Pete Smith N4ZR wrote:
> I feed my 160m receiving antenna with about 300 feet of RG-6, which
> supplies power to an amplifier at the antenna. Recently, I started
> seeing an high-resistance short across the coax (~30-50 ohms). I took a
> junction apart and discovered evidence of current flowing across the
> double-female, as wellas some water in the connector. I changed it out,
> and also changed both connectors on the cable, and in the process
> discovered that the braid and the first foil (it is quad-shield) appear
> to be somewhat corroded (there is a powdery grey residue that comes
> out). I reconnected the junction, and within a few days both the short
> and the moisture were back. In checking out the problem, I have cut
> back a couple of feet on the cable, and the grey residue is still evident.
>
> Is water ingress a known problem with RG-6? Any particular brand known
> to be better? If I'm going to replace the whole feedline I might as
> well do what I can.
>
>
Pete,
Yes some brands or off brands of RG-6 sucks up moisture like a sponge.
They are a foam product and.with diurnal sunlight heat and night time
cooling on the jacket laying out in the sun during the day, would IMHO
act as sort of a pump. You can mitigate this a bit with a cable that
has be flooded with a special sticky goo (flooding compound). I have
found out here that since VF and cable attenuation are not a big thing
that using good quality Belden RG-59 with a copper braid and solid
interconductor is a much better solution.
In CATV Foam, where UHF attenuation is always a factor, without the
flooding compound inside the jacked it doesn't take long to get to the
point where the foil is pulverize to aluminum oxide contaminating the
cable run.. If as in your case you run DC for pre-amps and/or
switching through the cable to relay boxes and switching of reversible
Beverages...expect even quicker deterioration as some forms as
electrolysis is certain. I have seen running DC make the female F
connector center pin actually dissolve. Always look for as tarnish on
the center conductor where it exist the RG-6 F-connector. Scrapping it
with a knife will only make things worse in the long run as now you are
down to steel and rather than copper plated steel. In fact DC over the
cable made things so bad here I now use separate feedlines for even the
DXE-Reversible boxes. Yes termination of the unused port is important
but this is done back at the shack with a self terminating Dynair 12x1
Video Passive switcher from a former TV station.
Regular RG-6 connectors are not water proof either except except some
are available with and O ring and sealing gel from gel from MCM for a
few pennies more.. Even tape wrap and then butyl over that will help
at the connectors. But changing a connector and removing it is very
bothersome when the connector is covered with Butyl.
Even protecting the connectors will not stop cable deterioration if one
little gnaw from of a small rodent, a peck of a robin looking for a
worm, or pin hole opens, or anything opens the flood gate for ingress
of moisture along the run. Using inexpensive RG-6 is fine if you plan
to change it every 6 months or less. If you need some long term
protection than the expensive self sealing flooded heavy jacketed RG-6,
direct buried in the ground is the way to go. This keeps the rats, weed
wackers, and lawn mowers from destroying the quality of the cable. It
also provides some reduction in heat effect.
There is one positive thing about poor cable and that on TB is the last
place where it really shows up. I have had complete outer shield breaks
and still worked some nice DX. In some case a few db's of attenuation
are not all that noticeable. But the dissimilar metals can be the point
at which birdies and signal re-rectification can make weak signal
reception very difficult.
The MFJ analyzer will not tell you the story and may even present a
false positive reading if there is significant attenuation in the
cable. I found the best test is to measure the Beverage feed point
transformer, which is hopefully floating and not grounded, and based on
the cable length I get about 1.2 ohms center to shield at the shack. If
it starts to creep up then you may have some shield deterioration
offering cumulative resistance over the length.
Most of the TB activists I have spoken to about this expect to do a lot
of Beverage maintenance both in the spring when the thaw melts and then
again in the fall before the contest season begins. Actually it never
ends as if you can get past the thunderstorms there is plenty of
trans-equatorial DX available. Nice thing about replacing Beverages is
the mental impact in thinking that you are going to hear something now
you didn't or couldn't hear before. You could be right.
73
Herb Schoenbohm
_______________________________________________
UR RST IS ... ... ..9 QSB QSB - hw? BK
|