> Jeff: I've used the Commscope smooth wall professionally, and it was
> hard to work with BUT it had significantly lower losses than the
> corrugated copper lines.At 2.4 GHz, we could use 7/8" smooth instead
> of 1-1/4" corrugated. I do agree, the corrugated copper is easier to
> bend and route, but one (hopefully) only has to do that once.
Even though Commscope had less loss for a given diameter cable, it was often
just as cheap to upsize regular Heliax, and then I didn't have to risk
having the tower crew put a kink in a line and eat the entire cost of the
run. Of course, a bigger line means more windload, so upsizing the cable
isn't always a viable trade-off.
> The
> aluminium cable that failed you-- was it Trilogy?
No, Andrew AL7. And a few years before, we replaced two long runs of older
Prodelin aluminum-shield air-dielectric cable that had "turned to powder".
The autopsy revealed that, over the years, water came in at a couple of
ground kits; those were from back in the day when ground kits used flat bare
copper braid that acted like a wick rather than the #6 THHN that most ground
kits use now. That cable was fairly old, on the order of 20 years. But if
it were copper, more than likely it would still be servicible today.
> Their construction
> methods differ substantially from other manufacturers. On one job
> where underground tunnels were flooded, all the copper coax (it was
> radiating coax, so not the best example) by Andrew and RFS had to be
> replaced, but the Trilogy did not.
There are many thousands of miles of buried Heliax in AM broadcast arrays
out there. Granted, most aren't sitting in pools of water (though those in
high-conductivity swamps may as well be), but they are, at the very least,
in contact with wet soil. I've had few failures of buried Heliax runs, and
those that I did have fail were always caused by some external trigger.
Like Comcast trenching across the array field without permission, chewing up
feeders, sample lines, control cables, and hundreds of radials along the
way...don't get me started about Comcast...but I digress.
> Some connectors had some water, but they cleaned up and the water did not
migrate down the cable.
So you're saying the water got in a connector, not along the cable itself.
That makes more sense. I'd never do an underground connection or splice.
In fact, last summer we had to dig up some buried 7/8" because an
underground splice had burned out. Fortunately the splice was not far from
the end of the run; the cable had been extended about 20' as part of a tower
replacement sometime in the past. TDR/FDR'ing the line made it easy to
figure out where the failure was.
> As for
> PIM, copper is usually a bit better than aluminium, but the difference
> is not particularly relevant in most ham use. I'm not even sure if
> Trilogy is less expensive than copper. I wasn't touting cost -- just
> resistance to water migration. Take a look at Trilogy cables and how
> their construction differs from other AL and CU cables.
I'm famiar with Trilogy, we have used it in the past when a client supplied
it, we just don't recommend or stock it. If you want an example of a cable
that was really impervious to water between the shield and jacket, look at
some old Prodelin Spir-O-Flex. The jacket was bonded to the shield; you
couldn't just slit the jacket and peel it off like you can with modern
cables, you had to chip and scrape it off. A couple of decades ago there
were many thousands of feet of surplus 7/8" Prodelin floating around, at
least in this area (northeast US). We still come across it from time to
time. It's a bear to work with, and the losses weren't anything to write
home about, but it definately was rugged and never had problems with water
ingress.
Regarding PIM, yes, for most ham applications it's not an issue. But in the
ham realm I do a lot of repeater work too, and anywhere you have duplex RF
on a cable, or when you are in a high-RF environment with other strong
emitters in proximity, PIM is a consideration. A dry aluminum cable isn't a
PIM risk in and of itself; PIM problems with aluminum cable are primarily
related to the connector-to-cable interface. Over the years, manufacturers
have gotten better at making connectors that minimize PIM problems on
aluminum cable, but again it's one of those things where, to me, the slight
savings isn't worth the risk.
For soft/braided cables, the direct bury LMRxxx-DB series with icky-pic
flooding the braid offers an added level of protection from water ingress.
For ham projects, any time something like LMR400 is called for, I use the
flooded direct-bury version.
Since we're on the topic of wet cables, in today's digital world, running
Cat5 up a tower or burying it isn't uncommon, and for that, I have only one
recommendation - Superior/Essex BBDGe series (or BBDG6A for Cat6A). Google
it for details. Good stuff.
73.
--- Jeff WN3A
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