On Thu, 10 Dec 2009 15:42:07 EST, RLVZ@aol.com 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
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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