A simple composite material structure is proposed which allows suppression of harmonic modes for microstrip edge coupled bandpass filters. The structure can be employed easily in the printed circuit board (PCB) fabrication process and gives a simple method to equalize the phase velocity of even- and odd-mode propagation, within a given range of frequencies. It is known that equalizing the even- and odd-mode phase velocities supports minimizing harmonic responses of microstrip coupled features. A quick overview of auxiliary concepts is given, followed by models using a commercial field solving software demonstrating the phase velocity equalization as well as filter performance with suppressed harmonics. Finally, measured S parameters from circuit evaluations are shown, comparing conventional edge coupled microstrip filters with composite material based filters as well as software model comparisons.