Impedance Ratio:

Maximum power is transferred from one circuit to another by a transformer while the impedances are equivalent or matched.  The transformer winding constructed along with a definite turn's ratio could perform an impedance matching function. A turn's ratio will establish the proper relationship among the primary and secondary winding impedances.  The ratio among the two impedances is referred to as the impedance ratio and is expressed through using Equation (13-2).

(N_P/N_S) ² =Z_P/Z_S                                          (13-2)

Another way to express the impedance ratio is to take the square root of both sides of Equation (13-2).  That puts the ratio in terms of the turn's ratio that is always provided for a transformer.

where

N_P   =  number of turns in the primary

N_S   =  number of turns in the secondary

Z_P    = impedance of primary

Z_S    =  impedance of secondary