Topband: Ladder line vs coax loss epiphany

[Guy Olinger K2AV]

On Wed, Apr 30, 2014 at 10:46 AM, Tom W8JI <w8ji at w8ji.com> wrote:

> Lines I measured here with heavy conductors were about 370 ohms,
>

Others have gotten 370 as well.  I got 360 and 370 off different rolls.
That's less than 3% variation.  The one up now is 360. The next roll, who
knows. I was always surprised at how often 50 and 75 ohm coax weren't
actually those values. I always measure the coax now, and construct series
matching transformers based on the measured Z0. Get much better results.

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.
>

As would any parallel line laid against things or inserted in PVC.

It is illogical to have a change in Vf without an accompanying change in
> loss or impedance.
>

Certainly. There was measured change in impedance, but proportionally not
nearly so much as the VF.  This was also noted on VK1OD's site (no longer
available), who carried dry and wet table entries for Wireman window lines
in his tables. So taking the trouble to get to 360 ohms did actually
minimize SWR change in the rain. I am certain the loss increased, from RBN,
but that was hard to separate from loss from wet trees in close proximity
to the L.

Standing in a heavy rain down by the creek with electronics was not really
an option with my equipment. Unfortunately, the box at the antenna feed
wasn't constructed with a relay to terminate the feedline to make that kind
of measurement easy. Next version of box will have that operation in mind.


> The odd impedance of "450 ohm" lines aggravates the issue of broadband use
> in matched systems.
>

It certainly does.


> 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.
>

I certainly considered it. However much "cleaner" that solution, the
downside was the need to fabricate *everything*, supports, the spacers for
the line itself, and in my case having to run it through the woods. The
Wireman #554 actually stands up to smaller trees falling on it (happens a
couple times a year) and instead tears up the supports. Out across the open
field I might have tried the homebrew bare wire stuff. Acres and acres of
open farmland certainly does have its advantages.

Surplus hardline is a much better option, IMO.
>

For all the physical reasons, yes. At the point it was going up, the main
energy was being spent developing the FCP. Have to say though, for all the
maintenance irritations, the 554 line performance is never an issue. An
Alpha 8410 with its tetrodes in AB1 and effectively no grid current, rides
the changes in the rain well enough.

73, Guy


> 73 Tom
>