Co-location of frequency hopping systems occurs both in civilian and military applications. For close co-location in such scenarios, out-of-band emissions can be of significant importance and therefore have to be considered in the analysis. Intersystem-interference analysis of scenarios with several systems using a large amount of frequencies can be complex and time consuming if one wishes to consider out-of-band emissions. In this paper possible simplifications is suggested and evaluated with respect to the resulting bit error probability. Our results show how different simplifications in modeling the out-of-band emission influence the resulting bit error probability.