# Calculating Distributed Capacitance of a Bifilar Coil?

1. Mar 25, 2011

### HMS-776

Hello everyone,

I have been tirelessly searching the net trying to find out how to calculate the distributed capacitance of a multi-layer bifilar coil.

Just wondering if anyone here can point me in the right direction?

2. Mar 25, 2011

### Bob S

For Hi-Q high frequency coils, you want Litz wire. The turn-to-turn capacitance is the same as single conductor wire, but the eddy current (skin depth) losses are less. Litz wire is multiconductor wire with insulated strands. What do you mean by bifilar coil? See

http://www.die.ing.unibo.it/pers/grandi/papers/ia99.pdf

Bob S

3. Mar 25, 2011

### Staff: Mentor

Bifilar windings are when the primary and secondary wires are wound together on the bobbin or core. It's generally done to minimize leakage inductance Lk, at the expense of much higher winding-to-winding capacitance Cww.

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

I'm not sure of a good way to calculate the capacitance, though....

4. Mar 25, 2011

### Bob S

The capacitance between the two conductors in a bifilar winding (neglecting turn-to-turn effects in a coil) is the same whether the bifilar pair is wound in a coil or in a long straight line. The capacitance per unit length is

C = πε0/[cosh-1(D/2R)]

where D is the center-to-center spacing of the two round conductors, each of radius R. (See Smythe Static and Dynamic Electricity Third Edition, Section 4.14).

It is also possible to get to this number by calculating the transmission line impedance of a twisted pair or a twin-line transmission line, and knowing that

a) The signal velocity in an air-filled transmission line is c = 1/sqrt(LC), and

b) The transmission line characteristic impedance is Z = sqrt(L/C)

where L and C are the inductance and capacitance per unit length.

Also look up Lecher Line capacitance in http://en.wikipedia.org/wiki/Lecher_lines

Bob S

5. Mar 25, 2011

### HMS-776

Thanks Bob S, I greatly appreciate it. although I am really hoping to find the equation one would use for a multi layer coil.