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jimbob
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Post subject: Spider coil forms Posted: Sep Mon 06, 2021 12:20 am |
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Joined: Feb Thu 19, 2015 12:49 pm Posts: 134
Location: Middleburg Hts. Ohio
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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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mrx
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Post subject: Re: Spider coil forms Posted: Sep Mon 06, 2021 12:46 am |
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Joined: Jan Thu 01, 1970 12:00 am Posts: 4184
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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.
_________________ Andrew
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Macrohenry
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Post subject: Re: Spider coil forms Posted: Sep Mon 06, 2021 3:27 am |
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Joined: Jan Thu 01, 1970 12:00 am Posts: 6062
Location: Austin TX
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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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mrx
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Post subject: Re: Spider coil forms Posted: Sep Mon 06, 2021 3:55 am |
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Joined: Jan Thu 01, 1970 12:00 am Posts: 4184
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jimbob
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Post subject: Re: Spider coil forms Posted: Sep Mon 06, 2021 2:23 pm |
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Joined: Feb Thu 19, 2015 12:49 pm Posts: 134
Location: Middleburg Hts. Ohio
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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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krystallo
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Post subject: Re: Spider coil forms Posted: Sep Mon 06, 2021 3:04 pm |
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Joined: Jul Thu 12, 2007 8:36 am Posts: 900
Location: Boston, Ma
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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
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Mike Toon
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Post subject: Re: Spider coil forms Posted: Sep Tue 07, 2021 6:03 am |
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Joined: Jan Thu 01, 1970 12:00 am Posts: 6621
Location: Burbank CA
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I have used the plastic CD "protectors" and cut them on the bandsaw. Even used a butter lid to prove it can be done.
Attachments: |

spider.jpg [ 141.98 KiB | Viewed 1645 times ]
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butter.jpg [ 182.43 KiB | Viewed 1644 times ]
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NW2K
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Post subject: Re: Spider coil forms Posted: Sep Tue 07, 2021 10:06 am |
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Joined: Jan Fri 27, 2017 7:41 pm Posts: 528
Location: Springwater, NY
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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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err ducks
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Post subject: Re: Spider coil forms Posted: Sep Tue 07, 2021 11:46 pm |
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Joined: Mar Wed 26, 2008 2:06 pm Posts: 899
Location: Cave Springs AR
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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.
_________________ Mark
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krystallo
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Post subject: Re: Spider coil forms Posted: Sep Wed 08, 2021 5:24 pm |
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Joined: Jul Thu 12, 2007 8:36 am Posts: 900
Location: Boston, Ma
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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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COM-NAV-ECM
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Post subject: Re: Spider coil forms Posted: Sep Sat 18, 2021 8:12 am |
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Joined: Dec Sun 25, 2016 4:53 am Posts: 626
Location: Santa Barbara, California
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How about plexiglass or lexan? Micarta? Where can I look up their dialectric characteristics? Thanks
_________________ 1970's Avionics Tech, USN Attack Squadron 147, "ARGONAUTS" A-7E A/C
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KB4QAA
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Post subject: Re: Spider coil forms Posted: Sep Sat 18, 2021 5:16 pm |
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Joined: Feb Wed 12, 2014 3:25 pm Posts: 350
Location: Fredonia KS
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COM-NAV-ECM
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Post subject: Re: Spider coil forms Posted: Sep Sun 19, 2021 9:48 am |
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Joined: Dec Sun 25, 2016 4:53 am Posts: 626
Location: Santa Barbara, California
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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.htmlThank you very much!
_________________ 1970's Avionics Tech, USN Attack Squadron 147, "ARGONAUTS" A-7E A/C
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