[TowerTalk] Ladder line through the wall

[Tom Rauch]

> With the help of my trusty ITT Blue Book, I can calculate
the impedance
> of a balanced line passing through a grounded pipe.  If
your line is 450
> ohms in free space with air dielectric, the balanced
shielded line has
> the following impedance:
>
> If shield dia. is 2 x the wire spacing, Zo = 388 ohms.
> If shield dia. is 4 x the wire spacing, Zo = 434 ohms

None of this matters, because to have a significant impact
on line SWR the impedance bump has to be a significant
fraction of a wavelength long.

On TV channels, a six inch long bump of 2:1 SWR would be
significant in a MATCHED line...but insignificant if the
line is already operating at high SWR.

Pass the line through the wall in 2" PVC, and don't worry
about a thing. If the wall is 3 feet thick and metal the
whole way through, and you operate the line at 1:1 SWR with
wideband video signals at 30MHz, you might see some ghosts
in the video.

For amateur applications and normal walls there won't be a
problem in the world. Pass it through the PVC conduit and
have fun. Just be sure to have a lightning gap outside.

73 Tom