----- Original Message -----
From: "Jim Brown" <jim@audiosystemsgroup.com>
To: <topband@contesting.com>
Sent: Wednesday, April 10, 2013 12:49 PM
Subject: Re: Topband: RG-6 coax
On 4/9/2013 9:19 PM, donovanf@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.
ALL CATV coax is designed for performance down to 5MHz as data, signaling
and other narrow band applications operate below the TV channels.
Many towns and cities have all their computers linked, traffic control,
alarms and more on cable these days.
The only real test of a cable is to terminate it at its characteristic
impedance, insert a swept high level signal and then walk it with a sniffer
such as CATV field crews use.
Terminating and listening on a receiver is acceptable if all you are
interested in is a narrow range of fairly low signal strengths.
Carl
KM1H
All good topband ops know fine whiskey is a daylight beverage.
_________________
Topband Reflector
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