Post couplers are available for 2 3/8" O.D. round sign posts.  Made from a high quality aluminum extrusion, these couplers are most often used to extend an existing post to accommodate additional or larger sign panels and maintain MUTCD minimum sign height requirements, without disturbing landscaping, paved surfaces or sidewalk panels.  The coupler should be placed where it will fall between the upper and lower attachment points of a sign panel, and not used unsupported beneath the signs or near ground level.  While not inexpensive, they save the time and expense of removing and replacing an otherwise serviceable sign post, and the surrounding surfaces.