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3/22/18 Transmission Line Taper Transition

On Page 6 of the pdf, when you talked about the transition from 50ohms to 68ohms transmission line, it is recommended not to use single step, and using smooth taper is the best option. My question is: what is the optimal length of the tapering transition? I would think it's longer the better because the transition is more smooth, and if it's too short, it will start to look like a single step. Is there any rule of thumb?