I've tried a lot of variations looking for a better match, including: Here is an example (using the same optimized board properties and parasitics as above) where the model matches quite well: Some component values match better than others, so the problem does not seem to be a fixed parasitic inductance/capacitance on the board. For this measurement, L15 = L5 = 2.2 nH and C16 = C13 = 1.0 pF. Solid lines show amplitude of S11 and dashed lines show phase. Here, the measured value is shown in blue and the model in red. For example, here is a plot comparing one measurement with its associated model: In particular, each set of component values shows a resonant peak at some frequency, but in my model the peak typically falls at a slightly different frequency. I have been unable to find optimized values (for board properties and parasitic elements) which match the measured S11 perfectly. Here's where I'm running into a bit of trouble. The basic dimensions for the transmission line components are specified in an equation: Note that I am using 2-port S-parameter models for each component from the manufacturer's website (Murata GRM1555 and LQG15). On the left, we have an ideal transmission line which represents the pigtail, then an inductor which represents the short length of center conductor which lies outside the dielectric of the pigtail. The Qucs model I've built for each permutation looks like this: I've made S11 measurements using 1.0 and 2.2 nH inductors and 1.0 and 2.2 nF capacitors. Here's what the board looks like with the pigtail and matching components installed.
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