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 Post subject: What makes a good coil form?
PostPosted: Mar Fri 23, 2012 6:22 pm 
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Location: Palm Beach County, Florida
What is a low loss material for winding RF coils besides air. Old "official coil forms" are not readily available and PVC pipe is uuuuugly. There are plastic supply stores and craft stores but it would be good to know what to look or ask for. There may even be coil forms hiding in everyday things. Any ideas out there in the land of experience and wisdom?


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 Post subject: Re: What makes a good coil form?
PostPosted: Mar Fri 23, 2012 6:41 pm 
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Lots of winders use pill bottles and have seen white plastic cores in some rolls of small printer paper, that didn't look horrible... I'm going to try the spindle from a CD stack, also have two I've wound on useless miniature tubes one a 9 pin the other 7pin... Inner capacitance is a consideration at higher freq, but something like a 1B3 has very little... Bakelite tube bases are another possibility...

Tom


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 Post subject: Re: What makes a good coil form?
PostPosted: Mar Fri 23, 2012 7:45 pm 
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Polystyrene pipe , not PVC , the thiner the beter on the coil form . Air dux coil formers do well . I tried alot of different former materails with litz wire , testing with a HP Q meter and the best foimer i found was the plastic form used under a paint roller . I removed the nap and wound the litz on the form .The next best was the rolls they use for the produce bags in Wall mart , i had them save me several of the empty rolls .

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 Post subject: Re: What makes a good coil form?
PostPosted: Mar Fri 23, 2012 9:06 pm 
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Location: Central Georgia
PVC can be painted satin black to look more acceptable......... not sure if that will significantly affect the coil's "Q" though.... Cardboard seems to be a poor choice unless sealed from moisture with wax, varnish, etc. I've been thinking about trying scarred up and unplayable plastic cylinder records (not the wax ones) - if I can find some cheap enough..... and I've wondered if old olive jars, wine bottles, etc could be used..... I think I still have one of those Ronco bottle cutter here somewhere.... :wink:

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 Post subject: Re: What makes a good coil form?
PostPosted: Mar Sat 24, 2012 5:46 am 
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Joined: Jan Tue 19, 2010 6:41 pm
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Location: Diana Texas
6GH8,
In the "old" days, the guys would look for ceramic forms to wind coils. However, ceramic is nether cheap nor plentiful. Air core coils are still popular (no form at all). For an air core coil, you usually find a form the right diameter (drill bits are some times used), wind the coil and then remove it from the form, and then solder it in place. The solid wire holds itself in place. As Tom mentioned above, pill bottles are popular forms. They come in different sizes, are easily drilled, and are usually free for the asking. They will fit on octal tube bases well for making a plug in coil system. Pill bottles have become my favorite coil form these days.
Jerry aka W5JH


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 Post subject: Re: What makes a good coil form?
PostPosted: Mar Sat 24, 2012 11:52 am 
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Location: Berlin, MA
It would be helpful to know the application - low power like receiver, high power like linear amplifier, frequency determining like local oscillators/VFO? As Jerry and others have said, pill bottles work pretty well for low power applications but not so well for high power applications. Frequency determining applications really want mechanically stable materials such as glass - something that won't move with temperature or mechanical shock.

For low power rf transformers I usually use powered iron toriods. Linear amps, air with maybe teflon or similar rods for spacing.

Can you describe your application a bit?

arnie


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 Post subject: Re: What makes a good coil form?
PostPosted: Mar Sat 24, 2012 6:34 pm 
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Location: Cleona, PA
For receiving applications crystal set experimenters have investigated many types of coil materials looking for the lowest loss. Best is pure air (i.e., no form at all) but that only works for very stiff and heavy gauge wire or tubing. You may have excellent coil forms in your kitchen cupboard or at the local supermarket. Look for plastic containers marked on the bottom as PS, PE, PP, HDPE with a "5" inside a triangle, or PS with a "6" inside a triangle. These are among the lowest loss plastics for this purpose. I have been saving the green plastic bottles that certain vitamins come in thinking they'll make cool looking forms. You can hacksaw off the bottoms and tops to get a pure cylinder if you want. Grated cheese containers are another source, as are peanut butter jars, etc.

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 Post subject: Re: What makes a good coil form?
PostPosted: Mar Sun 25, 2012 8:24 pm 
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I've had very good results from polystyrene tubing obtained from model shops. If you buy the 7mm lengths you can put an M6 tap down it to accept a ferrite slug.

Get some plain matrix board (or some strip board with the copper etched off), drill out a 7mm hole in the centre to accept the former, put in a few vero pins to terminate the coil windings and that's it pretty much. You can use polystyrene sheet to make a lid if you like. Poly sheet can also be used as a screening can if you paint it with that nickle-silver conductive paint, and cyanoacrylate will make a good job of bonding the polystyrene former tube to the matrix board base.

Another useful tip if you're into winding your own single layer coils. Put a thin length of double sided adhesive tape along the length of the tube - you'll find the windings don't spring off should you relax the tension on your wire whilst you wind it then ;)

To work out whether or not a material isn't lossy at RF frequencies, put it in a microwave oven for a few seconds. If the material gets warm, it's lossy - simple as :) Polystyrene does not get warm in a microwave, neither does candle wax.

Beware of anything black - including paint, the carbon content is likely high enough to effect Q.

A picture paints a thousand words, I may take a few and upload if what I've said isn't so clear :)

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 Post subject: Re: What makes a good coil form?
PostPosted: Mar Fri 30, 2012 7:57 pm 
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Location: Sheridan,Wy., 82801
I found a 1.250" diameter, thin walled drain PVC at a plumbing company. It is used for drains only. It may be used by sprinkler system installation companies too. I do not know for sure! It is not meant for high pressure. It fits very snugly into a Octal tube base, which can be bought in many places online. I just Crazy Glue it into the tube base. It fits perfectly snugly in the tube base. Any medium or fine sandpaper will sand off the markings, such as brand name, etc. within a few minutes of work. It can be painted to the color of your choice too! I gave up on the pill bottles awhile ago! Lenny


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 Post subject: Re: What makes a good coil form?
PostPosted: Mar Fri 30, 2012 8:05 pm 
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A lot of things can be used for coil forms but some are better than others. I like the idea of testing it in a microwave oven. If it gets warm not the best for a coil.

I use clear Plexiglass but it's more expensive, sold by the inch.

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 Post subject: Re: What makes a good coil form?
PostPosted: Apr Sun 01, 2012 10:10 pm 
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Location: Texas. USA
I ran across one of those "at the local supermatket" coil forms that may be of interest. Kroger "Value" brand 2.25 oz pepper comes in a pp (5) plastic bottle with a 'form' diameter of about 1.75" and 2 inches of form between the 'end bulge'. That should be just about right for a BCB coil, I would think, and at a buck it's not a bad deal even for just the form.


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 Post subject: Re: What makes a good coil form?
PostPosted: Apr Mon 16, 2012 2:21 pm 
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Location: Gainesville, Florida
there is black plastic pipe at the hardware store. the coupling fittings look best. what about ferrite core antenna coil :shock:

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 Post subject: Re: What makes a good coil form?
PostPosted: Apr Thu 19, 2012 6:15 am 
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Location: Texas. USA
I had mentioned the Kroger pepper bottle and here is a coil I wound on one. The second coil is scramble wound on an octal socket salvaged from a defunct tube. Both are intended to be BCB. The pepper bottle has, from the bottom, antenna coupling, primary, and 2 turn tickler. The octal coil is roughly 80 turns tapped 20 from the bottom. I say roughly because I have a LCR meter and, so, 'over wound' it, to make sure, and then backed off turns to get the inductance wanted.

They aren't works of art but, rather, an example of what even an amateur klutz like me can do.
Attachment:
coils sm.jpg
coils sm.jpg [ 42.94 KiB | Viewed 3538 times ]


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 Post subject: Re: What makes a good coil form?
PostPosted: Apr Tue 24, 2012 5:04 pm 
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Joined: Oct Sat 20, 2007 3:36 am
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Location: Southern NH, 03076
You can buy fiberglass tubing of several diameters and wall thickness at plastic supply stores. These are fully treated so there is no moisture ingress issues and RF performance is excellent.

Since a microwave oven only runs at those frequencies its not a good evaluation of what is the lowest loss material at HF or MF. For that you need a Q Meter which were made by Heathkit to HP or a satisfactory homebrew version could be built.

Carl


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 Post subject: Re: What makes a good coil form?
PostPosted: Apr Tue 24, 2012 11:49 pm 
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Joined: Jan Thu 01, 1970 1:00 am
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Location: Cleona, PA
FWIW I'd avoid painting any form with a pigmented paint, especially black paint, which may be carbon-black based, and which would lower the Q.

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 Post subject: Re: What makes a good coil form?
PostPosted: Apr Wed 25, 2012 2:43 pm 
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For BCB 2 liter soda bottles work well. Plastic peanut butter jars also. 3.5-4" diameter is perfect.

Doug


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 Post subject: Re: What makes a good coil form?
PostPosted: Apr Wed 25, 2012 9:29 pm 
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For 600M loading coils plastic 30-50 gallon barrels work well also :shock:

Carl


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 Post subject: Re: What makes a good coil form?
PostPosted: May Sat 05, 2012 5:00 pm 
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Joined: Feb Thu 02, 2012 6:09 am
Posts: 11
I have a local plastic store, here in Northern Calif., called "Tap Plastics". I hope there are similar stores around the country. Tap Plastics has some plastic tubes of various diameters. I had the shop cut me up some 5 inch length sections after buying one of their 2 inch diameters tubes. I makes for a nice coil form. Here is the 1-tube set I made a few years back. The schematic came from an article in the book, "Radio Receiver Projects You Can Build", by Homer Davidson.

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Dick/WA6ZFM


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 Post subject: Re: What makes a good coil form?
PostPosted: May Thu 24, 2012 12:55 am 
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Joined: May Tue 01, 2012 11:40 pm
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Location: Rainier WA
Wood turned to .125" wall thickness. And a wood mount gives it an "old school" look.

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 Post subject: Re: What makes a good coil form?
PostPosted: Jun Tue 05, 2012 7:34 pm 
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Location: Olympia WA USA
Coil forms are usually easily found at the local ham fairs.

I have made forms from pill bottles, PVC pipe, (Had threads cut on it at a machinest shop at just the right TPI I needed for a dual band mobile ham antenna project from '73 Magazine); ABS pipe, wood, masonite/ old TV backs (cut 2 rectangular pieces and notch the centers so they form an "X" when slid together), paper towel tubes (Soak in varnish for a couple if days & let dry), and various plastic pieces and objects. Nothing says the coil must be round either. I have a crystal radio I got at a hamfair where the coil was would on a rectangular piece of wood with the edges rounded over and then varnished.

I prefer old tube bases, and plastic tubing when I need to make a plug in coil, although I have many old ceramic plug in coil forms from some mil surp. stuff I bought back inthe early '70's that I use most frequently.

I have scrapped some old marine radios that used Plexiglass in an "X" pattern that were notched and slid together and some type of cement was used on the edges to keep the windings in place, although one I had did have small cuts in the edges for the wires to rest in, thereby maintaining the spacing.

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