[TowerTalk] fiberglass boom/elements

[Kurt Andress]

Bill Heinzinger wrote:
> 
> Has anyone ever experimented with fiberglass boom or elements.
> 
> Could you embed a conductor in the fiberglass as the radiator.
> Would this this lighter/stronger than aluminum?
> 

Hi Bill,
Good question! Sorry to take so long getting around to this. 
Like some wise guy said, "It depends." In this case it depends on what
you use and what you consider important. 

The RF guys can address the impact of embedding the RF conductor in a
dielectric lossy medium, my work experience says it can be headache
food. 

Every experiment starts with some pencil and paper. 

Let's pick a couple of readily available tubes:

(Aluminum Tube) 2" OD x .25 Wall  Material: 6063-T832 aluminum 	
Source: Texas Towers website 4-10-00.

(Fiberglass Tube) 2" OD x .25 Wall   Material: Undefined polyester/glass
Source: Max-Gain Systems website 4-10-00.


PRICE:
Aluminum Tube		$6.75/Ft	1.62 Lb/Ft	=$4.18/Lb

Fiberglass Tube		$8.63/Ft	1.40 Lb/Ft	=$6.16/Lb

STRENGTH:
Texas Tower lists their 6063-T832 at 40000 psi yield strength. Max-Gain
doesn't provide material properties, that is consistent with their
market niche, providing low cost materials. Other companies that make
composite tubing can provide values, and they are not all the same, and
are more expensive.
Fiberglass strength can range from ~25,000 - ~100,000 psi. If the
material has less strength than aluminum, the tube needs more material
and/or size to be as strong.  If the strength is equal, the fiberglass
part can be the same size and weigh less. If the strength is greater,
then the part can be smaller and/or thinner wall and weigh even less.
To whatever fiberglass solution is chosen, add the weight of the
conductor and the paint to UV protect it.

STIFFNESS:
Aluminum has an elastic modulus of ~10 Msi (million psi). Fiberglass
ranges from around 2.5 - 6.0 Msi, for the same reasons the strength
values vary, an entirely different discussion.

If we stuck 12' of each of the above tubes out in a 90 mph breeze and
fixed them at one end, the aluminum tube would bend 2.88" at the free
end. If the fiberglass was the real good stuff it would bend 4.81". If
it was average stuff the deflection would be 7.21".
If maintaining the physical relationships of the members of your antenna
is important, you might want at least equivalent aluminum stiffness and
need to use a fiberglass tube with a heavier wall or larger OD to get
there.
A solid 2" bar of the best fiberglass still wouldn't be as stiff as the
aluminum tube above (it would be incredibly strong). A 3" OD x .25 wall
tube with average fiberglass, at the same wind speed, would be real
close. It would weigh about 2.20 Lb/Ft and at the listed price per pound
would cost $13.55 per foot.

GENERAL:
After one knows what the material properties are, some decisions can be
made to determine what might be "better". 
Then it can become real fun. Fiberglass tubing comes in a limited number
of sizes and wall thicknesses. Sometimes, you can't find the size and
wall that will be an optimum aluminum substitute to fulfull the defined
"better" goal, but you can find something that will work (bigger,
heavier, and more expensive always gets any job done). 

I once designed an antenna with some fiberglass insulating sections in
it. I sized the insulators using a very modest strength value. I
couldn't get a stiffness value from the vendor, so I just blew off
looking at the stiffness thing, I knew better. When I built the turkey
the insulators deflected so much that I got to go do it all over again.
Of course, I was the one defining "best", might be ok for you, and YMMV,
etc....

I had another antenna, from the XYZ company, a trapped vertical with
coils formed around fiberglass insulators. Silly me, I fed it with legal
limit power and the heat generated by the insulator RF loss properties
created enough heat to soften the insulators, the antenna went dumb, and
looked like a deranged fishing rod. That was probably unconscious
antenna abuse (hadn't yet figured out why someone would design an
antenna incapable of legal power, now I get it), but I learned
something. Hoist a bottle to the value of experimentation, however it
comes!

Here's a related aside...Folks out there that are really keen on light
weight aluminum masts, by virtue of strength comparisons alone, ought to
try some out of high grade fiberglass (the 100 ksi stuff). Based on
strength alone, they can be very light, but I fear only suitable for
"moonbounce" when the wind is blowing hard enough from the right
direction. It's likely to hate leaning hard against the top tower
support and being clamped by the rotator. But, if bending strength and
weight is the only criteria, experimentation can provide plenty of
entertainment.

Fiberglass can be great stuff! It fed my family for 19 years. It is the
"best" material for lots of applications. Sometimes, the right metal is
still a cool choice.


-- 
73, Kurt, K7NV 

Visit http://www.freeyellow.com/members3/yagistress/

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