Topband: 1000 feet 5/8" hardline or 600ohm True Ladder line.

[Tom W8JI]

>> Paralleling two
>> runs of line does not decrease the insertion loss.
>
> That depends. Given the same source and load Z as with the single run, it 
> does decrease the loss, because the current divides between the two lines. 
> The condition that does not reduce the loss is the use of two runs of coax 
> wired in series to raise their impedance, so that the center conductors 
> are parallel wire line.

Any two cables in parallel have the same loss as a single cable under the 
same condition of mismatch. Power divides equally, loss remains the same in 
each cable. The losses add to the same amount whether in parallel or series. 
The only thing that can change loss is mismatch changes, and that should be 
small on 160.

>I wonder just how much different the loss is?
>

While the shield is important, I don't believe the shield is nearly as 
important as is sometimes made out. Someplace I have hard data on that, 
based on a test fixture that directly excited the shield with a transformer 
over the shield. The fixture was made with two feedthrough connectors and a 
loop of several feet of test cable. Both ports were terminated and measured 
with a selective receiver. The transformer was excited with a known current 
in the shield. I'll have to dig that data out, but I'm pretty sure it was 
somewhere around 80 to 90 dB for thin shield single foil single braid. I'm 
just not sure about the details, but it was a great deal of isolation.

>From practical experience, I certainly see no problems in my station. If 
there was a problem, I would be the first to worry about the cable I use.

When I terminate 500 ft runs of F6 style or 3000 ft long runs of single foil 
single shield F11 style cables, they are dead quiet on BC and 160 meters up 
through 15 MHz or more.

If there is a significant common mode, a few cheap beads and/or some grounds 
would be a better investment than new cables.  :)

Far more worrisome are ground paths through devices or improperly installed 
lines.

73 Tom