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Re: [TowerTalk] New Tower Installation

Subject: Re: [TowerTalk] New Tower Installation
From: Ian White GM3SEK <>
Reply-to: Ian White GM3SEK <>
Date: Wed, 19 Jul 2006 08:53:57 +0100
List-post: <>
K8RI on Tower talk wrote:
>>I almost agree with all that but I do think better of using Braded instead
>> of solid since it is the surface or skin that carries everything and that
>> being the case braded has a lot more surface or skin area than solid.
>The problem with braid is not the lack of surface area, but that it's 
>composed of many small wires that cross each other. These junctions 
>form... well... junctions if there is any corosion. Also the surface is 
>not one continuous surfaces as in solid or even coarse stranded cable. 
>So, although the stuff is easy to work with, it's not quite the low 
>inductance at the higher frequencies we expect and it's fragile to both 
>the environment and physical handleing.
Agreed. To add some detail: braid has a larger total surface area than 
solid if you count all the individual strands - but that is not relevant 
at RF (including the high-frequency energy component of lightning). The 
skin effect will force the RF current out onto the outside surface of 
the whole braid, so the current has to "hop" from strand to strand to 
stay on the outside.

That is why there are problems with contact resistance, which rapidly 
get worse if exposed to moisture. That's also why braided coax is ruined 
if water gets under the jacket.

Braid should only be used for RF in situations where the need for 
flexibility overrides everything else. Otherwise, use strap or large 
tubing with a continuous surface.

(Coming back to the earlier mention of braid for lightning protection of 
munitions stores, I'd bet that would only be for systems that can be 
deployed quickly in the field. Permanent installations would use strap 
like everybody else.)

73 from Ian GM3SEK


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