Topband: VF - It is what it is

[Tom Rauch]

> 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 
difference.

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?

73 Tom