The idea of this paper roots in the electromagnetic compatibility design of the whole aircraft system, in which out of band characteristics of airborne antennas are of equal importance as their characteristics in band. The definition of antenna characteristics for out of band is interpreted. Horn antennas are the main research objects in this paper. In order to make characteristic of this type antenna for out of band have a general, this paper is special to select pyramidal horn antenna and conical horn antenna. The characterization framework of horn antennas radiation characteristics for out of band is established, and the theoretical expressions of their radiation characteristics for out of band are given. In ensuring better antenna characteristics in band, four horn antennas that are different types of sizes, different frequency band width are simulated and analyzed by using electromagnetic simulation software HFSS. It is meaningful to analyze radiation characteristics and summarize different features for various performance parameters of these horn antennas for in band and out of band. Focus on characteristics of the changes for the maximum gain, 3dB beam width parameters of horn antennas within the working frequency band and out of working band. The results show that: radiation characteristics of horn antenna are the law within the working frequency band and out of working band.