Does my PCB require a 50 ohm impedance trace even if I am using an external antenna?
When you send a signal along a trace, if the length of the trace is of the same order as the wavelength of the signal, then you have to match impedances or, you will get a "reflection". What this could mean in extremes is that the power actually emitted by the antenna is only a fraction of the power being pumped out by the chip - in other words, you are not efficiently coupling the chip to the antenna and you get "reflections" - this can cause your chip to overheat and maybe it might damage it.

So, if you are transmitting 3 GHz (say), it has a wavelength of 100mm and, a rule of thumb is that if your PCB track is longer than 10mm (one tenth the wavelength), you need to ensure its characteristic impedance matches the antenna's impedance.

Further reading here and here