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A substrate integrated waveguide (SIW) loaded by embedded split-ring resonators (SRRs) with transversal negative effective permeability is proposed. It is shown that the structure can support propagation of backward waves below cutoff frequency. Therefore, the width of the SIW structure can be considered less than half a wavelength at the cutoff frequency, which means that the SIW structure is miniaturized. In this letter, design and modeling of a miniaturized SIW structure is proposed. An experimental SIW loaded with double embedded SRRs in 4.75-GHz frequency band has been designed, fabricated, and tested. The measured and simulated results show a passband for backward waves below the cutoff frequency. It is also shown that the phase at a certain frequency in the backward passband increases as the physical length of the loaded SIW is increased (opposite to forward wave propagation), which proves that backward waves propagate below the cutoff frequency.