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.
>
>_______________________________________________
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_______________________________________________
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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