How Long Should My Coax Cable Be?

   Question: I have a resonant antenna on my roof and my transmitter is located in my ham shack in the basement. How long should my cable be?  

    The key here is "RESONANT ANTENNA", when we have a resonant tuned antenna (50 ohms impedance) on the roof, and we use a matching 50 ohm coaxial cable and feed that into a matching 50 ohm radio, the laws of "Conjugate" are in effect and it wouldn't matter where you tested the SWR along the line, nor would it depend how many feet involved, there would still be a good SWR match with only slight variations due to natures pool of imperfections.  So that's why the calculator always spits back the same amount you typed in, because under these conditions the coax length should be just long enough to reach the rig.

    The length of coax to used to "repeat" the SWR of a non-resonant antenna (other than 50 ohm match to your system) at the other end would be 1/2 wave multiple length coax.  Learn How to calculate 1/2 wave multiples for use in special setups.

    For most cases when using a single antenna system with a 50 ohm coax feed ANY coaxial length will work providing the antenna is tuned properly at the feed-point of the antenna (where the coax attaches).