Pat, all of the conditions you cite contribute to increased loss, since
all of them affect the velocity factor and characteristic impedance.
Moist dielectric becomes conductive as well as changed in capacitance,
surface oxidation or reduction changes RF resistance and causes noise,
and re-soldering such conductors is a major chore.
When coax no longer meets its design specs, and if those specs were
required in its task, it should be replaced.
73,
Press Jones, N8UG, The Wireman, Inc., Landrum, SC, 29356
Sales(800)727-WIRE(9473) or cqwire@juno.com.
Tech help (864)895-4195 or n8ug@juno.com
www.thewireman.com
Our 21st year!
On Thu, 25 Jun 1998 13:40:49 -0700 (PDT) Pat Barthelow <aa6eg@cv.tmx.com>
writes:
>
>
>
>After discovering badly corroded shield inside an old RG-213 cable at
>the
>old Ft. Ord Army MARS station, I replaced it with available 3/4" CATV
>coax. The 400 foot run terminated at our new 4 element long boom, 20m
>Monobander
>Yagi at the top of the hill. Even with the old RG-213, that antenna
>is
>the sweetest antenna I had the luxury of owning and using regularly.
>The
>long boom, monoband design, 65 ft, and hilltop combines so well...When
>I
>replaced the coax, I was surprised to NOT see dramatic improvements in
>hearing. Some, but, not dramatic. The coax shield was BLACK with
>corrosion, and may have been only due to water intrusion (I found a
>splice
>had been penetrated by water, even though it was taped fairly well.
>On
>the other hand, the braid may have been corroded due to contamination
>by
>chemicals leaching from the jacket. The cable was old, evidenced by
>the
>dull, chalky black color of the exposed jacket. The dielectric was
>foam.
>I thought though, that copper antenna wire, corroded green for decades
>of
>exposure, only has insignificant loss, as the additional fractions of
>an
>ohm due to corrosion is small compared to the antenna radiation
>impedance.
> The question is, is corroded shield in old coax responsible
>for
>significant additional loss, or is the loss created by moisture
>captured
>by the porosity of the foam dielectric, or by lossy chemicals leached
>into
>the dielectric from the jacket? What is the mechanism for additional
>loss
>in old, contaminated, or moisture penetrated coax?
>
>73, DX de Pat, AA6EG/N6IJ
>"The Contest Station from the Government"
>
>Marina Amateur Radio Contest Station; N6IJ
>599 DX Drive
>Marina CA 93933
>
>aa6eg@tmx.com
>
>
>--
>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
>
>
_____________________________________________________________________
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
|