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#### Garion

##### New Member
I'm doing pre-tertiary electronics this year when I probably shouldn't be. You see I know nothing about electronics and have to take exams at the end of the year Anyway, that aside, as part of my moderation project I'd like to build an FM transmittor/reciever. I think I understand the most of it except the coil part (I think our teacher called it a tank?)

If anyone could provide me with some basic (very basic) tutorials as to how to wind the coil, the different measurements and what they relate to, and how it interacts with the capactitor (I'm running them in parallel)

I understand that the charge moves between the two (coil/capatitor) but what I really need to understand is how I get a frequency from a coil.

Garion #### audioguru

##### Well-Known Member
The frequency where the reactance of the coil (inductor) and capacitor are the same is their resonant frequency.

#### MrAl

##### Well-Known Member
Hi,

To add a little, there is a well known formula that goes like this:

F=1/(2*pi*sqrt(L*C))

That gives you the frequency for a given inductor L and capacitor C.
You can also solve for L or C knowing F and one other quantity so you
can calculate the required capacitance or inductance knowing the frequency
you need to work at.

One thing this doesnt show you however is the bandwidth, or frequency
selectivity. Depending on the absolute value of the cap (and thus inductor too)
for a given frequency, the bandwidth may be greater or more narrow.

#### k7elp60

##### Active Member
One reason it is called a tank is that when the capacitor is charged and the charging voltage is removed the capacitor will discharge into the inductor, then the inductor will discharge charging the capacitor. In doing so an ac wave is generated at the resonate frequency. This continues until the circuit losses use up the power generated by the oscillation. So it appears like a large tank of water with a small discharge.

#### Garion

##### New Member
@audioguru

Thanks for that, it helps to know what your actually searching for. @MrAl

But that formula will give you a rough idea as to where you need to scan, right? (Thats in the case of an FM transmittor, or whatever it's tuned into?)

@k7elp60

Thanks, thats a handy tip - I was thinking of the wrong sort of tank (I have no idea why though )

Cheers!

#### MrAl

##### Well-Known Member
@MrAl

But that formula will give you a rough idea as to where you need to scan, right? (Thats in the case of an FM transmittor, or whatever it's tuned into?)

Cheers!

Im not sure what you mean by "need to scan".
That formula simply tells you what frequency a capacitor and inductor
will resonate at if it is used as an oscillator.
Did you mean that you need coil winding formulas?

#### Garion

##### New Member
Im not sure what you mean by "need to scan".
That formula simply tells you what frequency a capacitor and inductor
will resonate at if it is used as an oscillator.
Did you mean that you need coil winding formulas?

When I meant was need to scan on a radio (to pick up the frequency).

Having the formulas would be handy thanks, it can only help my understanding of it #### MrAl

##### Well-Known Member
Hi again,

Here are some formulas for radio coils. The results are approximate.

Note the use of the capital letter "I" to indicate inductance. This is
because "L" is used to indicate length.

For the following three formulas:

L=length of coil (inches)
B=build (inches)
I=inductance (uH)

I=(R^2*N^2)/(9*R+10*L) single layer cylindrical

I=0.8*(R^2*N^2)/(6*R+9*L+10*B) multi layer cylindrical (Wheeler's Formula)

I=(R^2*N^2)/(8*R+11*B) multilayer of length one wire diameter (flat coil)

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