# Inductor specifications for AM circuits

I am working with the book "Build Your Own Transistor Radios" by Ronald Quan in which he has a lot of AM projects. I am having a hard time figuring out what I need to buy from his specifications. For example, he has the item L1 Antenna Coil, 470 mH primary 23 mH secondary which I assume means I need to buy two axial inductors, a 470 and 23. But there are a lot of possible parameters: tolerance, max DC, max current, self-resident frequency, whether it is shielded, etc. How do I know what to get?

Also, sometimes he talks about a "wrapped" inductor. For example, antenna coil 470 mH with 1 turn wrapped. What does that mean? If you go to Mouser you do not find anything with this description. Another example is antenna coil, 600 mH to 680 mH, 10 to 20 turns for secondary winding.

I'm a little surprised that the book tells you nothing about how to actually make the inductors. For AM, you generally make what's called a 'loopstick' antenna, like so:

(diagram from quazar31)

The idea is that you wind a primary and a secondary coil such that the flux in one affects the flux in the other, and vice versa, hence it acts as a transformer.

Whenever you wind a coil, the inductance is proportional to the number of turns and to what's called the permittivity of the substance inside the coil. Ferrite has a much larger permittivity than air, which is why it's frequently used in inductors. See the Amateur Radio SE for a more detailed discussion of ferrite parameters. Of course, you can also make air coils, but you will need many more turns.

A resource like the ARRL handbook will give you rules of thumb for getting the correct inductance. Having an inductance meter (or a function generator and 'scope) helps for double-checking.

• Thanks very much. The book should have included this information. – Tyler Durden Apr 7 '14 at 15:49

Well, first thing is units; you are talking about millihenries (mH) and almost certainly you should be talking about microhenries (uH or more properly $\mu$H).

Regards making coils, here's a short introduction. It's called "homebrew your own inductors".

Regards the word "wrapped" the only thing that springs immediately to mind is the spiral wrapping used when making planar transformers: -

I don't think you necessarily mean this so a little more context would help. Maybe you can take some photographs of the document you refer to?

Regarding the picture above, you can see that each turn will have capacitance to the next and this can be lumped to one capacitance in parallel with the whole coil (for convenience). This is now a resonant circuit and has a "self-resonant-frequency". This means you should try and avoid this area because it is hard to control. In fact, above the SRF the coil no longer behaves like a coil but a capacitor.

Shielding for AM coils is usually not a big deal except when it comes to superhet receivers and in particularly we are talking the IF stages - these need to be shielded to prevent the whole IF strip oscillating.

Any inductor with a primary and secondary is called a transformer and the two coils need to be wound together (usually small on top of big or by using a centre-tap).

L1 has two windings (primary and secondary) on the same former.

The wrapped (in tape) antenna coil also has two windings. One is just a single turn.

You will probably have to wind them yourself. Suitable formers can be made from plastic pipe, you will have to calculate the number of turns you need - formulae and instructions can be found on this site, and on the web.

• Are you talking about just the "wrapped" ones or the first mentioned one as well? The author does not describe how to make my own antenna coils. How can I find out how to do that? – Tyler Durden Apr 6 '14 at 18:40