In this paper, outdoor RF spectral survey results in a sub-urban area are demonstrated. this work was carried out to explore and define the ambient RF signal sources available in the populated environment so as to select the frequency bands most suitable for designing RF energy harvesting circuits. Referring to the Egyptian frequency allocation chart, the ambient RF power sources within the spectrum of 0.5 to 2.2 GHz has been detected and measured to cover the mobile downlink frequency bands (2G, 3G and 4G) and the digital TV broadcasting bands which represent the most favourable frequencies for ambient RF energy harvesting circuits which is convenient for outdoor applications that needs power supply source in the range of microwatts. According to the available parameters of the cellular network standards, the measurement results were compared with the analytical empirical path loss propagation models focusing on sub-urban areas which provide an estimation of the median path loss as a function of distance and the urban terrain classification. The models that are discussed in this paper are HATA Model, Ericsson Model and ITU-R Model. It is found that HATA model can be used to give the best predictions with minimum error for losses and received power levels. Based on the measured data, a designer can decide the maximum distance away from a cell-phone tower that meets certain detection sensitivity. This study is useful for ambient RF energy harvesting designers.