EMRFD Message Archive 10710

Message Date From Subject
10710 2015-01-15 15:28:35 bob_ledoux Making Ceramic Coil Forms
Anyone here make their own ceramic coil forms?

I've noticed there are a number of ceramic products available in the arts field.  Some of these fix at low temperatures.

Dick Blick Art Supplies has a number:

Clay and Modeling Materials - BLICK art materials

 




10711 2015-01-15 15:58:14 Cecil Bayona Re: Making Ceramic Coil Forms
There are many kinds of ceramic, the link is to clay pottery which
can absorb water and may or may not contain metals which makes them
lossy at RF. I have never seen Radio components using pottery, I used
Google to search all over the place and no mention of the ceramic
type used for RF components.

At 05:28 PM 1/15/2015, you wrote:
>
>
>Anyone here make their own ceramic coil forms?
>
>I've noticed there are a number of ceramic products available in the
>arts field. Some of these fix at low temperatures.
>
>Dick Blick Art Supplies has a number:
>
><http://www.dickblick.com/categories/clay/>Clay and Modeling
>Materials - BLICK art materials
><http://www.dickblick.com/categories/clay/>Clay and Modeling
>Materials - BLICK art materials
> From Dick Blick Art Materials - Find a variety of materials, tools,
> and modeling supplies, for use in clay working. Air drying clays,
> modeling clays, polymer ...

--
Cecil - k5nwa
< http://thepartsplace.k5nwa.com/ > < http://www.softrockradio.org/ >

Never take life seriously. Nobody gets out alive anyway.
10712 2015-01-15 17:26:15 augustinetez Re: Making Ceramic Coil Forms
Bit of a read, but this file from 1904 should give you a lot of information about ceramics for use in RF.

Terry VK5TM

Transactions : American Ceramic Society : Free Download & Streaming : Internet Archive
10713 2015-01-15 17:34:45 Dana Myers Re: Making Ceramic Coil Forms
10714 2015-01-15 18:35:13 Ronan McAllister Re: Making Ceramic Coil Forms
Off topic a bit, but for coil forms phenolic would be interesting..


PHENOLIC CORE
Phenolic is a common material used for inductor cores.
Many are made of a polyester base that have high
temperature characteristics. It is also common for phenolic
cores to have high flammability ratings such as UL94V-0.
Phenolic cores also provide high strength and are more
economical than ceramic cores.
Phenolic has no magnetic properties. Thus, there is no
increase in permeability due to the core material.
Phenolic core inductors are often referred to as “air core”
inductors and are most often used in high frequency
applications where low inductance values, very low core
losses and high Q values are required.

Here is a supplier, it's not all that expensive (3' long, $6.47)


I have no experience with this supplier.

I suspect the best kind of ceramic core to use would be a silica-based technical ceramic; however, for DIY,  I don't see why artists clay ceramics wouldn't work if the whole core were glazed/fired in a kiln......   Might make an interesting art/craft like inductor/coil!

Interesting topic!
Ronan/KB6NHQ



10715 2015-01-15 19:59:48 kb1gmx Re: Making Ceramic Coil Forms
Phenolic is very good choice.  Its easy to cut, glue and works well at RF.
Other material choices are PVC pipe, Lexan and Lucite rod and tubes
which are available from suppliers.  They have the advantage that they 
can be machined drilled and threaded.

Ceramics for RF use must be fired and if there are an trace metals
they are questionable. I might add that most clays change shape 
when fired so precision shapes are very difficult.  Many of the ceramic 
forms were cast then precision ground to get the right surface and size.

Ceramic forms are still available but very costly, mostly NOS material
and also some suppliers that cater to specialized needs.


Allison
10716 2015-01-15 20:13:31 farhanbox@gmail.c... Re: Making Ceramic Coil Forms

thin, old paper rolls. old paper has less moisture, tune them with brass bolts.


------ Original message------

10717 2015-01-15 22:51:59 mvs_sarma@ymail.c... Re: Making Ceramic Coil Forms
We get Nylon rod  in various dia..  i try to drill a stating and ending holes thro the as stoppers and wind over a snall cut piece of such .  perhaps bakelite tubing could also be possible

 
10719 2015-01-16 06:55:39 Ancel Re: Making Ceramic Coil Forms
For custom forms, I'd suggest hot glue. Of course your coil can't get
very hot then, so this isn't for power applications.
10720 2015-01-16 07:18:23 bob_ledoux Re: Making Ceramic Coil Forms
I was choosing ceramic over plastic for temperature stability in such applications as VFO's or antenna tuner s.

It also permits creating unique forms without lathe or milling work.
10721 2015-01-16 08:55:39 Nick Kennedy Re: Making Ceramic Coil Forms
My son just got one of those 3D printers. I'll bet you could knock out some nice coil forms with one of those.

73-

Nick, WA5BDU

10722 2015-01-16 11:04:43 afu Re: Making Ceramic Coil Forms
Hi,

do you know, that hot glue is PVC, which has a realy bad behaviour under RF ?!

Herbert

Am 16.01.2015 14:55, schrieb Ancel mosaicmerc@yahoo.com [emrfd]:
 

For custom forms, I'd suggest hot glue. Of course your coil can't get
very hot then, so this isn't for power applications.


10723 2015-01-16 11:26:06 kb1gmx Re: Making Ceramic Coil Forms
Hot glue is a variable product, there are three common formulations, and all three have been tested using my 4191A and to 100mhz its fine.  At 500mhz its no so good but then things tend to be self supporting there.

I've not found it to be unacceptable and all testing to date says if you can live with 
the low melt temps its fine.  

I'd not use it in VFOs however as temperature stability and mechanical stability are 
often the same thing and the low melt temperature may be a negative factor.  My
preference there is "Q-dope" (polystyrene dissolved in MEK).

Ceramic is variable with temps.  Those applications like VFOs  where its seen the 
ceramic has been formulated for temperature stability.  Most uses at RF are for 
low loss at high frequencies like UHF or high voltage withstand.

One thing to avoid is any material colored black as most often this color is obtained 
from adding carbon black or other colorants.  For this reason Natural or clear
Lexan, lucite, Polystyrene, and Delrin are good and Teflon. Most of those are 
available in rod or tube form from places like McMaster-Carr and Grainger.
There are also a few suppliers on the net that will sell odd pieces and cutoffs
for very attractive prices by the pound.

When searching for materials for building Google is a useful tool.  The other 
knowing or looking up alternate names for what you need as they may not 
be listed as coil forms but external threaded plastic tubes.


Allison


10724 2015-01-16 11:28:30 EricJ Re: Making Ceramic Coil Forms
I would also be suspicious of hobby ceramic materials. No particular reason other than I would not want to assume that all ceramic formulations behaved the same at RF frequencies and levels.

Eric
KE6US

10725 2015-01-16 13:40:29 kb1gmx Re: Making Ceramic Coil Forms
There are ceramics available from various sources such as ceramic spacers and standoffs. many of those are threaded making them easier to mought or to attach lugs to one or both ends to retain leads.  In the past I've used those as coil forms and the worked well.


Allison
10726 2015-01-16 22:23:25 peepholenz Re: Making Ceramic Coil Forms

One comment re unknown materials for RF use- get a sample and put it in the microwave oven - if it gets hot its probably not suitable. Not my invention- can't remember where I saw it.

Peter

ZL2AYX

10727 2015-01-17 08:54:44 James Duffey Re: Making Ceramic Coil Forms
Herbert - You wrote:

"do you know, that hot glue is PVC, which has a realy bad behaviour under
RF ?!”

PVC certainly isn’t a great dielectric, but it is far from really bad. It has a loss tangent comparable to Nylon, a little better than phenolic and about the same as porcelain. This is for PVC without the plasticizer. The plasticizer used for manufacturing PVC makes PVC flexible and degrades the loss tangent.

Here is a useful table of the dielectric properties of common materials

< http://www.kayelaby.npl.co.uk/general_physics/2_6/2_6_5.html >

The problem with PVC as a coil form is not its RF properties, but rather its poor thermomechanical properties. It has a low softening point, 150F or so, so it will melt if a lot of current is flowing in the coil, which makes it bad for loading coils at the base of very short antennas and it is not particularly dimensionally stable, so it is not really good for VFOs.

But one can wind coils on PVC forms that have moderately high Q, and if there isn’t a lot of current flowing in them they can perform well.

There is a discussion of some simple measurements made on PVC loading coils here “Loading Coils for 160-Meter Antennas” W7XC, APR 1990 - QST (PG. 28), so there is no need to speculate. W7XC concludes that PVC plumbing pipe from the hardware store will produce acceptable coils.

There was a picture in QST 30 or more years ago of a PVC coil that had melted under what hams at the time considered modest power, 150 Watts or so. It was being used as a base loading coil for a short, maybe 8ft or so, 160M antenna, had a very poor form factor, perhaps two feet long of 1.5 inch PVC plumbing pipe, and was wound with relatively small diameter wire, perhaps 18 gauge. That was largely the result of heating in the coil due to high currents and the low softening point of PVC and I suspect is the origin of the fear of using PVC in RF applications. - Duffey KK6MC
10728 2015-01-17 13:28:00 Brooke Clarke Re: Making Ceramic Coil Forms
Hi:

For coils with a high Q the electrical properties of the wire insulation and the coil form become important.
An ideal form would have a dielectric constant near 1 and the higher above that the poorer the coil will be because of increased capacitance.

Ceramic forms seem to be good where the coil generates a lot of heat, i.e. it's Q is not as high as it might be.

They still make "Air-Dux" coils, see:
http://www.bwantennas.com/coils/aduxlex.htm

Has anyone tried Aerogel as a form material?
 
10729 2015-01-17 18:54:58 kb1gmx Re: Making Ceramic Coil Forms
>>>For coils with a high Q the electrical properties of the wire insulation and the coil form become important.

This is very dependent on frequency.  Litz wire used for frequencies below 1hz is very effective and is many thin insulated strands.

At VHF and up enamel wire is less desirable as you go up.  Then things like silver plating 
become helpful.

>>An ideal form would have a dielectric constant near 1 and the higher above that the poorer the coil will be because of increased capacitance.

This is true but there are ways to get close to 1(air).  Methods like spiderweb and other mostly air wound coils deal with this.  

However I've built helical resonators supported on Teflon that exhibited excellent Q.
Teflon (PTFE) is an excellent insulator but is far from a dielectric constant of 1 (2.1).
Its useful as its loss tangent is also very good.

Generally its the loaded Q that is more important.

The key is that wire is affected by skin effect so greater wire diameter provides the lowest
RF resistance.  Again silver plating can be helpful at higher frequencies.

Wire wound even on poor material will have higher Q if the wires are spaced.  
The greater the spacing the better but there are limits.

If the end coil is enclosed in a shield or box spacing it well away from the walls
and other components helps.

With all that practical limits take over.  Structure, size, and  loaded Q tend to make
impose limits.

I have ceramic forms from old radios gear and the iron powder slug clearly
out weighs any benefit of the ceramic.

>>>Ceramic forms seem to be good where the coil generates a lot of heat, i.e. it's Q is not as high as it might be.

Generally the best reason to use it or when subjected to high voltages.

There are few applications that are aided by more than reasonable Q assuming
undesired effects like self resonance and resistance are avoided.


Allison

10734 2015-01-18 12:23:31 kx4om Re: Making Ceramic Coil Forms-glue gun stuff
Hot melt adhesive is *not* PVC. The most common type used in hobby glue glue contains Ethylene-vinyl acetate as it base. There are many other mixes for specialty use, but none seem to contain PVC.

Hot-melt adhesive - Wikipedia, the free encyclopedia

 

10739 2015-01-19 12:47:28 jorschei Re: Making Ceramic Coil Forms

Hi Bob,

To make your own RF coils may be the 3D printing people can make them for you. There are a lot of 3D printing shops around where you can use the printer yourself width help or they can do it for you. May be they can print your custom QRP housing to Hi......   May be dentist material is useful to make a ceramic coil  UV hardened   ?      

73" Joris Pe1KTH