This paper looks into the underlying RF energy harvesting issues at low input ambient power levels below 0 dBm where efficiency degradation is severe. The proposed design aims to improve the rectenna sensitivity, efficiency, and output DC power. In the same manner, we are using a straightforward and compact size rectenna design. The receiving antenna is a coplanar waveguide (CPW) slot monopole antenna with harmonic suppression property and a peak measured gain of 3 dBi. Also, an improved antenna radiation characteristics, e.g radiation pattern and gain covering the desired operating band (ISM 2.45 GHz), is observed. The rectifier is a voltage doubler circuit based on microstrip (MS) structure. Two architectures of rectenna were carefully designed, fabricated and tested. The first layout; antenna, and rectifier were fabricated separately and then connected using a connector. The peak efficiency (40% at -5 dBm) achieved is lower than expected. To improve the efficiency, a high compactness and simple integration between antenna and rectifier are achieved by using a smooth CPW-MS transition. This design shows improved conversion efficiency measurement results which typically agree with the simulation results. The measured peak conversion efficiency is 72% at RF power level of -7 dBm and a load resistance of 2 kΩ.