  In this paper, the Q-factor expression for periodic arrays over a ground
plane is determined in terms of the electric current density within the array's
unit cell. The expression accounts for the exact shape of the array element.
The Q-factor formula includes integration only over a volume of an element in a
unit cell, and can thus be efficiently implemented numerically. The examples
show good agreement between the proposed Q-factor, the full-wave-calculated
tuned fractional bandwidth, and the input-impedance-based formula by Yaghjian
and Best (2005) for the array of tilted dipoles and the loops array.
