Ford, VF is not a function of cable dimensions. Cable dimensions
effect stuctural impedance and losses. Vf is strictly a function of
the characteristics of the dielectric material, in this case perhaps
it the proportion of air versus dielectric base material in the foam.
More air in the foam means that Vf is closer to free space
(i.e. 100%). More air also means lower losses - both from the
dielectric and the fatter center conductor that is needed to maintain
the Zo of the line as Vf increases.
73 de Mike, W4EF...........................................
----- Original Message -----
From: "Ford Peterson" <ford@cmgate.com>
To: <Towertalk@contesting.com>
Sent: Saturday, August 10, 2002 8:58 PM
Subject: Re: [Towertalk] Bury - Flex Information
>
> ----- Original Message -----
> From: "Richard Karlquist"
> Sent: August 10, 2002 2:30 PM
> Subject: RE: [Towertalk] Bury - Flex Information
>
>
> > I bought some bury-flex and the
> > velocity factor was out of spec
> > (they claim 85%; it was less than
> > 80%). They sent me some more,
> > and it was still out of spec. I
> > see that the link you posted still
> > claims 85% velocity factor. Note
> > that lower velocity factor typically
> > is associated along with higher loss.
> >
> > Rick Karlquist N6RK
>
> I have noted the same thing Rick -- with just about every other type of
coax
> I've ever tested. It does not surprise me that BuryFlex follows suit.
From
> my experience, any coax, from any vendor, and sold under any type or
quality
> description, needs to be tested when you intend to use it in a VF critical
> application.
>
> Think about it. The VF is related to the physical dimensions of the coax,
> tollerance on the center conductor, the braid, the dielectric, etc. It is
> impossible to control the subtle nuances of the various factors accross a
> length of coax. Grab a spool of house wiring (or any other wire for that
> matter) and measure with a micrometer. Compare to the wire tables. Try
> various samples from the same spool, they will vary slightly. They are
> without exception, always out-of-tollerance--generally erring on a larger
> diameter. Coax is VF is tough to control. If it matters, check it first
> and use the actual VF instead of the "target" specification.
>
> Another related issue: When you test a coax that is new, remember that it
> has not been aged, flexed, or stretched. Now hang the 200' (or whatever)
> hank between two trees and leave it supporting its own weight for a week.
> Now test it again. It will not be the same.
>
> By the way, when you need to test the losses in a cable, don't use VF as a
> predictor of loss. Better to look at the loss of the coax.
>
> My $0.02
>
> Ford-N0FP
> ford@cmgate.com
>
>
>
> _______________________________________________
> Self Supporting Towers, Wireless Weather Stations, see web site:
http://www.mscomputer.com
> Call 888-333-9041 to place your order, mention you saw this ad and take an
additional 5 percent off
> any weather station price.
> _______________________________________________
> Towertalk mailing list
> Towertalk@contesting.com
> http://lists.contesting.com/mailman/listinfo/towertalk
|