The HPM antenna, still in development, consists of two reflecting surfaces, a feed-horn, and resonant window (bottom). It radiates microwave pulses that disrupt electronics to disable vehicles and weapons.

The HPM antenna, still in development, consists of two reflecting surfaces, a feed-horn, and resonant window (bottom). It radiates microwave pulses that disrupt electronics to disable vehicles and weapons.
The HPM antenna, still in development, consists of two reflecting surfaces, a feed-horn, and resonant window (bottom). It radiates microwave pulses that disrupt electronics to disable vehicles and weapons.
 
The Femlab model of the waveguide shows the horn and window. Driving the horn and window at an off-resonant frequency produces standing waves in the long waveguide. Simulations run on a 1.7-GHz P4-based computer with 1 Gbyte of RAM. The model has 89,000 elements and 115,000 degrees of freedom.
The Femlab model of the waveguide shows the horn and window. Driving the horn and window at an off-resonant frequency produces standing waves in the long waveguide. Simulations run on a 1.7-GHz P4-based computer with 1 Gbyte of RAM. The model has 89,000 elements and 115,000 degrees of freedom.
 
A comparison of the measured and modeled values for the voltage standing wave ratio on the HPM antenna shows a good fit. Researchers are confident they can successfully model other window geometries.
A comparison of the measured and modeled values for the voltage standing wave ratio on the HPM antenna shows a good fit. Researchers are confident they can successfully model other window geometries.

Scientists at Sara Inc., Cypress, Calif. (www.sara.com), have developed a weapon system that transmits microwaves at power levels reaching 100 MW. They hope the system will be able to disrupt electronic circuits it's aimed at, thereby disabling other weapons, vehicles, and communications systems. The system transmits only short pulses of microwave energy and, thus, should leave humans unaffected. The development team is using Femlab, a physics-modeling package from Comsol Inc., Burlington, Mass. (www.comsol.com), to simulate the antenna's operation.

Robert Koslover, a senior scientist at Sara Inc., says simulations with a finite-difference time-domain method (FDTD) did not produce good agreement between simulation and experimental results. In addition, all elements or cells in the FDTD method must be the same size. So if one area needs a fine mesh, the whole model must use the same density. This means more computer RAM and computation time.

The finite-element method, however, allows using a fine mesh for increased accuracy in key areas and a coarser mesh elsewhere. This lets a desktop computer perform simulations in less time than other methods. Femlab results have produced a closer match with experimental data, adds Koslover. The software also let him try various shapes and sizes for a window on the feed horn to find the lowest voltage standing-wave ratio (VSWR), an indicator of how much energy is reflected back into the horn. A perfect VSWR is 1:1. The Sara design has 1.2:1, a ratio Koslover considers quite good.

The window helps eliminate arcing by letting engineers either evacuate the horn or fill it with a gas that slows the electrons. The window must also let microwave energy pass with minimal attenuation and distortion.

-- Paul Dvorak