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Author Topic: PVC Center Insulator  (Read 6741 times)
KF7LCE
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« on: February 27, 2011, 03:35:25 PM »

I was building my first homebrew antenna (http://www.io.com/~n5fc/notebk_ant.htm; neat stuff!) when I hit an obstacle. My local Cellphone Shack does not carry center insulators, but the instructions mentioned that one could be fashioned out of PVC. How do I build this? Where do I drill? How do I insert the wire?

Thank you for the help!
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N3OX
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« Reply #1 on: February 27, 2011, 04:01:18 PM »

There's all kinds of stuff you can do. 

Some people get fancy and install a connector:

http://www.w0bxo.org/Dipole_multiband.htm

Sometimes you can just be really simple.  This isn't PVC pipe, but one big hole and two little holes in a piece of PVC a couple inches long would do the same thing:

http://degood.org/coaxtrap/center.jpg

No real reason why you can't replace the "dogbone" insulator in this picture with a piece of PVC with a couple holes drilled through at the ends:

http://www.skccgroup.com/nt9k/images/DipoleConst_A.jpg

Just drill straight through like this:

http://rogertango.com/articleimages/image.aspx?filename=mbdipole-21.jpg&width=450
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73,
Dan
http://www.n3ox.net

Monkey/silicon cyborg, beeping at rocks since 1995.
W5FYI
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« Reply #2 on: February 27, 2011, 05:14:55 PM »

The next time you're near a farm supply store, look there. I've seen both plastic and ceramic insulators there. I think I've even seen them in Ace Hardware stores, too.

Essentially any insulator will work; PVC pipe, plastic rod, the cut-off tops of 1- and 2-liter soft drink bottles, even plastic rope. Where you want to be careful is to avoid using lossy material at voltage nodes, like the ends of 1/2-wave dipole antennas.
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WB6BYU
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« Reply #3 on: February 27, 2011, 07:08:01 PM »

Basically the insulator has to provide strain relief for the wires while insulating them from
each other, and a way to connect them to the feedline.  There are millions of designs that
accomplish those goals.

I have some broken PVC outdoor privacy lattice that I have been cutting insulators from.
A good staring point is to cut a roughly rectangular piece, the drill a hole at each end to
tie the wires to and one in the middle for the coax to pass through.  I've done the same
thing with a slice of radiator hose (the wires and the coax tie though the hole in the
middle for strain relief, then I connect the ends.)  Lexan or polycarbonate are also good
choices.  I've had problems with acrylic sheet as it tends to be more brittle, but it should
work if you don't pull too hard on the antenna wires.  Dan's second link is a good example
of how simple it can be (though I'd secure the antenna wires a bit better by twisting them
rather than relying on the tie-wraps.)

For an indoor dipole the material isn't too important, but for outdoor use you want to use
a plastic that doesn't degrade in the sunlight.  For temporary use you could use the plastic
from a milk jug or peanut butter jar lid.  Just make sure it can withstand the stress of the
the pull on the antenna wires and the weight of the feedline hanging down.

Because I am often experimenting with different antennas, I often terminate the feedline
with a couple lugs under stainless screws, then put washers and wing nuts on the other
side of the insulator to attach the wires.

But remember - a key requirement is strain relief.  It is bad practice to put mechanical
stress on an electrical connection.
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W0BTU
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« Reply #4 on: February 28, 2011, 11:28:22 PM »

The next time you're near a farm supply store, look there. I've seen both plastic and ceramic insulators there.

Yup! Ask for electric fence insulators.
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W5DQ
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« Reply #5 on: March 09, 2011, 03:33:13 PM »

I was building my first homebrew antenna (http://www.io.com/~n5fc/notebk_ant.htm; neat stuff!) when I hit an obstacle. My local Cellphone Shack does not carry center insulators, but the instructions mentioned that one could be fashioned out of PVC. How do I build this? Where do I drill? How do I insert the wire?

Thank you for the help!

Although PVC tubing can be cut and drilled to make really handy center insulators or even using a 1" Tee connection works nicely too, one thing to be aware of is long term UV exposure. I live in the Mojave Desert and as such we get an abnormally high UV exposure levels during the summer. Standard white Sch 40 PVC tubing one might use for antennas doesn't last long out here if not protected. Non-protected it will turn deep dark brown in a couple of months and within a year it becomes brittle and will snap into easily. I do use it for some things and when I do, I drill the PVC and prepare it as normally I would and then I take PVC cleaner and give it a good covering. This makes the surface accept PVC 'weld' under normal plumbing use using PVC glue but in my case, after allowed to dry thouroughly, it make spray paint stick adhere very well to the PVC. I usually give the PVC I use at least 2 coats of a non-metallic based spray paint, enamel works well and then hang it up in the world's largest solar oven (outside) for a day in the sun to literally bake on the paint. Once I attach all the wiring, the PVC insulator will last several years with proper paint job.

Just something to consider....

Gene W5DQ
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Gene W5DQ
Ridgecrest, CA - DM15dp
www.radioroom.org
K3AN
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« Reply #6 on: March 14, 2011, 10:00:18 AM »

When we bought our house almost 6 years ago, the metal property corner stakes, which were almost completely buried, were highlighted with a section of 1" PVC pipe pounded part way into the ground right next to the stakes. These PVC pipes are as white today as they were when we moved in.
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W0BTU
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« Reply #7 on: March 14, 2011, 10:15:28 AM »

Not all "PVC" pipe is the same.

I haven't seen any turn dark, but I sure have seen white PVC pipe get brittle and crack, without any discoloration.
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WB6BYU
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Posts: 13356




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« Reply #8 on: March 14, 2011, 10:17:19 AM »

PVC pipe varies in UV resistance in different parts of the country - as does the exposure level.
The two don't necessarily correlate - UV resistance is more important when pipes are likely to
be left exposed, which won't be common practice in very hot or very cold environments.

The PVC lattice that I use is designed for outdoor use, and we've had many pieces up for
10 years.  They are showing cracks from mechanical stress (especially flexing in the wind)
but the plastic itself still seem stable.

When in doubt, talk to your local plastics supplier about what they recommend for outdoor
applications.  They probably also have a bin of scraps, often available by the pound, that
are a great source of insulator material.
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W5DQ
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« Reply #9 on: March 14, 2011, 10:53:22 AM »

Since I am not a chemical engineer nor a PVC expert, all I can say is from personal experience. Like I stated, I live in the Upper Mojave Desert (80 mile west of Death Valley) with summer temps over 110 for days on end normal and where summer daylight hours can exceed 12 hours with hot bright sun exposure (hence high UV exposure) can be very long, 8 hours or more depending on location and shading.

No doubt that in other parts of the country, PVC may last a long time outside (maybe forever) and be a nice clean white as first installed but every piece of PVC pipe I have ever used AT MY QTH externally for anything without some sort of protection from exposure has turned dark and the thinner stuff becoming brittle. Thicker walled pieces get brown but appear due to their thickness aren't brittle enough to crack.

" .... when pipes are likely to be left exposed, which won't be common practice in very hot or cold environments ... " -- I would agree under normal use transporting fluids this would probably spell failure and flooding depending on source but for antenna work, exposure in hot environments is exactly what is expected here. Again each QTH usage and results may (and most likely) be different than here.
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Gene W5DQ
Ridgecrest, CA - DM15dp
www.radioroom.org
N2EY
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Posts: 3895




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« Reply #10 on: March 15, 2011, 09:58:23 AM »

I have used white PVC DWV pipe for antenna insulators and traps for more than 25 years. Never had a problem with UV deterioration, even after more than 5 years of service in direct sunshine.

In all cases I have sprayed the finished insulators and traps (after all connections were made) with several coats of clear Krylon, to protect the connections.

My QTHs have been in EPA, which is quite a different environment than W5DQ has to deal with. We get hot humid summers, cold snowy winters, lots of wind, and some acid rain and smog. But I'm only a few hundred feet above sea level, so the UV exposure is probably a lot less. I have to worry less about UV and more about rain, snow, and worst of all ice.

I learned years ago that it's a good idea to check all components of wire antennas every so often, and replace anything that looks iffy. How long "every so often" is depends on QTH and materials used. (This is one reason I still keep a paper log book - the back side of the log sheets keeps track of such things).

I'm always on the lookout for sales on braided nylon rope (main halyard) clear Krylon spray, RG-8X coax, etc. Not because I need it "now" but because I'll need it eventually.

The one component which I have found to be UV sensitive, even in my environment, is nylon cable ties. The white/clear ones last only a few years before becoming brittle and failing, while the black UV-resistant ones haven't failed me yet.

OTOH, I've used plastic coated house wire (the conductors from NM-type cable) and the insulation just won't fail. Some pieces of wire aloft today have been recycled from antenna to antenna for over 20 years and it's still hanging up there. Go figure.

73 de Jim, N2EY
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W7ARE
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Posts: 16




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« Reply #11 on: March 15, 2011, 02:54:07 PM »

I have to second the fellow's comments about PVC exposure in the desert. I'm near the painted desert in Arizona, at 6000 ft. + elevation. Exposed PVC here does turn brown in places, becomes brittle and fragile, and the typical PVC glued joints give way in a year, or two at the most when exposed. We contend with extremes of temperature and high UV exposure. I'm not a chemist, but I attribute the deterioration to the sun and temperature. Whatever the actual agent, it does happen.
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KA1DBE
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Posts: 122




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« Reply #12 on: March 17, 2011, 10:43:26 AM »

If it is a coax feed antenna, I usually just use a small section of pipe and wrap the coax around it and then ziptie it together on the bottom and make the connection to the elements.  If it is for ladder line, I drill 6 holes in it and weave the antenna elements through the the outer two and feed the ladder line up through the inner two and make the connection on the top.  I usually paint the pvc before for some U/V protection and just general looks.

Good luck with your antenna project.

73's,
Jeff
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