Antonio Vernucci wrote:
I noted that the
measured SWR of all the antennas I have mounted (Yagis, dipoles)
slightly varies when the feedline length is changed by several meters.

If the characteristic impedance of the feedline differs from
the characteristic impedance of the calibrated SWR meter,
the indicated SWR will vary with length of feedline.

If the feedline is lossy, the transmitted signal SWR will
decrease between the antenna and the transmitter. All
feedlines have a certain amount of loss.

If common-mode current is present on the SWR meter case,
the SWR reading will vary because the meter has no fixed
ground reference.
--
73, Cecil, IEEE, OOTC, http://www.w5dxp.com