> Steve:
>
> Very interesting! Did this happen only under high power?
>
[Steve Katz] Yes, Kevin, it's a high-power phenomenon, which is
logical since the voltage across the line at 1.5kW (VSWR = 1.0) is 273.9V,
pretty close to the breakdown limit of cellular polyethylene for the
dielectric thickness used. Obviously, a mismatch antenna condition can make
the situation worse, as can high ambient temperatures or considerable
elevation (reduced barometric pressure). But I have "flashed" LMR400 at
lower power levels, too -- it takes longer, or more extreme conditions. As
I said, it's not a "receiving" phenomenon at all. The same thing
(dielectric breakdown from cable tying) happens with RG8 "foam" coax like
Belden 8214 (one of the very worst for this) and the old Times FM-8 which
was very similar to LMR400 but used only braided shield and no aluminized
mylar film. -WB2WIK/6
> Thanks,
> Kevin.
> ------------------------------------------------------
> Kevin Hemsley
> Systems Engineer
> Microserv Computer Technologies, Inc.
> kev@ida.net
> NF7J
>
>
> ----- Original Message -----
> From: "Steve Katz" <stevek@jmr.com>
> To: "'Kevin Hemsley'" <kev@ida.net>; <Towertalk@contesting.com>
> Sent: Wednesday, August 07, 2002 3:29 PM
> Subject: RE: [Towertalk] Coax opinions
>
>
> > Hi Kevin,
> >
> > I, too, have used tons of LMR400. Here's what happens with a cable tie:
> >
> > It would indeed be difficult to compress the dielectric so severely that
> it
> > shorts out due to mechanical failure. I certainly agree with that.
> > However, in my own home installation, here's what's happened, not just
> once,
> > but repeately:
> >
> > -Cable tie the LMR400 to tower leg. No superhuman strength, or even
> tools,
> > involved. Just pulling an 8" tie (like Panduit) snug, by hand.
> > -Run 1500W through the cable (50 MHz) on a hot day.
> > -LMR400 dielectric breaks down precisely at the cable tie point,
> creating
> at
> > first intermittently high VSWR, then, ultimately, complete failure at
> that
> > point.
> > -Examination and dissection of the cable indicates brown/black charring
> at
> > failure point, where cable tie was installed.
> >
> > Pretty self-evident. Mean Time To Failure: About five minutes, under
> > conditions described.
> >
> > I can repeat this, any time you'd like to come visit and see!
> >
> > I've had other problems with LMR400, too. Those have been documented in
> > various magazines over the years, but all are "transmitting" related,
> and
> > related to using high power. Never had a problem in "receive only"
> > applications.
> >
> > 73 de Steve, WB2WIK/6
> >
> >
> >
> > "If everything seems under control, you're just not going fast enough."
> -
> > Mario Andretti
> >
> > > -----Original Message-----
> > > From: Kevin Hemsley [SMTP:kev@ida.net]
> > > Sent: Wednesday, August 07, 2002 9:35 AM
> > > To: Towertalk@contesting.com
> > > Subject: Re: [Towertalk] Coax opinions
> > >
> > > I like LMR400 and I have used LMR600 for a longer runs. I use buryflex
> up
> > > the tower and for rotor loops.
> > >
> > > Steve Katz"<stevek@jmr.com wrote:
> > >
> > > > They cannot be cable-tied with metallic or nylon tie wraps; they
> > > > cannot be securely clamped; they they should all be gently
> > > > supported in several locations along a 150' run.
> > >
> > > This is the first that I have ever heard of not cable tying LMR400.
> LMR400
> > > is pretty tough stuff. You would need an extremely tight cable tie to
> > > crush
> > > the dielectric. I'm not sure it's even possible to tighten a nylon
> cable
> > > tie
> > > enough to make a dent in LMR400. Most wire ties are not installed
> > > bone-crushingly tight, so it would greatly surprise me if this were an
> > > issue.
> > >
> > > We've installed tens of thousand of feet of LMR400 using cable ties
> and
> > > I've
> > > never seen a problem. I searched Google and the Times Microwave site
> and
> > > couldn't find any mention of the limitations you list. Do you have a
> > > reference that describes the problems associated with using wire ties
> with
> > > LMR400 cable?
> > >
> > > Thanks,
> > > Kevin.
> > > ------------------------------------------------------
> > > Kevin Hemsley
> > > kev@ida.net
> > > NF7J
> > >
> > >
> > > _______________________________________________
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> > > http://www.mscomputer.com
> > > Call 888-333-9041 to place your order, mention you saw this ad and
> take
> an
> > > additional 5 percent off
> > > any weather station price.
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> >
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