The antenna efficiency relates the power transited from M2M module to the antenna and the power radiated or dissipated within the antenna. A high efficiency antenna has most of the power present at the antenna’s input radiated away. A low efficiency antenna has most of the power absorbed as losses within the antenna. Even when the M2M module transmits the power and supports perfect 50 Ohm impedance the loss in the coaxial cable can be already high. Think about that 3 dB is already 50%. With a good embedded antenna on reasonable ground plane, the antenna efficiency of an M2M system can be much better, because no coaxial cable or very short coaxial cable will be used.