Topband: Ladder line vs coax loss epiphany

[Guy Olinger K2AV]

On Tue, Apr 29, 2014 at 11:32 AM, Shoppa, Tim <tshoppa at wmata.com> wrote:

> It took me literally decades to realize this, but the low ladder line
> losses that show up in the ARRL graphs are not because ladder line is
> magical..snip...
>

>
It's because ladder line is used at 450 or 600 ohm impedance, so it of
> course has one tenth the resistive losses of 50 ohm coax built with similar
> amount of copper.
>

That tenuous advantage disappears if one popular 360 ohm heavy duty window
line variant of "450" line is operated at a mismatch. At that point the
stranded copperweld conductors (used for physical strength) start to lose a
lot of power at the current maximums of the standing waves. I confirmed the
360 ohms on my particular piece of the window line.

In my case, almost 500 feet of that running through the woods needed a
surprising amount of finagling the system to present 360 ohms to the
feedline. That SWR change people see in the rain apparently is a velocity
factor change, making the degree of change in the rain proportional to the
mismatch to the window line Z0. The 450 ohm baluns are not all that good a
match, and most of the baluns are poor at 160..

In my case trimming the big L and adjusting the series cap allowed me to
present 360 ohms resistive on the window line side of the 4:1 FCP isolation
transformer.

But none of that helps with branches and whole trees falling across the
window line. This of course pops the support points, and I've had to partly
redo the supports a half a dozen times after storms. Also, squirrels eat
the window line PE, which seems really weird. Buried Hardline or balanced
feed parallel coax increases in allure.

73, Guy