Co/Ti-substituted Sr-based M-type hexagonal ferrite containing composite sheets are used for the demonstration of electromagnetic noise suppression at and above 6 GHz. Three different compositions (SrCoxTixFe12 − 2xO19 with x = 1.0, 1.2, and 1.4) and two different size groups with the average diameter of ∼10 µm and ∼1 µm have been studied to understand the effect of Co-Ti substitution level and size on the high frequency magnetic characteristics of SrM hexaferrites. A drop in coercivity from 285 Oe to 40 Oe and 710 Oe to 90 Oe was observed as the Co/Ti substitution level, x, increases from 1.0 to 1.4 in the case of 10-µm and 1-µm particles respectively. Complex permeability was measured by two methods – by microstrip probe, and by all-shielded and shorted microstrip line. Ferromagnetic resonance frequency drops from ∼20 GHz to ∼3 GHz as x increases from 1.0 to 1.4, while it decreases a little as the particle size decreases. Electromagnetic noise suppression was demonstrated by measuring the ratio of power loss in the magnetic element to the input power by placing it on top of a microstrip line. With a 500-µm thick composite sheet, electromagnetic noise suppression up to 20% was achieved.