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 Post subject: Spider coil forms
PostPosted: Sep Mon 06, 2021 1:20 am 
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Joined: Feb Thu 19, 2015 1:49 pm
Posts: 114
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?


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 Post subject: Re: Spider coil forms
PostPosted: Sep Mon 06, 2021 1:46 am 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 3801
Location: Boston, Massachusetts
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.

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 Post subject: Re: Spider coil forms
PostPosted: Sep Mon 06, 2021 4:27 am 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 4954
Location: Texas
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.


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 Post subject: Re: Spider coil forms
PostPosted: Sep Mon 06, 2021 4:55 am 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 3801
Location: Boston, Massachusetts
this guy has been trying to sell these for awhile
https://www.ebay.com/itm/265243631223?h ... SwOyVg~1pw

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 Post subject: Re: Spider coil forms
PostPosted: Sep Mon 06, 2021 3:23 pm 
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Joined: Feb Thu 19, 2015 1:49 pm
Posts: 114
Thanks for the suggestions guys,I'll probably use Masonite . I use it all the time for radio chassis and panels.


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 Post subject: Re: Spider coil forms
PostPosted: Sep Mon 06, 2021 4:04 pm 
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Joined: Jul Thu 12, 2007 9:36 am
Posts: 570
Location: Boston,Ma
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 :D ). 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


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 Post subject: Re: Spider coil forms
PostPosted: Sep Tue 07, 2021 7:03 am 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 6274
Location: Beautiful Downtown Burbank CA
I have used the plastic CD "protectors" and cut them on the bandsaw.
Even used a butter lid to prove it can be done.


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 Post subject: Re: Spider coil forms
PostPosted: Sep Tue 07, 2021 11:06 am 
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Joined: Jan Fri 27, 2017 8:41 pm
Posts: 367
Location: Springwater, NY
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


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 Post subject: Re: Spider coil forms
PostPosted: Sep Wed 08, 2021 12:46 am 
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Joined: Mar Wed 26, 2008 3:06 pm
Posts: 846
Location: Cave Springs, AR
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.

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 Post subject: Re: Spider coil forms
PostPosted: Sep Wed 08, 2021 6:24 pm 
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Joined: Jul Thu 12, 2007 9:36 am
Posts: 570
Location: Boston,Ma
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


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 Post subject: Re: Spider coil forms
PostPosted: Sep Sat 18, 2021 9:12 am 
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Joined: Dec Sun 25, 2016 5:53 am
Posts: 338
How about plexiglass or lexan? Micarta? Where can I look up their dialectric characteristics?
Thanks

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1970's Avionics Tech, USN Attack Squadron 147, "ARGONAUTS" A-7E A/C


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 Post subject: Re: Spider coil forms
PostPosted: Sep Sat 18, 2021 6:16 pm 
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Joined: Feb Wed 12, 2014 4:25 pm
Posts: 209
Location: Southeast Kans. - KA0HCP
COM-NAV-ECM wrote:
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


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 Post subject: Re: Spider coil forms
PostPosted: Sep Sun 19, 2021 10:48 am 
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Joined: Dec Sun 25, 2016 5:53 am
Posts: 338
KB4QAA wrote:
COM-NAV-ECM wrote:
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!

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1970's Avionics Tech, USN Attack Squadron 147, "ARGONAUTS" A-7E A/C


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