In ADS, how can I change the electrical length on a transmission line to depend on the frequency sweep?
This effect is called dispersion, and ADS is able to model it. However, you might have trouble with it. The way that dispersion works is limited by the laws of physics, and the ADS simulator attempts to obey these laws. The consequence is that electrical length versus frequency cannot change in an arbitrary way. Using ADS for this type of analysis requires an understanding of dispersion and how it is constrained. This is an advanced topic. Start with the Kirschning and Jansen formula.