Re: [BULK] - [TowerTalk] RG-11 Source?

[Gary Schafer <garyschafer@comcast.net>]

Tom Rauch wrote:
> > cheaper -- and it accepts the correct connectors.  There
>
> really is a
>
> > difference with the extra shielding -- you can see it in
>
> much reduced
>
> > ghosting on signals, especially in metropolitan areas
>
> where multipath is
>
> > abundant.  -WB2WIK/6
>
>
> I don't think so Steve.
>
> In the field or outside a lab you can't measure the signal
> leakage through a single shield cable, unless the cable is
> defective or really junk.
>
> Ghosts and leakage were virtually always bad connectors and
> the remaining problems were broken shields (mostly by tree
> rats). I supervised engineering for a company that had
> dozens and dozens of systems and headends, and was always
> involved in field quality.
>
> We had a system someone wired with some special quad shield
> to prevent AM and FM BC ingress (it was next to a BC
> station) and wound up just ripping out all the fancy stuff
> and using a single foil cable with a single loosely woven
> braid. We did that because we could get better connections
> at the fittings, and that's where all the measurable ingress
> was rooted.
>
> Once a shield is several skin depths thick and has no breaks
> larger than a small fraction of a wavelength, adding more
> layers doesn't do a thing except help DC and low frequency
> AC resistance. Sometimes at GHz frequencies the foil wrap
> has resonances, and a second wrap that has integrity over
> the "slot" will help. TV channels are really in a sweet spot
> for not being especially critical.
>
> Consider twin lead! It has no shield at all, and it worked
> fine.
>
> 73 Tom

That's not entirely true. Ever try to make a vhf or uhf duplexer on a 
two way radio system work with only single shield interconnects on the 
cavities? Especially with a close spaced system. You will never get the 
isolation to stay put. Move a cable around and the isolation changes. 
You don't have that problem with double shielded cables.

73
Gary  k4FMX


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