Inductors Technologies: Molded vs. Air-Turned

[Ravaner]

Hello. Just a basic question : is there any difference ( Q, parasitic capcitance ... ) between molded inductors and air inductors turned by hand. Let say for instance for values in order of 1 µH ?

 

[Baluncore]

Hello. Just a basic question : is there any difference ( Q, parasitic capcitance ... ) between molded inductors and air inductors turned by hand. Let say for instance for values in order of 1 µH ?

Yes.
To identify the pros and cons of wound inductors you need to be more specific about your examples.

 

[Ravaner]

I don't really understand your reply. Assume my circuit is supposed to run at 50 MHz ( an amplifier or oscillator ... no matter)
I may wound a coil working in air or used a molded inductor. Except problems relative to room, is there a predictable difference of behaviour.

 

[tech99]

Hello. Just a basic question : is there any difference ( Q, parasitic capcitance ... ) between molded inductors and air inductors turned by hand. Let say for instance for values in order of 1 µH ?

You may find that an air cored inductor is large by today's standards if a good Q is to be obtained. And I expect you will also want it in a shielding can, which will exaggerate the size. A ferrite core will increase the Q for a given size.
To give you an idea, in the early days of television, which used these frequencies, we would use about ten turns wound on a pencil and then removed!

 

[Baluncore]

Inductors are extremely difficult to predict accurately. If you want accurate model values for an inductor, you will need a lot of experience and some heavy mathematics to account for the construction and environmental reality.

There is no such thing as a reference inductor, they are always “ugly”. Inductors have capacitance between the ends, and between all the permutations of turns. They have resistance and transmission line effects due to wire length. Air core coils have a greater stray magnetic field that, like an antenna, reaches out and reflects back from the universe around them.

A ferrite core gives a higher inductance for the same length of wire, so the inductor will be physically smaller, but the Q will be higher since X_L/R is greater and the self-resonant frequency will be at a higher frequency. But at some frequency the core will be lossy, so the core will get hot and and the Q will fall. Ferrite can be saturated, so permeability, Ur, is flux dependent. Ferrite has a permittivity, Er, that increases stray capacitance, but the Er of ferrite is highly frequency dependent.

To minimise external fields wind a wire evenly onto a toroid, then wind it back to the starting point so as not to make a closed antenna loop the size of the toroid. If you put an air cored coil in a metal box you must consider what happens to flux that reaches the metal wall. Does the reflected field phase increase or reduce the inductance, do eddy current losses reduce the Q.

The complexity of inductance calculation by Geometric Mean Distance, GMD, has been largely ignored since the time of Maxwell's 1865 paper, and the work of Rosa and Grover at the NBS. If you find yourself computing GMDs then you will begin to understand the problems.

 

[Rive]

I don't really understand your reply. Assume my circuit is supposed to run at 50 MHz ( an amplifier or oscillator ... no matter)
I may wound a coil working in air or used a molded inductor. Except problems relative to room, is there a predictable difference of behaviour.

No. And exactly that's the problem - the difference in behavior is not predictable. Those hand made coils in tuners and such are not just hand made, but individually fine-tuned after assembly. With machine made coils what you get is already sorted, values and parameters are guaranteed, you just pick them, put them on the PCB and it'll work within the known limits.

Sometimes you can't spare the manual tuning of coils, but why make it more difficult intentionally? If a stock one will do then it's better to use that.