Hi Tom -
I don't think you are missing anything, as long as we don't compare
apples to oranges. Solid dielectric stays constant in all respects much
longer, and thus, VF does the same, as does loss.
Foam is a completely different ball of wax - as a matrix, it is easily
compressible
by physical force, which can be bending, crushing by foot, rocks & dirt,
installation by electricians who think coax is romex, etc. Air pressure,
heat, and humidity jump in as well, causing significant changes in VF in
direct proportion to how well the stuff is protected from the elements at
what altitude.
Since the foam is not 100% polyethylene like the solid, the additives
that make it foam and stabilize it after it's formed don't last forever
either, hence the definable life expectancy. Oxygen combines with all of
the chemicals as well as the surface of the metal hastening their demise
or ability to perform up to spec.
What this all means is simply that the new hi-tech stuff with very high
VF when made and shipped does not stay that good "forever", and "forever"
is shorter than for the solid dielectrics. It doesn't mean that we should
not use them, but rather that we should treat them with care and respect
to get the most bang for the buck.
Hardline has the least permeable wall, so the foam stays constant longer
- if the terminations are done right.
It's all relative - and less important at the lower freq's - but boiled
down, anything that affects the VF, capacitance, etc, also affects the
loss. Hope I didn't ramble too far from your point!
73,
Press
On Mon, 13 Jul 1998 20:39:11 -0400 "w8ji.tom" <w8ji.tom@MCIONE.com>
writes:
>Hi Press,
>
>> the voids in the braid. Thus either coax can suck oxygen, water, or
>what
>> have you if the connector is sealed improperly. The hardline has
>only the
>> voids or bubbles in the foam, making it less at risk, but poor
>sealing at
>> the ends will still cause trouble through gas and vapor ingress.
>
>I'm a little puzzled by the worry about oxygen and the dielectric.
>
>I understand oxygen contaminates the surface of metals, forming lossy
>layers of oxides (except with silver) that might increase loss a tiny
>bit
>at HF and more as frequency is raised. Corrosion also increases loss
>dramatically (even at HF) if one or both conductors in the
>transmission
>line are braided.
>
>I had no idea oxygen contaminates dielectrics, or that DRY dielectrics
>are
>a major loss mechanism in coaxial lines. Last time I looked at the
>problem, the bulk of loss was in the conductors (at least at
>frequencies
>below 200 MHz I am interested in).
>
>Am I missing something?
>
>73 Tom
>
>
>
>
_____________________________________________________________________
You don't need to buy Internet access to use free Internet e-mail.
Get completely free e-mail from Juno at http://www.juno.com
Or call Juno at (800) 654-JUNO [654-5866]
--
FAQ on WWW: http://www.contesting.com/towertalkfaq.html
Submissions: towertalk@contesting.com
Administrative requests: towertalk-REQUEST@contesting.com
Problems: owner-towertalk@contesting.com
Search: http://www.contesting.com/km9p/search.htm
|