Compact design of an octagonal star-shaped UWB antenna using Ant Colony Optimization for Continuous Domains (ACO <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msub> <mml:mrow/> <mml:mrow> <mml:mi>R</mml:mi> </mml:mrow> </mml:msub> </mml:math> ) for wireless applications