Does antenna feed line length affect impedance
If your transmission line is exactly 0.5 wavelength, then it will have no effect on impedance.  If it has the same characteristic impedance as the antenna, then it will also have no effect.

However, it's quite difficult to calculate the effect with ≠ 0.5 wavelength (or multiples thereof).  

Read this guide: http://www.antennex.com/preview/New/quarter.htm  It should help you a lot.

I would quote it, but the site is huge.  Let me know if you want to put up with an answer that's almost as long as the transcript of the latest Congress session.