# What coil is this

#### ceidas

Joined Dec 26, 2011
50
I found a thread where audioguru had posted this coil and I have seen it in many high frequency projects so I want to ask about it. What material is this? Arent air cored coils good only for am radio frequencies? How do you measure the inductance of such coils? Is there a formula or something?

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#### DerStrom8

Joined Feb 20, 2011
2,390
That is just a simple wire-wound inductor. It is often used in RF circuits, like radios or radio transmitters. combined with a capacitor, it can create a high frequency oscillation, necessary for the transmission and tuning.

It is usually made of enameled copper wire. You can measure the inductance with some multimeters. You can also use an online calculator to determine its inductance.

#### ceidas

Joined Dec 26, 2011
50
Thanx for the answer. One last question, does the AWG number counts or whatever is good? Because i have heard that for ferrite rods coils you must use a precise number and for air coils everything is good.

Joined Dec 26, 2010
2,148
Thanx for the answer. One last question, does the AWG number counts or whatever is good? Because i have heard that for ferrite rods coils you must use a precise number and for air coils everything is good.
For any coil the wire thickness will have some effect. With a coil like this, where the wire diameter is appreciable compared to the other dimensions of the coil , you would be well advised to stick to whatever was specified to get the best chance that the tuning range will be correct.

The wire thickness for a ferrite cored antenna is not especially critical, in comparison with other coils. If winding a replacement coil for a radio, it will again be best to make the coil as close as possible to the original, to give the best chance of being able to align the aerial circuit properly.

Some long-wave antenna coils, not seen in North America, have many turns of special multi-strand wire called litz (or litzendraht). This is supposed to reduce the losses of the big 3.5mH tuning inductance. The coils are sometimes wound in a fancy wavy method. If you try to repair such a thing with ordinary wire wound in small piles, the results are not as good, but not completely useless if you know what you are doing.

#### debjit625

Joined Apr 17, 2010
790
Because i have heard that for ferrite rods coils you must use a precise number and for air coils everything is good.
I think you are asking about magnetic permeability of the core, in case of ferrite it have a higher magnetic permeability than air so the inductance of the coil will be also greater than a coil with air core. The core actually effects the inductance of the coil ,a core with more magnetic permeability will result in greater inductance than a core with less magnetic permeability.

http://en.wikipedia.org/wiki/Magnetic_core

The AWG stuff decides the thickness of the wire, more thickness results a larger coil but also less DC resistance. Normally where DC resistance of a coil could be a problem ,we design the coil using a thick wire i.e.. less AWG (In my country its SWG).

Good Luck

#### jegues

Joined Sep 13, 2010
733
How do you measure the inductance of such coils? Is there a formula or something?
Since you have some rough dimensions of the coil you could turn this sort of thing into an electromagnetics problems and solve for the inductances use a realtive permeability from a table for the type of material used in the winding.

#### SgtWookie

Joined Jul 17, 2007
22,227
That coil would measure about 116nH to 122nH.

#### Audioguru

Joined Dec 20, 2007
11,248
I used the thick 1mm enamelled copper wire from a speaker crossover coil so my coil will never get crushed. The speaker has plenty of wire so it still works fine when it is missing a few cm of wire. The thick wire allows my coil to have a high Q.

Of course its core is air because it operates at the very high frequency of 100MHz on the FM broadcast band. It is the same coil for AM aircraft radios at frequencies slightly above the FM broadcast band.

The AM broadcast band is at 1MHz which is about 100 times lower where ferrite is the core.

It is difficult to calculate the L and C for the very high frequency of 100MHz because of stray inductance and capacitance.

#### ceidas

Joined Dec 26, 2011
50
Actually, i want to use it in the 5000-1500 KHz AM band. Is it still good or this time I need the ferrite rod?

#### Audioguru

Joined Dec 20, 2007
11,248
I don't know where on earth you are. In North America the AM broadcast band is from 540khz to 1600kHz. A ferrite rod antenna and a tuning capacitor of 365pF max is used in the first tuned LC circuit.

My air-core coil is used on the FM broadcast band from 88MHz to 108MHz which is 100 times higher than the AM broadcast band.

#### Audioguru

Joined Dec 20, 2007
11,248
Deleted duplicate post.

#### ceidas

Joined Dec 26, 2011
50
I am sorry, my question was not clear. First of all, I live in Europe. Now, if i use enameled copper wire to make an air cored inductor, will i be able to tune in the AM radio band? (if i use the correct farads in the tunning capacitor).

Joined Dec 26, 2010
2,148
You would require a very much larger inductance to tune the Medium Wave AM band, which in Europe is 520 kHz to 1610 kHz: a little different from North America, but not much. An even bigger inductance would be needed for the European Long Wave band from 148.5 kHz to 283.5 kHz.

Typically these coils will be tuned with a maximum capacitance of a few hundred picofarads. Depending on what capacitor value is chosen, the inductances required will work out at about 300μH (micro-henries) for MW, and 3mH (milli-henries) for LW.

You can check this for yourself, using the formula f = 1/(2*pi*√(LC))*, which gives L = 1/ (((2*pi*f)$$^{2}$$)*C). Because of the square root relationship, the coils are hugely bigger than the hundred nano-henries or so needed for VHF FM, even allowing for a different tuning capacitance.

*(f is the resonant frequency in Hz, L is the inductance in Henries, and C is the capacitance in Farads)

There is no reason that you cannot use air-cored coils made with enamelled wire for this. In the past this was commonly done, but low-frequency coils made this way tend to be quite bulky. There are many on-line calculators for coils - here are a couple: http://www.crystalradio.net/cal/indcal2.shtml http://www.qsl.net/in3otd/indcalc.html

All things considered, winding a coil on a suitable grade of ferrite bar may be a lot easier.

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