On Sun, 14 Feb 2010 19:09:32 -0800, (Dave Platt)
wrote:

In article ,
Jeff Liebermann wrote:

I'm thinking of building a 3 element 20 meter quad (full size)
and I'm stuck on the spreaders. Many people use bamboo
or aluminum with insulators at the tips. Can anyone suggest
alternatives....hopefully inexpensive, common materials?
TIA
Steve

PVC pipe? Probably too heavy and too easy.

Tends to bend pretty easily under sidewise stress (including gravity).
Unless you fix the end of the spreader to the wire element so that the
wire takes up the bending stress, you'd find a PVC-based 20-meter quad
starting to sag badly quite quickly, I'd think.

I've seen plans for PVC structures that have two different methods of
stiffening. One is to run fiberglass rope inside to a pair of eye
bolts. The threaded part of the eye bolts goes through a hole drilled
into the end caps. A nut at each end tightens the fiberglass rope.
Ordinary rope can be used but it tends to stretch and absorb water.
Once tightened, the PVC pipe is substantially more rigid.

Another method is to build a cross braced 3D structure. For a 2
element quad, that would be a box. Suitable end caps can be bought or
fabricated out of block of plastic (or a mess of PVC adapters).
Putting the box under tension between opposite end points should be
sufficient to provide structural stiffness.

Also, much standard PVC tubing doesn't stand up to sunlight very well.

True. Per the advice of Dr Barry Ornitz, spraying the PVC with Krylon
acrylic clear plastic spray is sufficient to protect the PVC (and any
electrical tape wrapped around the connectors) from UV embrittlement.
However, the really good stuff has been recalled:
http://www.cpsc.gov/cpscpub/prerel/prhtml09/09036.html

Maybe vinyl tubing or thin wall pipe. For stiffness, fill the tubing
with expanding foam urethane fence post compount, cement, or epoxy.

I'd be concerned about the weight of the filling.

Ok, forget the concrete antenna spreaders.


--
Jeff Liebermann
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558