Topband: RG-6 coax
jim at audiosystemsgroup.com
Wed Apr 10 12:49:59 EDT 2013
On 4/9/2013 9:19 PM, donovanf at starpower.net wrote:
> CCS RG-6 may be a problem for powering some remote devices such as preamps or relays through the cable because CCS RG-6 has significantly more DC resistance than SC RG-6.
If you can find a proper (that is, complete) technical data sheet for
whatever coax you are considering, it will include values for the DC
resistance of the center conductor and the shield. Likewise, a proper
technical data sheet for a remote device should include the DC current
and the minimum voltage needed at the remote device for it to work
Having this data, and the length of the coax, it's a simple matter to
predict whether the cable resistance will be a problem.
It's important to remember that the center conductor is not the only
issue -- MOST RG6 coax is designed for use at VHF and UHF by cable TV
systems, and their shields are optimized for use at those frequencies.
The foil + braid shields of these cables tend to be relatively thin, so
their DC resistance tends to be fairly high.
I don't worry a lot about RF loss in RX antennas for topband and 80/40M,
but I do worry about shield resistance, because high shield resistance
(at the frequency of interest) degrades shielding. The parameter of
interest here is the Transfer Impedance, the lower limit of which is the
resistance of the shield. Transfer Impedance is a measure of the
conversion of shield current to a differential voltage (that is, signal)
on the inside of the coax. High Transfer Impedance means more signal
intrusion. There's a fine discussion of this in Henry Ott's classic
text, Electromagnetic Compatibility Engineering, published by Wiley
In my experience, Commscope cables tend to be good stuff, well
manufactured, and you can believe their tech data. Ditto for Belden.
But both companies make a wide range of cables, designed for a wide
range of uses, so it pays to study their data sheets and choose one that
meets your needs.
73, Jim K9YC
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