[TCML] Secondary form material

[Ed Phillips]

    I don't understand your comment about PVC.  As several others have 
pointed out already that PVC makes a perfectly dandy coil form if 
handled with reasonable care in selection and preparation.  I've had 
excellent luck with something far older and simpler - cardboard tubes 
coated [and to some extent impregnated with] shellac.  The cardboard is 
coated with thinned shellac on both the outside and the inside [takes a 
little doing] and the process repeated until the whole form is solid and 
quite rigid.  The largest form I've made was for a "radio coil" and it 
was 7" in diameter.  Measured RF losses were negligible and the coil has 
an excellent Q.  My biggest TC secondary is wound on a 5-1/4" mailing 
tube treated the same way.  I've used this approach because I had a good 
supply of thin-wall mailing tubes but if I were to build a coil now I'd 
probably go the PVC route, same as I do with VDG columns.

Ed

Andrew Robinson wrote:

> As many have already discovered, PVC is apparently a terrible material 
> to use as a secondary coil form. About a week ago my engineering group 
> also discovered that acrylic is extremely hard to work with. We ended 
> up cracking our only secondary coil form which we slaved over to find! 
> So my question to everyone out there is what on earth should we use 
> for the secondary coil form. We need a 4" OD coil form and if 
> possible, if you can name a material that can also be an 1/8" thick we 
> have a nice cutting tool that makes the most perfect end caps. So 
> ideally 4" OD and 3.75" ID is perfect for what we need. I've called 
> numerous plumbing companies with no help what so ever. Thanks everyone!
>
> Regards,
> Andrew Robinson