Terahertz communications demonstrate advantages of wide bandwidth and low latency compared to the microwave counterparts but suffer from high propagation loss in the air. Little research focuses on long-distance (i.e., distance>1 km) terahertz propagation characteristics in the air, posing challenges on long-distance terahertz wireless applications. In this paper, the terahertz long-distance propagation characteristics are explored and studied with the help of the measurement system operating at 220 GHz. Based on the measurement systems, we conduct out-door experiments with a propagation distance of 2.5 km to obtain the terahertz propagation loss in the air. Additionally, a comparison between the measured propagation loss and the calculated one based on models issued by the international telecommunication union (ITU) shows an intrinsic loss gap between these two results, which is found and explained for the first time in the above-100 GHz long-distance communication applications. With the analysis of the collected data, we propose a gap-compensation method to obtain a more accurate prediction for the propagation loss in the air. With the help of the proposed systems and corresponding measurements, we could give insight into the terahertz propagation and pave the way for future terahertz long-distance applications.
