Topband: VF - It is what it is
w8ji at contesting.com
Thu Mar 29 20:23:13 EST 2007
> Tom, what I suspect is the fact that the foam is not tough
> and that the center conductor can slowly ooze away from
> it's center alignment, especially if the coax is strongly
> coiled or bent... High temperatures, sunlight heating,
> etc., will soften the foam and make the drift happen more
> rapidly... I have seen the occasional piece of foam coax
> on my own station where the center conductor was pushed
> sideways almost to touching the braid <one was a piece
> that was accidently kinked around a rotor... While I did
> not bother to do a VF measurement I know that the changed
> geometry will have to affect the VF... I suspect this is
> what is being warned about in VF critical applications...
Hi Denny and all,
I think this is much ado (or doo doo) about nothing. Let's
get rid of this myth right now.
About water ingress...
Perhaps there is a false impression water can penetrate or
soak through the foam. Virtually any foam cable is a closed
cell. I can't recall ever seeing an open cell foam. This
means it is no more susceptible to moisture than any other
cable. CATV foam cables and even 50 ohm foam cables are
commonly available in direct burial types, so obviously
people planning on using cables underground aren't worried.
Neither are people who use foam cables outdoors in the
weather that are regular cables.
The problem with moisture is it wicks through the stands of
the center or shield and contaminates the metal. This leaves
a poor connection between strands and increases the loss
(and might change the VF) even after the cable has dried.
That's why some cables have the center and shield bonded to
the dielectric. We can take a section of foam insulation
Heliax with bonded shield and center and soak the end in a
bucket of water for weeks and there is generally no water
penetration at all past the first inch or less, and that
ingress is only at the conductor to dielectric interfaces
when there is imperfect bonding.
About VF drift with temperature...
The dielectric, unless defective or abused, isn't any issue
at all. The very small issue is a cable changes
characteristics with temperature. As a general rule HIGHER
velocity of propagation cables are more phase stable with
temperature change, not less stable. The can be exceptions,
like Teflon dielectrics that can go through a molecular
change in the dielectric that can amount to perhaps 500 or
1000 parts per million drift with a 20 degree C change, but
there isn't a person on this reflector that has a 160
antenna directive enough to ever begin to notice that
To put this all into perspective I had a phasing system that
used 2500 feet of coax back to an antenna and another 2500
feet coiled up in a closet in the house in a phase
equalizing system. This put the cables at different
temperatures as outside weather varied and inside
temperature stayed within perhaps ten degrees. I used those
cables to set a null on my own transmitted signal. Over all
that length of cable the accumulated phase drift was less
than 30 electrical degrees out of around 3300 electrical
degrees... and I used CATV foamed cables.
I wouldn't spend one nanosecond of time worrying about the
drift in a 90 degree long line, especially since the general
trend is high VF lines drift less for a given length. Who
really cares if it drifts off .05 degrees?
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