Re: Alternate sources for large secondary coil forms (fwd)
---------- Forwarded message ----------
Date: Mon, 6 Jul 1998 00:07:30 -0700
From: Bill Noble <william_b_noble-at-email.msn-dot-com>
To: Tesla List <tesla-at-pupman-dot-com>
Subject: Re: Alternate sources for large secondary coil forms
why not roll your own?? wrap your choice of material around something of the
right diameter - for example, roll some vinyl around a pipe and you can have
a vinyl form - remove the pipe of course after gluing the vinyl.
From: Tesla List <tesla-at-pupman-dot-com>
To: 'Tesla List' <tesla-at-pupman-dot-com>
Date: Friday, July 03, 1998 11:43 PM
Subject: 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??
>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