"Antonio Vernucci" wrote in
:

....
I knew that the resonant point does not precisely coincide with the
minimum SWR point, but I would not have suspected such a big
difference (2 MHz shift at 29 MHz!).

Any comment?

VSWR is not defined in terms of the conditions for resonance.

The characteristic of some kinds of antennas (including half wave dipoles
and quarter wave monopoles over ground) with resonant impedance near 50
ohms is that the R component of feedpoint Z varies slowly with frequency
around resonance (X=0) and X varies relatively quickly with frequency
around resonance. Because of this, in the region of resonance (X=0), X
tends to dominate VSWR(50) and the VSWR(50) minimum will be quite close
to where X=0.

Whilst many folk equipped with MFJ259Bs or the like, and with less
understanding, tune such an antenna for X=0, it is likely that the higher
priority for system efficiency is to tune for VSWR minimum. Worse, they
often do it at the source end of some length of transmission line.

I canvass the issues in the article "In pursuit of dipole resonance with
an MFJ259B" at http://vk1od.net/blog/?p=680 , you may find it
interesting.

Owen


Tony I0JX
Rome, Italy