Re: [TowerTalk] Ladder line through the wall

[Martin Ewing <martin@aa6e.net>]

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

(If you're willing to spread the balanced line - increase the 
wire-to-wire separation - you can tweak the balanced shielded 
configuration to give 450 ohms.)

A couple of factors probably help you: (1) if you are using dielectric 
ladder line, the impedance change is less because the E field 
concentrates in the dielectric, and (2) since you are only passing 
through a thin mesh opening, the effect is smaller, too.

I would say that the 2X opening is probably sufficient, but if you are 
really conservative, you might choose 4X.  If you're running high power 
& high VSWR, you might want at least 2X spacing to avoid arcing, though 
your PVC should help.

Of course, the "ham" answer is just do it any way you like and adjust 
your antenna tuner to match whatever impedance situation you get.  And 
avoid smoke if possible.

73, Martin AA6E
----------

"Steve Forst" <kw3a@snip.net> schreibt:

What is the best way to run 450 ohm ladder line through a block/stucco wall,
where the stucco has  steel mesh behind it?   There is no wood, vinyl, or
window pane to  use for access.



I was thinking of a piece of pvc pipe, a few inches longer than the wall
thicknness, a cap on each end, with a slit cut in each cap  for the ladder
line to pass through.

How much clearance  is required beetween the ladder line  and the  steel in
the  wall to prevent  problems?

Tnx, 73 Steve KW3A

kw3a@snip.net



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