Consider the law of thermodynamics that states “Energy cannot be created or destroyed, only changed in form.” Power goes in, power comes out. The total power, manifested by all of the charges in the radiating field, is equal to the input power, less any loss incurred in the conversion from input power to ERP.

You can think of this as if you had a garden hose and water was flowing out of it at a rate of 2 gallons per minute. You would still get 2 gallons per minute worth of water if you attached a lawn sprinkler to the hose and distributed the water over a 500 square foot area. If you put a nozzle on the hose to wash the windows on your house, you would still get 2 gallons per minute; it would just squirt out farther in one direction and, of course, there would be some loss due to the resistive characteristic of squeezing the flow through the nozzle.

The effective radiated power from an antenna can, also, be shaped and directed using high-gain and directional antennae. When we talk about antenna gain we see that ERP can remain the same, but when itʼs “squirted out” in something other than an isotropic sphere the effect is that the field, in a specific direction, has more power than the same field would have had in that direction in the isotropic situation.

The Near Field and the Far Field

Considering the field equation (from Figure 5.2 above) it can be seen that the aspects of the field calculated in first two terms falls off inversely with the square of the distance (r2 in the denominator) The last term does not fall of with the distance but, rather, is differentiated with respect to time. This gives rise to two different parts of a radiating field. Very close to the source there are effects that decrease very quickly. As you get further away these effects become so small that they no longer cause measurable changes in the field. Further away the effects of the last term of the field equation are more significant. These two areas are called the near field and the far field. Sometimes these are also referred to as the Fresnel Zone (the near field) and the Fraunhofer Zone (the far field). It should be pointed out that weʼll see the use of the term “Fresnel Zone” when we discuss interference patterns later in this paper. The two terms are the same, but their meaning is slightly different.

When the terms that vary inversely as the square of the distance are removed from the field energy equation the result is the equation for energy in the far field and is shown below in Figure 5.3. You can see how the energy equation from Figure 5.2 has been transformed into the far field version by the removal of the two middle terms and the consolidation of the common factors in the denominator.

Copyright 2003 - Joseph Bardwell