[TowerTalk] RE:[towertalk]Coax - LONG POST
Press W Jones
n8ug@juno.com
Wed, 18 Mar 1998 17:39:24 EST
Hi Dick - your problem shows up quite often, and we've tried a number of
things to help. Coming back to this point from the distant end of what
I've written, I've decided to post it, hopefully with your approval,since
a number have asked, and your story includes an excellent description of
what many have gone through.
I will warn all that, unless you have a curiosity about "flexible" coax,
this is the time to hit the "delete" key.
I am putting your story first, here, followed by my "mega" reply, and
will tack the original post answering the 9913F/LMR400UF query at the
end.
INSERT 2/17/98 DICK GREEN E TO PRESS JONES:
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~Hi
Press,
I'm interested to know just how flexible the 9913F really is. How does it
compare to RG213?
I think I've brought up the flexibility issue with you before, so please
forgive the repetition if so. I have a U.S. Tower 70-foot tubular
motorized
crankup. When I first put it up, I used LMR400UF from the base of the
tower
to the anetenna (TH7.) The cable comes off the antenna balun, is taped to
the boom, and snakes over to the uppermost coax standoff where it is
attached in two places with cable ties (yeah Kellum grips would be
better.)
>From there, it drops straight down the tower through the eyes of the
remianing coax standoffs. When the tower is retracted, the coax simply
falls
though the eyes of the standoffs and coils on the ground.
I tried creating loops by fixing the coax to each standoff arm, but the
tubular isn't really wide enough to permit enough spacing between the
loops
when the arms are staggered, and there's a lot of stuff on the tower that
the loops can get caught on (like the limit switches, motor housing, coax
arms, etc.) I think this would be a problem with any coax, but the
LMR400UF
was so stiff that it made the situation worse -- the big loops would sort
of
walk around the tower looking for something to get caught on! That's why
I
settled for the coax dropping through the eyes of the standoffs. Later,
U.S.
Tower confirmed that this was the right way to do it (Written
instructions?
Not a word.)
Unfortunately, this turned out not to be the ideal solution. Not very
long
after I installed the tower, I had a near-disaster when the coax snagged
on
part of the motor housing. As best as I could determine, the stiff coax
migrated around the tower when it was being retracted and got hung up in
the
motor housing (more flexible coax would have simply coiled on the
ground.)
The next time the tower was raised the coax snagged hard just as the
tower
was reaching the top. The tower started gyrating wildly and, lucky for
me,
the balun housing broke before the tower did, releasing the SO-239 and
the
coax. The balun and coax were destroyed, so the cost was about $100. Not
bad, considering that $8,000 worth of tower and antenna could have been
destroyed instead.
While the stiffness of the coax was a major cause of the problem, the
real
culprit was the placement of the lowest coax standoff -- at 15 feet above
the motor, the coax was allowed to snake around the tower and get hung
up. I
corrected this by adding two more standoffs, one above the motor and one
below. That means the coax is guided until it falls well below the motor.
I
also built a cage out of hardware cloth around the lower three feet of
the
tower to prevent the coax from getting hung up in the rotor or on the
bolts
securing the tower base. Finally, I replaced the LMR400UF with RG213,
which
is far more flexible.
This setup has worked flawlessly for months. I'm reasonably satisfied
that
there is no way for the coax to get caught on anything anymore (although
I
still check it regularly just in case.)
As you might guess, the only fly in the ointment is the increased loss of
the 80 feet of RG213. I suppose that in itself wouldn't be a big deal,
but
the tower happens to be located over 250 feet from the shack. I already
have
substantial losses in my 250' run of LMR400UF (in a buried conduit), so
the
extra losses in the RG213 just add fat to the fire. I'm considering
replacing the LMR400F with hardline or an equivalent, but in the meantime
I'd love to put a cable up the tower that has losses comparable to the
LMR400UF but flexibility comparable to the RG213. Is 9913F the answer or
is
it too stiff?
73, Dick, WC1M
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
There is an advantage to high flexibility such as the 9913F has, but it
is lost in the velvety rubber type jacket, which doesn't slide easily
across metal, so it, as well as the LMR400UF, aren't the best choice in
your example, unless, perhaps, you provide hi-lubricity eyes in the
standoffs(teflon, delrin, etc) - not tested, as far as I know. Bending
radius minimums are easily violated in the very flexible coaxes, too, and
foam dielectrics, while much better than the old 78%VF class, are still
very compressable than solid, and thus more prone to center conductor
migration.
The seven strand, 9.5 to 10 AWG conductor in the LMR400UF is quite a bit
stiffer than either the 7 strand 13 AWG in RG213 or the 19 strand
9.5-10AWG in the 9913F, so you see the differences are effected in great
part by the jacket.
We've played with an RG213/U type with polyethylene jacket for high
lubricity, and the results are quite promising, if not the loss
characteristics, which stay the same as any RG213(40 to 50% greater than
the 9913 types, str or solid) - no problem to most HF installations of
reasonable length.
An excellent new member is the Davis RF "Bury-Flex"tm, which has
construction similar to the 9913F, but with a tough and slippery
polyethylene jacket, and loss figures approaching the Belden product.
Perhaps overkill for the HF short runs, it is a great choice for VHF -
UHF for all "flexible" installations, at $0.59/ft - a real bargain
considering double the price for the next halving of the loss per
foot(example: LMR600UF).
A major point that is often overlooked in a coax that can be flexed many,
many times is the definition of the terminology. "Flexible, Flexi,
Flexi-flexi, Ultraflex, Superflex, Extraflex, etc, etc, etc.," are all
excercises in semantics and have little significant definitive meaning
without further explanation and specification. Hardline with solid center
conductor is considered "flexible" as compared to "rigid," and levels of
flexibility in hardline become further defined with wall corrugations and
the number of them per foot, still with a solid center conductor,
advertised as the "most flexible coax of its type."
The basic design of flexibility in coax revolves around ease and
convenience of installation with minimal effect on its RF-carrying
characteristics.
A whole 'nother reason for some degree of flexibility is that of
unlimited bending capability, wherein stranded center conductor and
shielding come into play. This obviously applies to rotor applications,
pulling through conduit, tower lowering, and again to ease and
convenience of installation. Here appears a further class, where
"flexible" means "limp as cotton rope," "perfect for rack mounts," test
leads, and so on.
Throughout, the MAJOR point is construction that allows the DESIGN use
without impairment of the signal carrying capability.
Our personal choice of use is met in the design, and we are often
inclined to err. The best example is rotors - too often we use coax that
is too limp, make too big a loop, and otherwise violate bending radius
limits and shorten coax life with misguided good intentions.
This reflector is the very best place I've ever seen for the thorough
airing of such subjects, and if we carry if further by making good
"Elmers" of ourselves to those not blessed with "Towertalk," we become
the true strength and future of the hobby. This, then, is my apology for
this essay if it is a problem to anyone. We do it regularly on the phone
and by E, enough that I thought it might be of value here.
73,
Press Jones, N8UG, The Wireman, Inc., Landrum, SC 29356
use n8ug@juno.com or (864) 895-4195 for tech help
orders only use 800-727-WIRE(9473) or cqwire@juno.com
www.thewireman.com Bargains + Hamfest schedule at
THE WIRE LINE(http://thewireman.com/wireline.html)
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
~INSERT PRESS JONES POST TO TT 2/15
Hello, John - we can offer a bit of insight on the two excellent cables.
They are of similar construction as you probably know, and would both
last a nominal 10 years. The 9913F has more flexibility, if that is
important to you, with a 19 strand center conductor. The 400UF has 7
strands. The loss characteristics differ in that the 9913F is quite
similar to regular 9913 which is quite similar to LMR400. Almost too
close to call, so flexibility is the major factor there. The LMR400UF is
rated at 15% more loss, and is at least that, so if loss is really
important, it comes in second. Price differs quite a bit - we currently
sell the 9913F at $0.70/ft and the LMR400UF at $0.80/ft in 100 ft to
499ft lengths, custom cut.
Both are top quality products from highly respected manufacturers.
Will be happy to supply greater detail on loss, etc if required.
73,
Press Jones, N8UG
On Tue, 17 Mar 1998 13:10:39 -0600 John Kjos <KB9RPM@centuryinter.net>
writes:
>Hi,
> I am putting up some new antennas and I was wondering if anyone
>knows where I can find comparison's between the LMR-400UF and the
>Belden-9913F.Thank you, John KB9RPM
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
END OF POST, 2/18/98
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