Simulating a sensor swallowed by a cow to aid in the study of effects of different types of feed on the cow's digestion.

The FDTD method is widely used for predicting performance of radiating devices implanted in human bodies. The high fidelity human body meshes offered by Remcom are useful in such applications.

FDTD may also be applied to animals. For example, cattle feed is tested in part using data from sensors that are swallowed by the cow and radiate information about the stomach contents. Designers of these devices were concerned that there might be a cow "resonance" such as occurs when animals are exposed to incident plane waves. At a resonance frequency the radiation from the internal radiating device might be more attenuated than would be predicted by simple formulas. It was desirable to determine any possible resonances before the device was designed.

Since the results are not as critical as for human implanted devices, a very simple mesh for the cow was made using the XFdtd^® mesh editor. The mesh size is 1.5 cm, and the entire cow was assumed to be made of fat tissue with permittivity 62, conductivity 0.1 S/m. The implanted device is a metal case 5 x 3 x 3 cm with a 3 cm wire antenna. The device is entirely coated in dielectric material. The antenna was excited with a band-limited modulated Gaussian pulse. From the FDTD results, the antenna gain (including the effects of the loss due to the location inside the cow) was calculated. The antenna gain for the device in free space was also calculated for comparison. Results show a drop in gain of about 11 dB at 150 MHz, indicating that this frequency should be avoided.

Figure 1, Figure 2, Figure 3, and Figure 4 are 3D and 2-D views of the geometry and electric field displays as rendered in XFdtd. Figure 5 shows the antenna gain of the implanted device when radiating in free space. Figure 6 shows antenna gain of the implanted device when radiating inside the cow.

Further calculations for different antenna types and for different radiation directions were made during the design process. These changes were very easy to make using the XFdtd mesh editor capability.