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Re: [TowerTalk] Coax Loss -- RG-Numbers Don't Tell us Much

To: "Tower and HF antenna construction topics." <>
Subject: Re: [TowerTalk] Coax Loss -- RG-Numbers Don't Tell us Much
From: "Jim Brown" <>
Reply-to: "Tower and HF antenna construction topics." <>
Date: Thu, 04 Feb 2010 21:13:41 -0800
List-post: <">>
On Thu, 04 Feb 2010 20:45:09 -0800, jimlux wrote:

>Tricky, because you need enough length so you're not trying to measure 
>0.001 dB differences. 

Yes. 100 ft is sort of a minimum length, and more is better. My 
measurement setup is quite simple -- an HP generator and an HP spectrum 
analyzer. Nothing automated. To eliminate calibration errors in the 
equipment which can easily be of the same magnitude as what we're trying 
to measure,I must write down numbers and subtract the difference between 
a very short cable and the cable under test. 

>As to reasons for things varying all over the place:
>perhaps skin effect and cladding of various sorts, as the current 
>density profile moves across the boundary.   Most of the loss is in the 
>center conductor, 

I wouldn't be so sure of that. DCR is split between center and shield, 
in many cables the DCR of the shield is significantly greater than the 
center, and the composition of both center and shield can be complex. 

>but the effect of cladding is easily modeled, and 
>probably doesn't account for everything. How uniform is the cladding? 
>What about the losses in the shield, particularly for those cables with 
>both braid and foil? How much effect on loss is there from braid moving 

Exactly the sort of thing that needs to be measured. 


Jim K9YC


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