Using simscape to simulate a finite power AC source

I'm trying to make a simulation of an AM radio detector circuit. These circuits draw power from the antenna, which has a finite amount of power to provide. As the circuit loads the antenna, either the voltage or the current reduce (probably both). I can't use the standard AC sources to do this modelling, since they are ideal. As my circuit loads my sources, they increase either their voltage or their current. How do I model a finite power source in simscape?