Alternate sources for large secondary coil forms

From:  Jim Lux [SMTP:jimlux-at-earthlink-dot-net]
Sent:  Friday, July 03, 1998 10:27 AM
To:  Tesla List
Subject:  Alternate sources for large secondary coil forms

> 	If no one is aware of probs with HDPE coilforms, I suggest that 
> those of you looking for larger coilforms try commercial pipe shops
> specializing in sewer mains...that's where I found some.  Unfortunately,
> I'm still looking for an appropriate scrap (it comes in 40 foot
> :-(
> Also tends to be thick-walled and very heavy; one example of a 20 inch
> specimen had a 0.6 inch thick wall, and the guy said it was 16 lbs/foot!!
> On this basis, it might be a bit impractical unless one could find
> stock.  Anybody out there finding large diameter PVC??
> 						Clay

The thick wall PVC is what you are going to find. Sewer pipes have to be
strong enough to withstand the pressure of the soil (ask any civil
engineer, there isn't anything called "dirt") that the pipe is buried in,
which essentially determines the strength requirement. Sewer pipes usually
run at atmospheric pressure, so the "bursting strength" requirement is

For a thinner wall tube, a bit of fabrication might be required, but thin
wall polyethylene drums are available. A couple of 50 gal drums  welded (PE
is a pain to glue, but easy to weld) together end to end, perhaps after
removing the heads. You could make an insert out of 1/8" or 0.100 LDPE
strip to use as a coupler.

For an even bigger fabrication challenge, but one that would certainly work
well, why not get a sheet of 0.100 PE and wrap it around a form (like
sonotube) and weld the seam. It comes 4 feet wide and 10 feet long, so you
would need to either lay vertical strips (along the axis of the cylinder)
or spiral it (much like the sonotube is made).

And, having just worked with a radome manufacturer (where dielectric
properties are very, very important) to spec out a 4 foot diam, 6 foot high
cylinder, here are some more thoughts. They make radomes a couple of ways.
One way is to start with a paper honeycomb cellular core (from Hexcel, for
instance) which is laminated with fiberglass prepreg layers on the inside
and outside. They vacuum bag it and cure it in an oven. A typical structure
is .030 fiberglass, .100 honeycomb, and .030 fiberglass. Our 4x6 foot
cylinder will weigh about 50-60 pounds and be extremely rigid.

Another way is to use foam and laminate the fiberglass over it. The key is
to get that center layer fairly thick, which gives it strength once you put
the inner and outer layers on it.

Both of these schemes have very low RF loss (at 13 GHz, the dielectric loss
is down in the hundredths of a dB, so at 1 MHz and lower, it probably has
an unmeasurably low loss).

You could just use a big solid cylinder of foam and then laminate
fiberglass over it.

Foam is a bit pricey in big blocks suitable for a form, but you might be
able to find something. You could glue some 2" thick foam slabs together
into a hollow octagon or something, and then turn the outside circular. Rig
up sort of a giant lathe (i.e. the same rig you are going to wind the wire
with later) to turn the foam down to a cylinder. Laying up fiberglass isn't
particularly difficult. You would want to use a very dry layup to reduce
the amount of resin. I would suggest checking out the literature on making
airplanes with foam fiberglass composites (like the Rutan VariEZ). 
Companies like Aircraft Spruce and Specialty in Tustin, CA (maybe it is
Santa Ana? (714) area code, anyway) sell kits for this, as well as the foam
and stuff.