Several factors come into play: Band, power emissions at TX site, number of sites repeating same signal just to name a few. Rule of thumb is the lower the frequency the further the signal propagates, but there are exceptions to this rule. VHF generally propagates much further than 700/800mhz when conditions are favorable. Also geography, topography between you and the transmit site(s). Atmospheric & tropospheric conditions, your & TX site's elevation. Also the antenna you're using will have an impact on what your scanner is able to receive. Theoretically the higher the antenna the more you'll be able to receive. Many many different things, I'm sure I'm missing others