Miniature directional coupler

Differing from conventionally-used directional coupler being implemented on a planar board having a small size like coin, the present invention stacks a bottom substrate, at least one phase retarding unit, at least one reference ground unit, a coupled layer, a main layer, and a top substrate to form a miniature directional coupler. This miniature directional coupler would not occupy too much circuit area if the miniature directional coupler is mounted on a main circuit board of a mobile communication product, such that miniature directional coupler can meet the requirements of light weight and compact size demanded by modern mobile communication products on the internal electronic components thereof. It is worth explaining that, because the said phase retarding unit consists of a plurality of end-to-end connected transmission wires, engineers skilled in designing microwave circuit are able to carry out the modulation of coupling flatness of the miniature directional coupler by changing a total length of the e