Does antenna feed line length affect impedance

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.

• An important property of an antenna is the frequency (band) at which one intends to use it. Unfortuantely you omit this crucial information. Feb 16, 2016 at 22:22
• Have you done multiple tests, it could also be the connector affecting readings depending on what kind it is. Also as @Paul said, length matters when reflections come into play. Feb 16, 2016 at 22:23
• Probably your antenna is not matched to the source, therefore the feed line is radiating and acting like additional antenna. Feb 16, 2016 at 22:24
• At those frequencies the effects of a quarter of an inch difference can be enormous. First of all make sure you get rid of any reactance in your antenna, then the length of feedline becomes less important. You'll also make your antenna more efficient if you can get rid of the reactance. Feb 16, 2016 at 23:41
• If the coax is 50 ohms you can try using 4 quarter-wave radials as ground plane and drooping them down 45 degrees. This will increase your antenna's impedance closer to 50 ohms, provide a better match with the coax, reduce SWR, make the length of feedline less critical, minimise possible higher voltages at the transmitter and overall increase your radiation efficiency. Pretty much anything can be used as an antenna but doing it right I find much more rewarding. Thanks for the link to the article btw. Feb 17, 2016 at 21:12

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).