This is a good illustration of one of Reg's hot buttons -- that "SWR
meters" don't actually measure the SWR on a transmission line, but
rather are reporting the degree of match or mismatch at the meter's
insertion point.

In the case of 50 ohm line, it turns out that the "SWR" reported by the
meter is actually the SWR on the 50 ohm transmission line, assuming that
the meter is designed for use in a 50 ohm system and reads properly in
that environment. When you substitute the 72 ohm line, the SWR meter
will still read 1:1 because the impedance at its insertion point is
still 50 ohms, but the SWR on the 72 ohm transmission line is actually
1.44:1. So the answer depends on what you mean by "what would be the SWR":

The SWR on the 72 ohm transmission line will be 1.44:1
The SWR meter will read 1:1
If there is a 50 ohm line between your rig and the 72 ohm line, its
SWR will be 1:1.

Roy Lewallen, W7EL

Fred W4JLE wrote:
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...