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Re: [TowerTalk] Resistance of RG-8 cable?

To: "" <>
Subject: Re: [TowerTalk] Resistance of RG-8 cable?
From: "Jim Brown" <>
Date: Thu, 10 Dec 2009 14:17:09 -0800
List-post: <">>
On Thu, 10 Dec 2009 15:42:07 EST, wrote:

>I'm wondering what the typical resistance should measure on a 100'  length 
>of RG-8 coax with one end shorted?  I'm measuring 1.5 ohms when  shorting 
>one end of a 240' length of RG-8 ... is that about normal?

The answer is simple, but one size does not fit all RG8s. First, RG8 is NOT 
a specification for coax, it generically describes a 50 ohm cable of about 
0.4" diameter. That's ALL!  There are wide variations in the size and 
composition of the center conductor, the shield, the dielectric, and the 
outer jacket. 

So to answer your question, you must go to the spec sheet for the particular 
cable that you are using and see what the two conductors are on YOUR cable. 
What you are measuring is the DC resistance of the series combination of the 
center conductor and the shield. RG8 designed for transmitting typically has 
a center conductor of #10 copper, and better grade coaxes designed for 
transmitting or video typically have a braid that is equivalent to #10 
copper. The data sheet for any coax worth using should specify the DC 
resistance per unit length of both center conductor and shield. For #10, 
it's 1 ohm per 1,000 ft (x2 for a round trip). For such a cable, 240 ft 
should look like about 0.5 ohms at DC. 

DCR (and MF/HF loss) will be significantly higher for copperclad steel 
center, foil shields (without braid), or with a less robust braid. 

>Question 2: Measuring in the above manner, should the resistance increase  
>as the coax gets a few years old?

A higher resistance measurement generally indicates corrosion of the braid 

These DC resistance measurements are a very good starting point. For all 
practical purposes, loss from DC through VHF is nearly all due to copper 
losses, taking skin effect into account. 

Diectrical losses don't come into play until you hit UHF, OR at HF, until 
the dielectric gets wet. If there's enough moisture to cause corrosion, 
there's a fair chance that the dielectric has degraded. 


Jim Brown K9YC


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