A new Waffle-iron Ridge Guide (WRG) structure that has the ability to control both wavelength and impedance is proposed. With the proposed structure, not only can the wavelength be controlled over a wide range for both fast- and slow-waves in free space but the impedance can also be controlled. These features can improve the performance of array antennas in terms of reducing grating lobes and side lobes. In this paper, we discuss and evaluate a design scheme using equivalent circuits and EM-simulation. This paper also discusses how the conductivity and dielectric loss in the WRG affect the total gain of the array antenna.