Jian said:

I know what you say. What I'm looking for is a good procedure

to calculate the L(f) based on L and M (there are formula for

straight wire, etc...). So, let's assume:

wire radius: r

distance: d

length: l

from ground: g

they are all in a couple of cm range.

According to the Allied/Radio Shack Electronics Data Handbook

(1970,1983), a parallel conductor line (ie: twin-lead) impedance is:

Z0 = (276/sqroot(k)) * log(2D/d) where the dielectric coefficent of

the surroundings is k, the wire diameter is d, and the seperation is D.

That is all i can find so far.

As a matter of note, if one makes a "gimmick" by twisting two wires

together, one gets roughly one pF per inch.

Obviously, the actual value will vary according to the insulation type

and thickness, along with the wire size.

Adding a ground plane will slightly decrease inter-wire capacitance if

it is realatively close.