Or, if you know the length of your coax, you can calculate the actual
impedance at the feedpoint from the measured value at the shack end
of the coax using VK1OD's transmission line calculator:http://vk1od.net/calc/tl/tllc.php
No need for a cable that is 1/2 wavelength long.
But if you are looking for the frequency of the minimum of the SWR curve,
that will remain constant regardless of mismatch when you add coax, as
long as the coax is the same impedance as the analyzer. While theimpedance
varies along the feedline, the SWR
for a reduction due to cable losses.
In most cases, adjusting an antenna for minimum SWR at the shack end
is the most effective (and less error-prone) method.