An incident time-varying electromagnetic signal (pulse) is now assumed to illuminate the structure. The incident pulse and the output voltage at the device port are related by a linear, time-invariant, continuous-time system. In this case, the system consists of the feed wire, the box, the PCB trace, and any linear loads that the PCB trace terminates in before arriving at the device port of interest (which is assumed to be open-circuited). Figure 1 shows once again the general system, although results will be presented for the structure of Figure 2. The incident electromagnetic field is now assumed to be a time-varying plane wave of the form
where p(t) is an arbitrary time-varying signal (two specific pulse shapes will be considered for the results presented later), PW is a unit vector in the direction of propagation of the plane wave and r is the vector to the observation point. The real-valued unit vector E0 gives the polarization of the incident wave. The voltage signal v(t) at the device port can be written as [Lathi, 1998]
where h(t) is the unit impulse response (the response when p(t) = δ(t)) and ‘*’ refers to convolution. In the frequency domain, the relationships can be written as
where P(ω) is the Fourier transform of the input pulse, V(ω) is the Fourier transform of the port voltage, and H(ω) is the Fourier transform of the unit impulse response of the system. The transform definitions used here are
Because the voltage signal v(t) is real valued, the output signal can be obtained from its Fourier transform as
The incident signal can be represented in terms of its Fourier transform as
where k = k0PW and k0 = ω/c, the propagation constant in free space, and c is the speed of light.
 Comparing (7) and (8), it is concluded that H(ω) in (7) physically represents the frequency domain voltage at the device port due to a unit-amplitude incident plane wave at a radian frequency ω, of the form
Therefore the transfer function H(ω) of the system can be calculated by the hybrid method with a unit-amplitude incident plane wave excitation. The magnitude of this transfer function is shown in Figure 3 for the canonical structure in Figure 2.