Antonio Vernucci wrote:
. . .
Another interesting observation is that, at 29 MHz (i.e. where the
antenna impedance is 76 + j32 ohm and the SWR on a 75-ohm cable shows
the minimum value of 1.95) one can find a cable length at which the
impedance appears to be purely resistive and equal to 1.95*75 = 146 ohm
(or 75/1.95 = 38.5 ohm). This fact is deceiving as, seeing a purely
resistive impedance, one could be led to concluding that the real
antenna resonant frequency is 29 MHz, whilst in reality it resonates at
27 MHz (although knowing what is the real antenna resonant frequency may
not be so important).
. . .

No one with a basic understanding of transmission lines would think that
the frequency at which resonance occurs (X = 0) at the input end is the
same frequency at which the load is resonant, except for two special
cases -- if the line Z0 equals the load resistance at the load's
resonant frequency, or the line is an integral number of quarter
wavelengths long at the load's resonant frequency. And, as you imply,
the resonant frequency of the antenna itself has no significance.
Transmission lines have been used for over a hundred years for impedance
matching, transforming a load of complex impedance into a purely
resistive impedance of a desired value.

I raised the above arguments just as a confirmation of the fact that
understanding what to do before attempting to adjust antennas is not
that easy.

The way to begin is to gain a basic understanding of how transmission
lines transform impedances. The ARRL Antenna Book is a good resource. If
a person's knowledge is limited to only vague understandings of SWR and
resonance, antennas and transmission lines will be a constant source of
mysterious and unexpected results.

Roy Lewallen, W7EL