The problem is that if the antenna is not matched by design, it probably has a complex impedance. For example 20+j15 ohms. So you can not just hook up a 20 ohm transmission line to it and expect to see 20 ohms resistive at the far end....the reactive part makes it spin around on the smith chart.
So, ideally you would match right at the antenna. If not, you need to figure out the transformed impedance at the far end of the line, and match to it. The longer the transmission line, the harder it will be to obtain a good broadband match, though.