I built my own antenna but when I measure the impedance (VNA) I see they are off. One is 30+25J, the other 15 + 10J. I built these by hand, the only difference I see is the feed line to one is about 1/4 inch loner than the other one. Of course there could be other differences I don't understand yet.
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.
What frequency is this? The feedline length definitely affects the impedance if you have a load that is not equal to the characteristic impedance of the feedline. But a 1/4 inch difference won't be measurable until you are way up into the GHz region.