An internal wireless wide-area network (WWAN) antenna suitable for GSM850/900/1800/1900/UMTS operation in the clamshell mobile phone with reduced ground plane effects is presented. Small variations in the performances of the antenna for the clamshell mobile phone in the open state (talk condition) and closed state (idle condition) are obtained. This is owing to the reduced effects of the upper (cover) ground plane on the performances of the internal antenna obtained by embedding a slit at the edge of the upper ground plane close to the connecting strip between the two (upper and main) ground planes of the clamshell mobile phone. In this case, large surface currents excited around the slit are achieved, with other portions of the upper ground plane showing much weaker surface current distributions, which makes the embedded slit behaves like a current trap for the excited surface currents. This condition results in reduced effects of the upper ground plane. This also makes the near-field radiation of the clamshell mobile phone with the proposed antenna easily meet the specific absorption rate (SAR) and hearing aid compatibility (HAC) specifications required for practical applications.