Implantable Antennas/Power Amp Design

I attended the AnSys/Ansoft Smart Engineering Simulation Application Workshops earlier this week in Newton, MA. They had a complete RF/microwave track including sessions on Electrically Large Antennas and RCS; Statistical Modeling, DOE and Optimization; HFSS Tips and Techniques; RF Power Amp Design Using Coupled Circuit, EM and Thermal Analysis; Method of Analysis for Implantable Antenna Systems and RF Product Mgmt. Road Map and Dialog.

The Power Amp Design was very interesting as they showed how you can model the chip and package separately and merge the results to simulate the device. Then with a link to AnSys Mechanical, you can make a full model for thermal analysis and evaluate the performance. This allows you to trade of electrical and mechanical/thermal parameters to optimize device performance which is pretty powerful.

I also was intrigued by the Implantable Antenna Systems presentation given by Cambridge Consultants local here to the Boston area in Cambridge, MA (and headquartered in Cambridge, UK). They discussed how most of the radiation energy from a wireless signal is lost just by exiting the body due to losses in the tissues it goes through (especially fat) and reflections due to the changes in interfaces (fat, skin, tissue, etc. have large changes in permittivity). This loss also varies tremendously person to person depending on their build, etc. so over 99% of the energy is typically lost.

Therefore it is very important to have a optimized antenna system to maximize the gain and sensitivity of the signal. They use magnetic coupling for these applications since most of the energy is lost but the typical range requirement is 2 m. The FCC medial band used is 401-405 MHz but sometimes they used the unlicensed 2.4 GHz band for wake up since the FCC only allows 25 micro Watts ERP (if using an agile signal) for the medical band. Another design concern is a shift in frequency from the fat layer which varies by individual so they have to design for a broader bandwidth.

They have designed patch antennas that greatly improve sensitivity over the typical loop antenna currently used in many applications. The patch antenna results in about 3.4% of the radiated power while the loop antenna is only about 0.6% and the patch has better coverage. All of this has to fit into an enclosure about an inch square.

There were some very interesting applications that were presented here and it is very good to see the interaction with their customers as they strive to solve to their problems.