Buck wrote:

W5DXP wrote:
You're right, neglecting losses, the antenna
impedance is repeated every half wavelength. Neglecting losses,
that 100 ohm antenna feedpoint impedance will be repeated at half-
wavelength intervals for *any* feedline Z0, including 50 ohm coax.

So far, so good. If the antenna is fed with 50 ohm coax, there will
be great losses, ...

A 100 ohm antenna causes an SWR of 2:1 on 50 ohm coax. Lots
of hams don't even bother with a tuner when the SWR is 2:1. The
additional losses in 50 ohm coax due to an SWR of 2:1 are pretty
much negligible on 80m for reasonable lengths of feedline.

I have a yagi with about 25 ohms at the antenna. I connect 600 ohm
twin-lead and run it 1 1/2 wave lengths to my radio. This, I assume,
gives me the highest current at the end of the twin-lead. So I
connect it directly to the radio with a PL-259 and get an acceptable
load for the transmitter with low-loss in the feedline?

The SWR on the 600 ohm line will be 600/25 = 24:1

The SWR on 50 ohm coax will be 50/25 = 2:1

For a 25 ohm single band antenna, I would be inclined to go with
the coax.
--
73, Cecil http://www.qsl.net/w5dxp