"Cecil Moore" wrote in message
...
Antonio Vernucci wrote:
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!).

There's a thread over on eHam.net dealing with this same subject.
Many complex antennas exhibit this effect to a certain extent. The
reason is obvious. Our SWR meters are calibrated for 50 ohms and
an antenna may be resonant with a e.g. 9+j0 ohm feedpoint impedance.

That's a 50 ohm SWR of 5.6:1 where almost 1/2 of the RF is rejected
at the antenna when 50 ohm coax is being used. If the 50 ohm SWR
drops below 5.6:1 somewhere else it necessarily must exhibit a
higher resistance and reactance than exists at the 9 ohm antenna
feedpoint.

Moral: There is nothing magic about 50 ohms. If you were using
a transmission line with a Z0 of 9 ohms with a 9 ohm SWR meter,
you wouldn't notice anything worth reporting.
--
73, Cecil, IEEE, OOTC, http://www.w5dxp.com


Actually, there is something 'magic' about 50 ohms. An air-dielectric
co-axial cable has minimum loss per metre when its characteristic impedance
is 76.7 ohms and the relative permittivity of polythene is 2.26 so a
polythene-dielectric co-axial cable has lowest loss when its characteristic
impedance is 76.7/SQRT(2.26) = 51 ohms, which is most often rounded down to
50. This is on the basis that the conductor loss greatly exceeds the
dielectric loss, which is true over most of the frequency range for which
solid polythene dielectric is appropriate.

Maximum power handling, for a polythene-dielectric cable, occurs at a much
lower impedance: 30/SQRT(2.26) = 20 ohms.

Chris