On 10/19/2010 1:27 PM, dalej wrote:
> I am not convinced that in my case, coax would work better than open wire
> feeders regarding the noise problem.
As I understand the problem, the answer is, it depends. :) In this
case, it depends on the location of noise sources with respect to
feedlines and the antennas themselves, the electrical length of the
feedlines, and whether the coupling is common mode or differential mode.
It also depends on the antennas connected to the noise sources, and
their directivity and orientation. That's a lot of variables. No magic,
but not easy to compute, and one size clearly does not fit all.
One of the principles at work here is that each design problem involves
trade-offs, and is best approached by thinking through all of the
variables. What would be the loss of coax in a given situation, how
important is it that one antenna work all bands, what are the noise and
RFI issues in THIS installation, how easy is it to do it in the more
ideal way, etc.
As to the validity of N7WS's work on the effect of moisture on ladder
lines, and on other twin leads, I would expect that to depend a lot on
the construction of the line (how much material other than air in the
dielectric space and what that material is), and upon any degradation of
that material due to UV and weather that might cause it to be more
porous. Wes's is the only work I've seen on this topic, nor have I seen
any serious peer review of it other than the comments that N3LI just
posted.
73, Jim K9YC
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