Does anyone know where spider coil forms can be found? I know pebbles used to sell them,but I understand they are no longer taking orders. If I have to make my own what materials can I use? Does it have to be plastic or could I use thin plywood or cardboard?

would have to be rigid, plastic, bakelite, masonite, phenolic, CDROM, etc... Crosley and others used cardboard, although I think you would have to impregnate it to make it stable and rigid.

Is this for crystal radio? If so you want to minimize your losses. High Density Polyethylene (HDPE) has less loss than just about any other material. For example, cardboard (paper) has a dielectric constant of 3.7 while polyethylene is 2.25.

Does anyone in your family have cats? Cat litter containers produce sheets of it big enough for a spider coil. The 40 lb containers are rigid enough to be used single ply. The 20 lb containers may need to be doubled or tripled up.

Also you can use a corrugated election or garage sale sign. Harder to cut cleanly, though.

this guy has been trying to sell these for awhile
https://www.ebay.com/itm/265243631223?h ... SwOyVg~1pw

Thanks for the suggestions guys,I'll probably use Masonite . I use it all the time for radio chassis and panels.

Hey Jimbob,

Masonite used as a coil form may be fairly lossy. On a good antenna /ground , it "should" work on local strong stations.

If you are going for any type of Dx, I might shy away from Masonite and go with a formless basket weave or other type of coil.

Will you actually NOTICE and HEAR a big degrade with Masonite forms- "maybe".

FWIW, ie with the Crosley Model 50, the coils are wound on wooden dowel "spokes" that radiate from a center hub.You'll notice that the wire has a somewhat restricted direct contact with the dowels. There is a lot of AIR in the coil ( a good thing ). IMHO- In ANY type of coil form : the less coil form /wire contact , the better.

In a crystal set (if that's what you're building) EVERY theoretical loss should be minimized. Multiple sources of loss within the "stages" can add up ( ie type of coil form used, det/tank Z match, det phone Z match, etc, etc, etc.). In an amplified set, particularly one that has RF amplification, it might not be so critical.

So it depends. I sometimes intentionally use "less than optimal" techniques for construction convenience or simply for looks.

K

I have used the plastic CD "protectors" and cut them on the bandsaw.
Even used a butter lid to prove it can be done.

Here is a link to what is used in the Antique Wireless Association's Radio Fab Lab, which is an excellent hands-on program for ages 8-15. The coil forms are laser cut in acrylic or polycarbonate...no attempt is made to reduce RF losses.

http://awa.canadice.net/

AWA's Radio Fab Lab is getting ready to start with several new classes for area youth, which include an Introduction to Analog and Digital Computers and Build an AM/FM Radio.

73 Dean

I always wanted to try tinker toys. The only drawback is that they only have an even number of spokes. An odd number would help with stray capacitance.

Hey Mark,

GREAT to see you in here. It's been a LONG time !!!

I too was thinking Tinker Toy for reproducing a Crosley type coil. I forgot how many spokes it had though.

One could drill press out a section of a wood and glue in very small radial dowels (I suppose ). Personally I usually go 13 point basket weave formless and call it a day .

de N1NQC

How about plexiglass or lexan? Micarta? Where can I look up their dialectric characteristics?
Thanks

Extensive list of Dielectric Constants.

https://www.clippercontrols.com/pages/D ... alues.html

Thank you very much!