----- Original Message -----
From: "Tom W8JI" <w8ji@w8ji.com>
To: <topband@contesting.com>
Sent: Wednesday, April 30, 2014 10:46 AM
Subject: Re: Topband: Ladder line vs coax loss epiphany
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..
The ARRL has had some goofy measurements. They had one article that showed
almost no change in loss with a line laying right on wet dirt!
Lines I measured here with heavy conductors were about 370 ohms, and loss,
velocity factor, and surge impedance changed with water. They also changed
substantially when the line was laid against things, or a line enclosed in
PVC pipe was buried.
It is illogical to have a change in Vf without an accompanying change in
loss or impedance.
The odd impedance of "450 ohm" lines aggravates the issue of broadband use
in matched systems. I would stay away from ladder lines for low loss
impedance matched systems, and stick with real open wire line of a modest
planned impedance such as 450 ohms.
Surplus hardline is a much better option, IMO.
73 Tom
These days even surplus 50 Ohm hardline (Andrew Heliax and other brands) in
1/2" and 7/8" is often available at low scrap cost as 2 way shops and sites
are going out of business and towers dismantled.. Many come with connectors
and price can be free if you do the climbing. Let your fingers do the
walking and call around.
Carl
KM1H
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