"Fred W4JLE" wrote in message
...
Assuming I have an antenna that is perfect on 3.8 MHz. Perfect being
defined, as I am feeding it with exactly 1/2 electrical wave length of 50
Ohm feedline and it is 1:1 SWR measured at the source end.

What would the SWR be if I substituted the 50 Ohm feedline with a 1/2
wavelength of 72 Ohm feedline?

The head of the pin is now open for dancing...

Hi Fred,

Assuming that when you say the antenna is 'perfect' with a 1:1 SWR on a 50-ohm
line it means that the input impedance to the line is 50 + j0 ohms. Since the
line is 1/2 wl,and presumed lossless, the terminal impedance of the antenna is
also 50 + j0.

However, when you replace the 50-ohm line with one of 72 ohms the SWR on the
line changes from 1:1 to 1.44:1. But since it's length is 1/2 wl the input
impedance is still 50 + j0 ohms. The 1.44 SWR results from the 1.44 mismatch
between the 72-ohm line and the 50 + j0 ohm impedance of the termination, the
terminal impedance of the antenna.

The 1/2 wl consists of two 1/4 wl lines in series. At the point 1/4 wl toward
the input the line impedance is transformed up to103.68 + j0, (72 x 1.44) which
is then transformed back to 50 + j0 (72/1.44) along the 1/4 wl section returning
to the input of the line.

Walt, W2DU