Playing the Field

Furthermore, there are two basic shapes of field: the purely source field, such as you would see traced by the particles of an exploding grenade, and the purely vortex field, as you see when water flows down a drain.

From studying these concepts, we learn that electric fields are sourced by charges that are on conductors and can easily be terminated by charges on another conductor. Magnetic fields flow around in closed curves and can be bent to go around conductors, but cannot be terminated by them.



Closely related to these concepts are the electromagnetic field boundary conditions for fields near conductors. When an electric field meets a nearby conductor, it does so at right angles, whereas when a magnetic field approaches a nearby conductor, it slides past tangentially. So, if you put a tag antenna on metal, it had better be designed for the type and direction of field you expect to find there.

Our analysis has led to the definition of appropriate measures of strength of exciting field and tag sensitivity to fields that can be applied to both near and far fields. These measures are called reactive power density per unit volume and coupling volume.

Our conclusions are:

• When considering power transfer to tag antennas, it is essential to know whether near or far field excitation is in play.
• If an antenna is very small, only the intense fields of near-field excitation will likely overcome its disability of very low efficiency regarding radiated fields.
• Antennas are not sensitive to fields of all types and directions. An antenna must be designed to couple to the type of field that exists near the object to be tagged.
• The object itself can significantly modify the nature of the field in its vicinity.

Our research also has derived various designs for both transmitter and tag antennas for various contexts.

Peter Cole is the research director of the Auto-ID Lab at Adelaide, Australia, which is part of the School of Electrical and Electronic Engineering at the University of Adelaide.