Hi Jon, I disagree with you entirely.
The "failure" has nothing to do with frequency, or an impedance bump. The
failure is dielectric breakdown that occurs with high power operation and a
pinched dielectric. It's happened thousands of times, to lots of people,
and it's one of the several reasons that no "soft dielectric" cables are
included in MIL-C-17.
Most recent experience: A friend reported the VSWR skyrocketed on his KT34XA
when he added a new amplifier -- he figured he blew something out on the
beam, but when you think of it, there's very little to blow out on a KT34XA
(maybe the 6kW balun-?). I asked what kind of cable he used, and it turned
out to be Belden 8214, which is a cellular poly (foam) dielectric cable
that's notorious for failing. I advised him, without ever seeing the
installation, to eyeball every place he had taped to coax to the leg of his
tower, or mast, or antenna boom, and that if he untaped anyplace that looked
"pinched," under the tape he'd find the problem.
He did. The cable shorted by dielectric breakdown directly beneath one
point where he had pulled PVC tape too tightly when wrapping the tape around
the cable and tower leg.
Ask anyone who's installed a few hundred antennas, and I think you'll find
we all share such experience.
WB2WIK/6
"If everything seems under control, you're just not going fast enough." -
Mario Andretti
> -----Original Message-----
> From: Jon Ogden [SMTP:na9d@speakeasy.net]
> Sent: Monday, October 07, 2002 2:09 PM
> To: Steve Katz; xppq; Towertalk@contesting.com
> Subject: Re: [Towertalk] coax on new tower ( high desert )
>
> I've not seen this problem ever. I've tie wrapped or taped all my cables
> over the years and have never seen an issue. It's hard for me to believe
> a
> little tie wrap could hurt a dielectric.
>
> Even if you deformed the dielectric a little, you would not notice
> anything.
> Perhaps you would at 10 GHz, but then again, you wouldn't use any of these
> cables at 10 GHz! Come on, let's be practical. The little impedance bump
> seen from a slight deformation of the dielectric will be unnoticeable at
> HF.
> And how will it be a "failure waiting to happen."?? A tiny deformation
> isn't going to make the cable fail. That's just not true.
>
> 73,
>
> Jon
> NA9D
>
> on 10/7/02 9:53 AM, Steve Katz at stevek@jmr.com wrote:
>
> > The problem with RG8 "foam" cables is the same problem that all "soft"
> > cables have, that being it's mechanically fragile. If handled with
> care, it
> > should last a long time. But it should not be clamped, ever, to
> anything,
> > and I'd avoid the use of even nylon tie-wraps with foam dielectric
> cables
> > other than hardline. When taping foam dielectric cables (or spiral, air
> or
> > gas filled dielectric cables that are soft, including 9913, LMR400,
> CXP1318,
> > etc, etc), PVC tape may be used, provided it's not pulled tightly during
> the
> > wrapping, and the tap wrap loosely spirals a few inches of cable to its
> > fixing (boom, mast, tower leg, whatever). If PVC tape is pulled and
> > stretched during wrapping, that is a failure waiting to happen.
>
>
> -------------------------------------
> Jon Ogden
> NA9D (ex: KE9NA)
>
> Life Member: ARRL, NRA
> Member: AMSAT, DXCC
>
> http://www.qsl.net/ke9na
>
> "A life lived in fear is a life half lived."
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