Topband: RG-6 coax

[Jim Brown]

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 
reliably.

  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 
Interscience.

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