I need to understand the meaning of this word "2 1/2 turn ferrite bead" ?? What exactly this 1/2 means ? Does this means that the wire is turned twice inside the bead?

• Related: Curious-looking inductors The OP's picture shows a 2.5-turn bead. Apr 7, 2019 at 14:01
• Does the image in your head picture a one-hole toroid, or a two-hole toroid, or a multi-hole ferrite? Half-turns don't make much sense for a one-hole toroid. Apr 7, 2019 at 14:23

In this terminology you have encountered,

• One turn of a coil is the wire making a full loop and ending up where it started.

• Half a turn is the wire passing through the core without looping.

So for a 2 1/2 turn coil, you put the wire through the the core/bead three times. A half turn by itself is a ferrite bead on a straight wire.

In general, an “and a half” coil has the two ends of the wire exiting from opposite sides. A “whole number of turns” coil has both ends on the same side.

Example of 2 1/2 turn coil (photos from this question); observe that the ends of the wire are on opposite sides of the board:

Example of 1/2 turn coil, a bead on a wire (taken by Wikimedia Commons user Omegatron):

I'd like to add some photos of "whole turn" coils but couldn't find any suitably licensed and I don't recall which piece of my own equipment to disassemble to find one :)

• Odd, I'd count your black-wire photo of that one-hole "bead" as one turn. It is carrying current, which has to form a complete loop....hence one turn. Apr 7, 2019 at 14:26