The ``raw'' output from a source will not produce a perfect waveguide mode, unless someone labors to derive the exact field values for a given waveguide (figure 8 and configures the sources appropriately.
An alternative is to inject a signal using a simple block source. The fields produced by this source (figure 9) contain a bunch of modes: one mode is the propagating mode that we want (figure 8) and the other modes will either propagate at a different velocity or not propagate at all. So, if we let this block shaped signal travel down the microstrip, many of the unwanted modes will simply decay and die out. Other modes will propagate at different velocity; typically the strongest mode is the one we wanted in the first place.
Using this process to generate the desired modes requires that the signal propagate a distance away from the source. This requirement means that the source cannot be placed directly next to the incident wave probe or the structure we are studying (in this case, the discontinuity). So there must be a short propagation distance between the source and the probes/structure.
The effect of placing a source too close to the probe is shown in figure 10. The distance from source to probe has a drastic effect on the high frequency, less so at low frequency. This indicates that the source block is injecting a wide frequency range into the microstrip, but much of the higher frequency content quickly decays. Since S-parameters are only relevant for propagating signals and not decaying modes, we must place the source at least 14mm away from the first probe - and possibly further if the high frequency S21 parameter is wanted to any accuracy.
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