Transmission lines are generally one of two
types: coaxial cable and waveguide. A break in a coaxial cable does not
normally represent a serious RF hazard. The one exception is if one were
to look into the end of an energized RF cable. Don't think that this
hasn't happened! The result can be serious eye damage.
Waveguide leaks are more common than many people realize. Although it
is rare for a waveguide leak to pose a serious whole-body heating risk, a leak
in a waveguide represent a serious eye hazard. Leaks commonly occur at the
connection points. Flexible waveguide is prone to failure anywhere along
its length. Instrumentation that detects an air leak often provides the
first warning. But rubber coated flexible waveguide will usually
hold air in a pressurized system. The area where the field level is
intense is often confined to an area just a few inches away from site of the
leak.
Even a few Watts of energy can create a very intense RF
field over a small area―an area the
size of your face, for example. If a 100 Watt system is losing ten percent
of its energy to a leak, the system monitoring equipment may not detect a
problem. Ten Watts doesn't sound like much. Assume that the energy
radiates into a cone shape and that your face is placed at a point where the
diameter of this cone is about eight inches in diameter. It is simple to
calculate the field intensity. The area of a circle 8 inches or 20
centimeters, in diameter is about 314 cm². With an input power of 10 Watts
or 10,000 milliwatts, the field level equals 10,000/314 or nearly 32
mW/cm². This is more than six times the FCC's Maximum Permissible Exposure
limits for Occupational/Controlled exposure. And that is with only a 10
Watt leak!
