6.2.6   Power Available from the source

QuickWave allows the user to obtain a precise information about the actual spectrum of the selected excitation pulse. This is available through the post-processing called Power Available. It invokes calculations of the Fourier transform of excitation waveform of all sources. The result will be a square root of time-maximum spectral power (sqrt(Pav)) at each frequency point; when displayed in square scale (available in the software), it will be time-maximum spectral power in Watt. Power Available essentially amounts to applying FD-Probing on source signals. The results are available in QW-Simulator in Results window.

Having the curve of square root of time-maximum power available versus frequency becomes very convenient when it is needed to compare the resulting field values in the structure to the input signal.

Power Available post-processing enables also the phase of the source signal.

It is worth noting at this point that for a plane wave box, the curve will be named sqrt(Sav) instead of sqrt(Pav), and denote the available power density of the incident wave. Sqrt(Pav) and sqrt(Sav) denote time-maximum values of the corresponding available power and available power density. Time-averaged values can be obtained by dividing these results by two. Note that sqrt(Pav) and sqrt(Sav) are calculated analytically in the FD-Probing post‑processing, based on the analytically given excitation waveforms. In the FDTD implementation, the numerical available power tends to slightly higher and to increase with the increasing frequency, up to about 5% (with respect to the analytical prediction) at the discretisation of 10 cells per wavelength.

The Power Available post-processing is a convenient tool for verifying spectrum of the applied source signals. User Guide 3D: Joint use of contours with frequency-domain monitoring of field and source signal example, visualises the effects of the Power Available results depending on the duration of an exciting pulse.

Note that the results produced by Power Available do not have direct physical significance for source signals that are not Fourier-transformable. Among QuickWave defaults, these are sinusoidal, step and step with finite rise time pulses, which do not decay to zero in time. Their Fourier transform will be continuously increasing in time, at the sinus frequency f1 and at DC, respectively. For example, with the step pulse of amplitude 1 the result of Power Available will be equal to time [ns] covered by the simulation (e.g., at iteration 2600 and f=0 we obtain |sqrt(Pav)|=2600*dt, where dt is time step visible in Simulation Info).