[regenfreak]

The explanation for the strange dip of S21 in the resonance point of the helical cavity filter is simple. When i measured the S21 with an active probe sampled on an open tranmission line, there is a bigger impedance mismatch and energy is reflected from the port 2. One end of filter was not terminated. Once i repeated the measurement with the transmission line terminated with a 50-ohm load, then the dip disappeared. However, there was a certain degree of mismatch. As i mentioned before, the helical cavity filter may have non-50ohm I/O impedance as the S11 is close to -10db no matter how hard i tried to fine tune it. In any case, i didnt see obvious evidence of negative resistance of both Poorman's and Tek probes when i probed both the high- and low-impedance nodes of moderately high Q filters. The effect is not easily quantifiable.