Creating inductors

[gastonanthony]

how do you know or measure the inductance of a coil of wire?just wondering how to make an good inductors by coiling wires (especially useful in transmitters). is there any site that gives you the physical specs of a coil (how many turns, circumference, wire diameter etc) that asks for the inductance value needed?

 

[fsahmed]

The fastest way of making inductors is looping 7mm wire on a ferrite core & monitoring the inductance using a henri meter. A 1 inch diameter toroidal core would give a very large range with a 7mm wire. Practical RF handbooks will provide more references.

 

[chemelec]

An inductor wound on a Torroid ferrite core Isn't suitable for a Transmitter. Especially if its a High Frequency transmitter in the FM Range.

If you give me MORE Info as to What Frequency?, I can give you Much better help on this.

Gary

 

[Miles Prower]

how do you know or measure the inductance of a coil of wire?

The best way to do that would be with an RLC bridge. If you don't have one, then another way to find out is via a test oscillator, a known capacitor, and an o'scope or some other RF voltage meter. Simply tune the coil to resonance with the known capacitor. This will tell you what the inductance is to a reasonable degree of accuracy.

If you are using a core of some sort, then you can make the calculation based upon the manufacturer's A(l) numbers (usually so many millihenries per 1000 turns or microhenries per 100 turns).

In the vast majority of cases, getting the indictance exactly right is neither necessary nor possible. For example, if you calculate that your coil needs 5.345678 turns, how, exactly, do you put .345678ths of a turn on the coil form? That's what trimmer capacitors are for.

The only good way to see if your coil is right is to install it in the project, see how it works, then make whatever adjustments are required to get the proper operation: add/remove turns and/or adjust the spacing of the turns (if you aren't using low frequency ferrite cores -- these have such high permeabilities that turns spacing becomes irrelevant so far as determining indictance.)

 

[tansis]

For value for a single layer coil can be calculated thus
(apologies for awfull looking formula but am doing this in a hurry)

Henries = mu *(((N^2)*A)/l)

l = length of coil
N= number of turns
A=cross section area
mu is the permeability of the core material.
vacuum= 8.85 * 10^12
air=8.85* 10^12
glass ranges from 45 to 90 * 10^12
rubber 3 to 35 times ten to 12
mica 27 to 54times ten to the 12


Ps: Gary can you confirm those numbers for me

 

[Miles Prower]

^
|
|

That doesn't look right:

L= (uSN^2)/l where:

L: inductance in henries
u= u(rel)u(0)
Where: u(rel): Relative permeability of core (from data sheet); u(rel)= 1.0 (free space)
u(0): Permeability of free space: 4*pi X 10^-7 (H/m)
S: Cross-sectional area of coil in sq. m.
N: No. of turns
l: Length of coil in m.

Of course, there are some simplifications: the turns are close-packed, and fringing is ignored. Rarely is a close-packed winding a good practice as this maximizes the stray capacitance. That, in turn, reduces the coil's Q, and if the operating frequency is high enough, may prevent the coil from functioning as an inductor at all. Stray capacitance can also promote destructive series resonances, and annoying parallel resonances that can "take over" a circuit, causing oscillation way off frequency. If the turns are spaced, this formula won't be as accurate since the coupling coefficient between turns will be reduced. It does, however, give a good starting place for an initial trial run. Actual coil performance will need to be determined by empirical observation and testing.

 

[Dean Huster]

I just posted an answer to a similar question on Tony's forum yesterday!

From the ARRL Radio Amateur's Handbook --

For single-layer, air-wound coils, the equation is,
L = (a²n²)/(9a + 10b), where

L is the inductance in microhenries
a is the coil radius in inches
b is the coil length in inches
n is the number of turns

Dean

 

[solidhelix]

air wound coils means it has no core???

 

[Nigel Goodwin]

air wound coils means it has no core???

That's correct - if you have a core you need to know the EXACT specifications of the core, most formulas give air cored inductors for that reason.

 

[pandaemonium]

Theres some coil calculators here: www3.telus.net/chemelec/Calculators/Calculators.htm

Anyone know how to measure the magnetic permeability of a core material?

 

[chemelec]

I Do, But No time to answer you at the moment.

Maybe Later today.

 

[pandaemonium]

chemelec, I didn't even realize that was your page I posted a link to, funny!

 

[Miles Prower]

Anyone know how to measure the magnetic permeability of a core material?

That's usually not necessary. If you have some anonymous core that you'd like to use, simply wind a test coil and measure the inductance of that coil. Then all you have to do is figure your design inductance thusly:

N= N(test) * sqrt(L/L(test))

In effect, you are creating your own A(l) number for the core. With known cores, the manufacturer provides these A(l) numbers: so many uH per 100 turns for high frequency cores, or mH per 1000 turns for the high permeability, low frequency cores.

For most applications, the A(l) number method of designing inductance is accurate enough. There will always be some inaccuracy since it's very difficult to impossible to wind fractional numbers of turns anyway.

 

[chemelec]

pandaemonium Posted: Mon May 02, 2005 11:15 pm Post subject:
chemelec, I didn't even realize that was your page I posted a link to, funny!

No Problem.

If you go to:

**broken link removed**

You will see AL-Value-F********.jpg

It gives you some of the Necessary Formulas.
I Should Reprint this Properly and Post it on my Site. I'll try to do that soon and also put a Better Step by Step Instruction with it.
But For Now, EXCUSE MY POOR PRINTING and the Rather poor Photo from my Notebook.

Basically as Miles Prower said, You need to wind a Test Coil on the Unknown Core. Than Measure its Inductance.
Sorry, You DO NEED An Inductance Meter for this.

Once you have that you can calculate the AL Value to Determine Other coil Inductances on that Inductor.

But Other Inductors will be Different.
Additionally If its a Gapped Ferrite, Changing the Gap, Changes the AL Value and Inductance.

 

[chemelec]

Here is my Revised Article for Calculating Coils on Ferrites or other core Materials.

**broken link removed**

I Hope this helps some of you.
And Hopefully I did it Correct and easy to follow.

Take care.........Gary

 

[gastonanthony]

actually, I would like to create coils for rf frequencies greater than 10MHz for a transmitter. would a microstrip antenna serve the purpose better?
if yes, could you give me a link on designing one?thanks to all of you guys

 

[Nigel Goodwin]

actually, I would like to create coils for rf frequencies greater than 10MHz for a transmitter. would a microstrip antenna serve the purpose better?
if yes, could you give me a link on designing one?thanks to all of you guys

No, it's far too low a frequency.

 

[gastonanthony]

What's the minimum frequency that is practical for the use of microstrips?do you have any links on how to design RF transmitters, the theory behind it, sample designs with explanations, practical step-by-step guides, etc..

 

[Nigel Goodwin]

What's the minimum frequency that is practical for the use of microstrips?do you have any links on how to design RF transmitters, the theory behind it, sample designs with explanations, practical step-by-step guides, etc..

I would suggest UHF and above, I don't have any links I'm afraid!.

 

[Miles Prower]

do you have any links on how to design RF transmitters, the theory behind it, sample designs with explanations, practical step-by-step guides, etc.

No links, but here's a suggestion: get yourself a copy of The ARRL Handbook. It's by no means completely comprehensive, but it does include everything you asked for here. Even at $25+, it's a good investment. See if there are any stores in your area that deal with amateur radio supplies and they'll probably have it. Or write:

ARRL Headquarters
225 Main Street
Newington, CT 06111
Tel: (203) 666-1541

to see about ordering one.