Shield theory, design and construction

• Pages: 32-51

Appendix A – Shield theory, design and construction

1. Basic principles

Before attempting to shield a radiofrequency (RF) heater, one must have a basic

understanding of the concepts and theories used in RF shielding. The applications of RF

heating and the types of equipment are so numerous and varied that not every case can be

discussed individually.

Basic principles, as discussed later, can be used to design shields for specific

applications. Detailed instructions for applying these principles are included in section A2.

Because an understanding of static DC field concepts is basic, these are described and then

extended to the AC field concepts used in RF heating.

In static fields, an electric field can exist without a magnetic field and vice versa. With

alternating fields, both the electric and magnetic fields exist, but the relationship between them

may vary. Thus both fields need to be evaluated in terms of biological interaction.

If a static electric field exists in a volume in space, certain effects can be detected (see

Appendix B). The electric field effects can be detected by observing the action of a free

electron inside the volume. For example, between the two parallel plates of a capacitor, an

electron near the centre of the plates would be repelled from the negative plate and attracted to

the positive plate. The path of motion would be a straight line that starts perpendicular to the

negative plate and ends perpendicular to the positive plate. Near the edges of the two plates, an

electron still leaves in a direction perpendicular to the surface of the negative plate. This path of

motion is called a "line of force" because there is a force on the electron causing it to move.

The directions of lines of force depend on the size and shape of the conductors that set up

the field. Near the conductor, the lines of force are always perpendicular to the conductor,

regardless of the size or shape of the conductor.

To gain a better understanding of the lines of force, current flow and electric fields,

consider the following theoretical example and how these parameters change under different

conditions. Assume that a metal conductor is completely surrounded by a rectangular metal box

(figure A-1). This "shielding cabinet" (metal box) has no openings. All electric and magnetic

fields are confined within its interior. The rectangular metal plate is similar to the top of a thick

metal table on wooden legs, with the edges of the top rounded.

32

Appendix A

Figure A-1. General principle of shielding

Closed metal box

(shielding cabinet with

no openings)

Metal plate

Wooden

support leg

Voltage generator

Return current

(denoted by dotted

lines with arrows)

33