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).