|Subject:||Re: [TowerTalk] Ladder line through the wall|
|From:||Martin Ewing <email@example.com>|
|Date:||Wed, 25 Aug 2004 21:18:47 -0400|
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" <firstname.lastname@example.org> 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
See: http://www.mscomputer.com for "Self Supporting Towers", "Wireless Weather Stations", and lot's more. Call Toll Free, 1-800-333-9041 with any questions and ask for Sherman, W2FLA.
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