[Towertalk] Coax opinions

[Steve Katz]

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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