HFSS15: Incident Power

Incident power for an antenna structure is defined in terms of accepted power under special conditions. In HFSS the incident power flow on a transmission structure is defined as the accepted power when that structure is terminated in a conjugate-matched load. Thus we write a general definition:

(1)

For a transmission structure that may be modeled with a real characteristic impedance, this definition is equivalent to the zero-reflective case where the transmission structure is terminated in a matched load. For example, in the simple example discussed in this section on accepted power, setting S₁₁ = 0 produced the incident power as:

(2)

However, there is an important difference in the case of Lumped ports with complex impedance, and Wave ports renormalized to complex impedance. In these cases, the conjugate-match and zero-reflection definitions of incident power give different results, but only the conjugate-match definition used by HFSS enforces the important property that P_acc<P_inc.

In the Edit Sources window of a terminal project, you specify total voltage V instead of the modal incident power. Using the underlying modal S-matrix, HFSS converts the terminal excitation to total fields at the ports, and then applies the general definition. As a result of this procedure, the incident power in a terminal project depends on both the user-specified stimulation as well as the model itself.