[Antennas] RE: Expanding foam and antenna tunning

2002年8月27日 00:16:47 -0400

Demetre Koumanakos <[email protected]> asked about 
expanding foam in a particular antenna application:
> I constructed and installed a 50 Mc vertical coaxial 
> center fed antenna this past weekend, the antenna itself 
> is housed inside a fiberglass fishing rod in order to 
> keep the coaxial vertical. The antenna tuned nicely 
> throughout the band with less than 1.7 SWR.
>> The fishing rod itself seems to handle the wind nicely 
> but the base of it where it is secured on two wooden 
> planks with U bolts may need some reinforcing so it 
> doesn't crush from the wind pressure at the U bolt 
> contact points.
>> I thought of the Expanding Foam (Poly something....) used 
> for insulation, but I was thinking, would this effect 
> tuning ? has anyone had any experience with expanding 
> foam on antenna applications ?

Several other people have already addressed some points on 
this issue, but I would like to add a little more from a 
safety and material science perspective.
The typical "expanding foam in a can" materials are 
urethane foams. Based on isocyanate esters, they 
polymerize when exposed to the moisture in air. The 
foaming comes from dissolved gases in the liquid monomer. 
Dow Chemical's "Great Stuff" is the material most commonly 
found in home improvement stores but other manufacturers 
make similar materials. These are typically used in many 
applications to provide sealing, caulking and thermal 
insulation. The foams are available from low expansion 
varieties to those that expand considerably before 
hardening.
As others have noted, these foams provide minimal 
structural properties. They crush rather easily, so do not 
consider using the foam to strengthen the base of the 
antenna. A wood dowel, drilled to pass the inner coaxial 
cable, and sized to provide a snug fit to the fiberglass 
should be adequate to strengthen your base. While you 
might consider some other lower-loss plastics here, the 
base of the antenna is a low impedance point and dry wood 
is probably quite adequate. Wood certainly has the 
advantage of being easy to work with - an electric drill 
and pocket knife are all the tools needed. Boiling the 
wood in paraffin wax like us old-timers used to do will 
waterproof it quite well. [Danger: Fire Hazard!]
Now Demetre might also be considering filling the entire 
hollow fiberglass tube with the foam to hold the antenna 
element centered in the shell. At this point the 
electrical properties of the foam become quite relevant. 
Urethane is not a good low-loss dielectric as someone else 
noted. Also the foam has a dielectric constant greater 
than unity so it will affect the tuning of the antenna. In 
this application, the higher expansion foams will be 
preferred as more of the finished foam is gas-filled void 
space (and the gas has unity dielectric constant).
I would recommend against using this foam here for several 
reasons beyond the electrical loss of the foam itself. The 
first is that the installation is rather permanent. You 
will never be able to remove the antenna from the 
fiberglass cover once the foam is cured, so any adjustments 
in the future are impossible. With the antenna being fed 
with coaxial cable, you also have to worry about 
unpolymerized monomer attacking the coaxial cable's 
polyethylene dielectric as well as the cable's PVC jacket.
As I noted earlier this isocyanate monomer cures in the 
presence of moisture in the air. Trying to get a deep 
section cure inside the fiberglass tube may be difficult. 
One approach is to spray some water into the tube as you 
squirt in the foam. Yes it helps the cure, but it is not 
particularly good for the electrical properties or the 
corrosion of the antenna materials.
I think at this point, adequate reasons have been presented 
to avoid using the foam in this particular application. 
But there are many other applications around the home where 
we might wish to use this material so let me add a few 
other points.
I have already noted the problems with the need for 
moisture if the foam is to cure in deep sections. For most 
home applications, this is not a problem at all. But 
ultraviolet resistance may be an issue in outdoor 
applications. The cured urethane foam has virtually no 
ultraviolet resistance. It rapidly deteriorates in the 
presence of sunlight. You will need to paint over it after 
curing if you plan to use it outdoors.
And now for some safety issues...
First the foam is very flammable until cured. Always work 
well away from sparks and flames. Also wear protective 
eyewear and gloves when working with this material. This 
point cannot be emphasized enough. You do not want to 
breath the fumes or get the uncured material on you skin. 
The ingredients are all irritating to the eyes, skin, and 
respiratory tract. Follow the manufacturers precautions 
carefully.
Sadly there is an important point that the manufacturers 
seldom discuss about these materials in their safety 
precautions. This is the fact that isocyanates are what 
are known as "sensitizers". Generally most people do not 
have a problem with the initial exposure to these 
materials. But after several exposures, some (but not all) 
people become EXTREMELY sensitive to these materials to the 
point that many allergic reactions are seen ranging from 
itching skin and watery eyes to coughing, wheezing and 
shortness of breath. Rarely, fatal asthma attacks can 
occur. Minimizing your exposure to isocyanates is the best 
way to prevent future problems so be careful and wear those 
gloves.
 73, Dr. Barry L. Ornitz WA4VZQ [email protected]