Jeff Liebermann wrote:

On 10 Sep 2011 12:57:07 GMT, dave wrote:

I have worked some of the premiere sites (Cedar Hill, Mt. Wilson, South
Mountain in Phoenix, Mt. Harvard, Senior Road in Houston, the John
Hancock building, the router room at Channel 4, etc.) and I have never
seen a blanket ban on LMR because it leaks.

It's not leakage. The problem is the plated steel wire used over the
foil wrap on the shield. The steel is non-linear and subject to PIM
(Passive Intermod) problems. The aluminum foil to steel junction can
easily become a diode if the mylar coating is penetrated. I've seen
it with LMR-400 on a lab test similar to the YouTube video that you
apparently didn't watch. The problem was bad enough that Times had to
conjure a special mutation of LMR-400 with low PIM:
http://timesmicrowave.com/products/lmr/downloads/126-129.pdf
I think (not sure) that the only difference is that the braid over the
foil is now aluminum.

The initial reaction of most techs is that the PIM is sufficiently low
level that it would not have an effect on receiver performance. Wrong.
In cell sites, where squeezing every dBm of sensitivity out of the
receiver is necessary to deal with perpetually marginal cell phone
handset signals, that install cryogenically cooled front ends and
tower mounted preamps to do this, can definitely see the effect. Look
at a cell site install and try to find anything other than Heliax.

"SITE MANAGEMENT EQUIPMENT INSTALLATION RULES" (sample)
http://www.repeater-builder.com/antenna/site-stuff/radiositerules.html
"C. All cabling from the building to tower including on the
tower to the antenna, shall consist of a minimum of 1/4 inch
jacketed corrugated copper "Heliax" type cable. Semi-rigid
"LMR-400", "LMR-600", etc. cable and non-rigid cable, such as
RG8, RG, 213, RG-214, RG8X, etc. will NOT be used as
transmission cable exiting the building."

In most cases, this has been extended to include internal coax cabling
that carry transmit RF.


Cell sites are a different animal . We were talking about 2-way,
point-to-point, VHF/UHF broadcast type sites. YouTube is pretty intense.