[TowerTalk] Shields

Tue Oct 12 21:40:43 EDT 2004

Just an an anectode about Coax problems.

Years ago when I had a big HF System, which included
large monobanders and 80 meter vees and quads.

I was trying to work OH2BH on the short path on 75.
It had been a very cold winter up til then and I was
cranking 1500w to him so he could hear me S3.  Just a
West Coast problem!!
 As I was transmitting to him, I noticed drips of
water coming out of the coax switch that controlled 75
meters!! Mind you the SWR still looked good and I was
getting full output!
I am still amazed today to think that I didn't blow
anything up, more so that the impedance didn't change
and cause a SWR fluctuation that I could easily see
and look for a problem.. When I saw the Drips I ceased
my transmission. After inspecting the coax. There was
a split in the outer jacket caused by the incredible
cold weather we had gone through. Then our normal
rainfall decided on it's path down the coax.
Maybe the water caused a perfect dialectric, and
stayed away from the center conductor??? I don't know.
But having good shielding is very important, I do
know!

Johnny
KE7V

--- Tom Rauch <w8ji at contesting.com> wrote:

> > I'm sure I'll take some heat on this, but oh well.
> 
> Hey, just put the heat shields up. There seem to be
> enough
> to go around.
> 
> > First, I would consider LMR-400 to have a dual
> shield, not
> to be double
> > shielded.  It has a foil shield which likely
> provides 100%
> shield
> > covered with a braided shield.  Personally I
> believe this
> provides a
> > very good shield for a reasonable cost.  Double
> shielded
> coax such as
> > RG-214 and RG-142 have two braided shields over
> the
> dielectric, my hunch
> > is that no braid is tight enough to ensure 100%
> shielded
> so one over the
> > other pretty much gives you that while still
> maintaining
> the
> > flexibility.
> 
> I think the problem comes from the fact we confuse
> light
> with 14MHz signals Daron. We can see through the
> shield, so
> we assume RF can leak through. Fortunately our eyes
> don't
> run at radio frequencies, and that large .005 inch
> gap looks
> pretty damn small to a wave that is 2 meters long,
> let alone
> one that is 40 meters long.
> 
> > varying frequencies and power levels.  There are
> reasons
> that higher
> > power (lower loss) feedlines go to a solid shield.
> 
> A large part of the reason, if not the largest, is
> resistance in the shield. Woven conductors even with
> a good
> lay and untarnished conductors has a few times the
> resistance of a smooth solid conductor.
> 
> Get any moisture in the cable and the loss becomes
> much
> worse. As a matter of fact shield corrosion is the
> bulk of
> the reason a cable gets lossy after moisture
> ingress.
> Current tries to stay inside the shield, and every
> time the
> lay wraps outside the current tries to migrate
> across the
> poor connection to the incoming stands.
> 
> The braid of RG-8U cable, when pressure is removed
> between
> lays, has about the same resistance per foot  as 
> #14 or
> smaller solid wire.
> 
> If you lay smooth foil against the dielectric,
> corrosion
> doesn't matter nearly as much. As matter of fact, it
> makes
> almost no change at HF.
> 
> > I think Tom's point is that you don't need to have
> dual
> shield or 100%
> > shield for many applications, and frankly spending
> the
> extra money for
> > it may be a waste.  I've had electrical engineers
> spec
> quad shield RG6
> > coax inside a grounded metallic raceway to 'reduce
> interference'.  I'm
> > no PE, but what will that 5th shield catch that
> the first
> 4 don't?
> 
> Nothing. It probably doesn't catch anything the
> first shield
> doesn't in most cases.
> 
> > outer jacket.  My experience with CATV and other
> services
> taught me that
> > a dual shield was considerably better at keeping
> signal in
> the wire than
> > was a 90% shield, so it makes sense to me for some
> installations.
> 
> I use a dual shield too. The inner foil makes the
> nearly
> perfect shield, and the light outer shield allows my
> connectors to get a "bite" on the foil. Think about
> the end
> connection problem. It is impossible to guarantee a
> good
> connection to the foil, but the braid is easy and
> reliable
> to sandwich in a crimp connection. The trusty foil
> that does
> all the shielding only lays inside the connector
> shell. The
> metal shell crimps down only on the braid, and the
> braid
> actually couples to the foil.
> 
> I think we are mostly on the same page. If you can
> see
> through the shield light can certainly get through.
> Lower
> frequencies still have a pretty tough time.
> 
> Save the extra shield money for something useful,
> like a
> gallon of gas.
> 
> 73 Tom
> 
> 
> _______________________________________________
> 
> See: http://www.mscomputer.com  for "Self Supporting
> Towers", "Wireless Weather Stations", and lot's
> more.  Call Toll Free, 1-800-333-9041 with any
> questions and ask for Sherman, W2FLA.
> 
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