In software defined radio (SDR) systems, it is desirable to down-convert multiple RF signals simultaneously by placing an analogue-to-digital converter (ADC) as near the antenna as possible. Radio frequency (RF) subsampling is one of the established methodologies used by radio receivers to directly down-convert and digitize RF signals. In this paper, we propose a novel direct RF sampling method to find out the minimum sampling frequency for an evenly spaced spectrum comprising multiband RF signals. However, it can also be applied to nonuniformly spaced signals with slight modification. The proposed methodology describes a set of rules to achieve the lowest possible sampling frequency rates without the compromise of spectrum folding or overlapping of aliases in the baseband after down-conversion. Moreover, the proposed formula is general, and flexible to the number of input signals or bands and to their positions in the desired spectrum. Simulations verify that by using the offered minimum sampling rates, the desired signal is extracted without the mentioned constraints.