At 01:04 PM 7/6/2005, you wrote:
>Thanks, Jim. Most of that is way beyond me, but I get the general idea....
>don't use PVC for RF work and expect it to work properly.
Well.. I'd be more optimistic.. Use PVC for RF (I do all the time), but
test it first.
Mostly, you're worried about loss.
One way is if you have an air core inductor that you can measure the loss
of (say you've got it resonated with a capacitor at 14 MHz). Shove the
suspect hunk of plastic in the middle and see if the Q of the coil
drops. If not, you're good to go.
(The other popular test, but not one I use, is to put it in the microwave
and see if it gets hot or melts)
If you need to test HV breakdown, then some sort of lash up HiPot tester
(Oil burner ignition transformer or neon sign transformer) will work.
PVC is kind of hygroscopic, and when it absorbs water, it does change
dielectric properties. If this is important, dry it and
paint/polyurethane/varnish it first (think of it as wood).
It also gets soft when it's hot.
>73, Jerry K3BZ
>
>----- Original Message -----
>From: "Jim Lux" <jimlux@earthlink.net>
>To: "Jerry K3BZ" <k3bz@arrl.net>; "Tom Rauch" <w8ji@contesting.com>;
>"(Reflector) TowerTalk" <towertalk@contesting.com>
>Sent: Wednesday, July 06, 2005 2:05 PM
>Subject: Re: [TowerTalk] Bazooka 1/4 wave balun
>
>
> > At 08:51 AM 7/6/2005, Jerry K3BZ wrote:
> > >Tom...read your website, thanks. I'm looking for a good balun at 6M. I
> > >suppose a 1/4 wave length of braid from an old section of large-diameter
> > >coax could be slid onto 50 ohm coax feedline, but you suggest a greater
> > >spacing than that provided by the coax jacket thickness alone. Couple
> > >questions:
> > >
> > > To provide a larger distance from coax braid to sleeve, would a length
>of
> > >PVC pipe slid over the coax be a suitable material? With, say, a 1.5" or
>2"
> > >OD? If not, can you suggest something?
> > >
> > >Velocity factors for standard coax are available, but how would one
> > >calculate the VF for the sleeve if it's on a 2" OD PVC pipe?
> > >
> > >73, Jerry K3BZ
> >
> > Velocity factor for pretty much any transmission line (including a single
> > wire) immersed in a dielectric is 1/sqrt(epsilon).
> >
> > Something with a polyethylene dielectric (epsilon around 2.2 to 2.4) will
> > have a velocity factor of 0.67 to 0.65. Teflon has an epsilon of around 2
> > (although various dielectric loaded versions can have much higher
> > epsilons), for a velocity factor of .70-.71.
> >
> > (if you ignore the resistive part, and just assume soil with an epsilon of
> > 13, then the velocity factor in a buried wire (i.e. a deep radial) is
>about
> > 0.28.
> >
> > If the dielectric isn't homogeneous, then it gets a bit more complex
> > (hah... really complex is more like it), but for things like foam, where
> > the "grain size" is very much smaller than the spacing or wavelength, you
> > can use the "average". Foamed dielectrics have an epsilon much less than
> > the dielectric itself, because you're really looking at a mixture of air
> > (or nitrogen) and the plastic. Say it's a mixture of air and PE, then the
> > epsilon will be around 1.4 so the velocity factor might be 0.85.
> >
> >
> > If the dielectric is layered, etc., then the analysis gets a bit more
> > tricky, and well beyond the scope of this.
> >
> > SO, Looking at one of my tables, it gives the epsilon for PVC as being
> > 4. If you build a coaxial line with PVC as the dielectric, the velocity
> > factor will be 0.50. But this assumes that it's a "snug fit".. no air
> > gaps, because if there are, the analysis gets a bit more complex.
> >
> > Be aware that many plastic pipes, particularly in larger sizes, are not of
> > uniform internal construction. The outer part may be nice white PVC, but
> > the core might be grungier, and contain carbon black residue from
> > recycling. Other large diameter PVC pipe (particularly for low pressure
> > applications, e.g. sewer or drain lines) has a foamed core, between inner
> > and outer skins. Even if you look at a cut end, it's hard to tell whether
> > you've got one of these varieties.
> >
> > The internal contaminants can make the pipe much more lossy, even though
>it
> > looks the same. The foamed core can make for a very odd and inconsistent
> > dielectric properties, both from RF propagation and from breakdown
>standpoints.
> >
> > Some normal looking plastic also has fillers added to it for various
> > reasons (mass, color, thermal properties, strength, manufacturing
> > processes, cost). There might be a whole raft of fillers with the same
> > properties as far as the pipe maker is concerned, but with radically
> > different RF properties. Black colorants are a good example. Both
>synthetic
> > dyes and carbon black are used. The first has almost no RF effect, the
> > second makes a fine dummy load.
> >
> > Folks building HV equipment, for which plastic pipe is very attractive as
>a
> > structural material (cheap, good insulator-normally, etc.) run into this
> > all the time
> >
> > This variability might account for the variety of accounts of using PVC
> > pipe for RF, ranging from "it works great" to "it's horribly lossy and it
> > melted".
> >
> > It's water pipe after all, not a precision RF properties controlled
>extrusion.
> >
> > >----- Original Message -----
> > >From: "Tom Rauch" <w8ji@contesting.com>
> > >To: "Jerry K3BZ" <k3bz@arrl.net>; "(Reflector) TowerTalk"
> > ><towertalk@contesting.com>
> > >Sent: Wednesday, July 06, 2005 9:23 AM
> > >Subject: Re: [TowerTalk] Bazooka 1/4 wave balun
> > >
> > >
> > > >
> > > > > What is a "bazooka 1/4-wave balun" ?
> > > > > An article I'm reading makes reference to using a
> > > > "balun... or even better, a bazooka 1/4-wave balun" at the
> > > > feedpoint of a quad element, using a 50-ohm coax feedline.
> > > > > Is this a 1:1 balun?
> > > > > Is a "bazooka balun" better than a balun? If so, why? and
> > > > where can I get info on building one?
> > > >
> > > >
> > > > http://www.w8ji.com/Baluns/sleeve_baluns.htm
> > > >
> > > >
> > >
> > >
> > >_______________________________________________
> > >
> > >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.
> > >
> > >_______________________________________________
> > >TowerTalk mailing list
> > >TowerTalk@contesting.com
> > >http://lists.contesting.com/mailman/listinfo/towertalk
> >
> >
_______________________________________________
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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