Jim Kelley wrote:
The Bird manual shows a section of 50 ohm transmission line. But even
the N connectors are 50 ohms. I assume even one of those would show up
on a TDR.

Probably would, but would it cause appreciable reflections?
I don't think it would have |rho|=(75-50)/(75+50)=0.2
when used with 75 ohm coax. Owen said something about 40mm.
I just don't think 40mm (1.5") is long enough to establish
a 50 ohm environment but I could be wrong. I've never owned
a Bird. I suspect |rho| would calculate out to be much less
than 0.2. And of course there would be two discontinuities,
one from 75 ohms to 50 ohms and one from 50 ohms back to 75
ohms. Given the short length between those two discontinuities,
one probably interferes with and tends to cancel the effects
of the other as would happen with a zero length of feedline.
--
73, Cecil http://www.qsl.net/w5dxp