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Author Topic: Using PVC for Mast  (Read 1078 times)
KD2FAH
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« on: April 10, 2009, 04:04:50 AM »

I need to build a mast on a limited budget and was thinking of using PVC. I have a G5FRV antenna that I would like to string from my house to a PVC mast that would be used to support the other end of the G5FRV. A tree was my first choice but no suitable trees near the house. I was intending to use 2.5"-3" PVC in three or four ten foot section. I also intended to reinforce the PVC with 2" circular wooden poles that I have in ten foot sections. The base of the mast would be mounted to a 6 foot section of 4"x4" wooden post that is already sunk into the ground. To join the sections together I would use oversized PVC and screw into place. What are your thought on even using PVC in this manner? Any suggestions on this design or an alternative? I thought the wood inserts would stiffen up the PVC and reduce any listing.
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N2EY
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« Reply #1 on: April 10, 2009, 04:18:19 AM »

For all that trouble and expense, you'd be better off with metal TV masting or an all-wood mast. PVC pipe simply bends too much.

btw, the popular antenna you refer to is a G5RV, named for the inventor.

73 de Jim, N2EY
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WA3SKN
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« Reply #2 on: April 10, 2009, 05:05:01 AM »

As a center support, you probably could get away with it.  But using it for end supports on a commercially made G5RV (102 ft of 14-12ga wire) probably won't... even guyed I think the stress will be too much.
Maybe if you used three supports, both ends and the middle,... but that would raise the cost some.
73s.

-Mike.
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KB9CRY
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« Reply #3 on: April 10, 2009, 05:32:03 AM »

What are your thought on even using PVC in this manner?

Bad idea.  PVC pipe is designed to hold water.

Any suggestions on this design or an alternative?

Metal mast
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WX7G
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« Reply #4 on: April 10, 2009, 05:39:44 AM »

The wooden poles alone make a nice mast. I use 1 x 2 lumber for masts up to 32'. Guying at one point a few feet from the top works well.

The idea with a light mast like this is that if it does fall down it doesn't damage things.
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KB1LKR
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« Reply #5 on: April 10, 2009, 07:44:58 AM »

Use stainless steel, steel, aluminum, wood or fiberglass instead, if possible, PVC won't hold up well in this service. If you must use PVC, the larger the diameter and shorter the unsupported length the better also, use gray UV resistant PVC electrical conduit vs. white plumbing tubing.
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KB1LKR
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« Reply #6 on: April 10, 2009, 07:50:18 AM »

And: G5RV was R. Louis Varney, 1911-2000 (SK).
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N3OX
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« Reply #7 on: April 10, 2009, 08:04:04 AM »

PVC is heavy and flexible.

I wasted a fair bit of money on it in the past.

Good for 2m antennas, coil forms, that kind of thing.

But not great for a tall support for any HF antenna structure.

You can probably get away with it if you can guy it in several directions at several levels.

You essentially need a small tubular tower to do what you want to do in a self supporting fashion.  If you think about your typical decent antenna tree, which would be adequate, by the time it gets to thirty feet high it's possibly five or six inches in diameter at the base tapering down as it goes up, and it's got literally tons of root mass.

Steel is stronger and stiffer than wood so a few inch diameter steel pole is probably sufficient for 30 feet, but you'll basically need a concrete base.

If you hung the center of the G5RV at your house and just had to support the weight of the wires and stand up in the wind at the ends, it would be a bit different and I could recommend some fiberglass poles that might do the trick.

But to hang it full span between a house and a self-supporting mast and keep the center up at a reasonable height?  Not so easy, and not a job for PVC & wood

73
Dan
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73,
Dan
http://www.n3ox.net

Monkey/silicon cyborg, beeping at rocks since 1995.
N1LO
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« Reply #8 on: April 10, 2009, 11:30:01 AM »

As already mentioned, pvc is completely unsuitable.

Chain link fence top rail makes nice masting and has a far better stiffness-to-weight ratio. It's nicely galvanized for years of outdoor service. You should be able to walk up 3 sections (30') maximum.

I recommend a pully at the top for your antenna support line, two back guys around the 25' level and no more than about 15 lb tension.

If you're rigging the antenna flattop, you'll need a lot more than 15 lb tension to control the sag. In that case, I'd recommend 20' max, with the pulley and guys at the top.

GL,

Mark
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WB2WIK
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« Reply #9 on: April 10, 2009, 12:32:11 PM »

Mark, N1LO's right about "a lot more than 15 lbs."

A standard G5RV is 102' long and should be at least 35' above ground to let the ladder line portion of the transmission system hang down from the center.  The coax can lay on the ground if you wish, but the ladder line shouldn't, and that part is normally about 31-1/2' long.

To pull such an antenna reasonably taut so the center doesn't sag 10' or more requires a pull tension of about 50 lbs.  If you're pulling up not only the antenna and ladder line, but also the line coupler and some coax, it can get to 100 lbs really quickly.

I'd use at least thickwall aluminum or steel masting, and guy it.  To prevent antenna-to-guy wire coupling, the guys can be strong rope tensioned with turnbuckles, and re-tensioned now and then since rope stretches.  I use doubled braided Dacron or Kevlar rope for a similar installation here.

WB2WIK/6



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KB3RHV
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« Reply #10 on: April 10, 2009, 01:41:51 PM »

For a short mast under 10' that will have a small VHF antenna mounted on it, PVC is ok but above that, it bends and sways like mad. And I am talking the 3" diameter stuff. Also, UV light kills PVC fast and makes it brittle over due time.

I would look at building some thing like this......
http://www.wv7u.com/mast/mast.html

We built one for may buddy and it is working out pretty well for him.
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K6REA
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« Reply #11 on: April 10, 2009, 07:42:34 PM »

for the use you are intending, i think that you should purchase one of the fiberglass masts from ebay.
i just bought one in the last few weeks for that exact reason/use.
it works just fine.
i think the total cost was about 70.00 with shipping.
here is the one i bought.. it works perfectly for that.
http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=320343927620

Kevin
K6REA
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KZ1X
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« Reply #12 on: April 10, 2009, 09:18:24 PM »

http://www.tmastco.com

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N3JBH
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« Reply #13 on: April 11, 2009, 04:14:37 AM »

Being frugal is a good thing. Being cheap is not. Jeff
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KD2FAH
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« Reply #14 on: April 11, 2009, 06:17:48 AM »

I have seen these military surplus fiberglass poles being sold on eBay and wondered if it was too good to be true.

I am also rethinking the use of a tree and house, but I may be using up to 100 feet of coax to connect tail of G5FRV and radio. I think I can knock it down to 75 feet.

Feedback on PVC by all is much appreciated and will nix that idea of building a reinforced mast out of it.
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