries work best for the HPM antenna — thickness, shape and curvature — is by screening the designs for those with an output radiation pattern that yields a small voltage standing wave ratio (VSWR). The VSWR is the ratio of the maximum voltage to minimum voltage.

A window giving VSWR close to 1:1 is optimal because it indicates low energy loss; i.e., a minimum of energy reflects off the window, returns back into the horn, and burns up as heat rather than proceeding to the target. A good design also minimizes the potential for air breakdown (and arc formation) just beyond the window.

Design testing with COMSOL Multiphyics

Dr. Koslover designed and tested a range of window geometries and dielectric materials for the pyramidal-horn antenna using a computer simulation approach with COMSOL Multiphyics. An advantage to this approach is that working with simulations in addition to physical testing not only minimizes the time, material and laboratory costs of prototyping, but it also adds new insights about the physical processes. Another distinct advantage to his method is his design criteria-calculating the VSWR offers an objective means to rank design efficiency.

In a typical problem set up (Fig 1), Dr. Koslover simulates HPM propagation throughout the waveguide for each window geometry. Absorbing, PMC (perfect magnetic conductor), and PEC (perfect electric conductor) boundary conditions are applied as appropriate. The VSWR is calculated as a postprocessing step for each frequency simulated (Fig. 3). During calibration, the VSWR results are compared with experimental data. Otherwise, the VSWR is used to evaluate new window geometries. Why he chose COMSOL Multiphyics to zero in on the best pyramidal horn design was easy for Dr. Koslover to explain: meshing and accuracy.

"As to accuracy, the VSWR estimates from COMSOL Multiphyics simulation are a near perfect match to the experimental data"

With the finite-element method that COMSOL uses, he can vary the size of the mesh, inserting tiny elements in critical zones, near the window for example, and switch to a coarser mesh where properties and conditions are relatively uniform, beyond the horn. With the FDTD (finite-difference time domain) codes he also considered for this job, getting high accuracy necessitated extending the fine mesh he used near the window throughout the entire model. This would have amounted to prohibitively long run times and enormous computational cost. Given that his COMSOL model consists of 89,000 elements or 115,000 degrees of freedom (Fig 1), it runs in reasonable time on his personal computer - a 1.7-GHz P4 machine with 1 Gbyte of RAM.

As to accuracy, the VSWR estimates from COMSOL Multiphyics simulation are a near perfect match to the experimental data (Fig 3). The VSWR estimates from the FDTD code, however, are relatively poor. With the good fit for the one geometry, Koslover gained confidence that he could use COMSOL Multiphyics for continued modeling and yield reliable results for other designs. In fact, after a number of simulations he arrived at a design of particular promise, one with a low VSWR, a ratio of 1.2:1, which he considers to be especially good.

The HPM antenna is adding to what already is an impressive list of non-lethal methods that SARA is developing to disable unwanted threats. Consider the possibility of a sonic fire hose, multisensory grenades, or a vortex launcher. For more information on SARA and its work with nonlethal weapons and HPMs, visit www.sara.com.