Low frequency astronomy — The challenge in a crowded RFI environment

Low frequency radio astronomy is a hot topic at the moment. Many large arrays of antennas are built to facilitate the astronomical research on low frequencies. Building an instrument for the frequency band below 30 MHz on Earth will run into some problems. One of the issues is the instable and sometimes even opaque ionosphere at low frequencies. Another issue is the man-made Radio Frequency Interference (RFI) at low frequencies. In this paper we will address the later one. An overview will be given of propagation models at lower frequencies; we will present measurements and will summarize the impact of RFI on radio astronomy at frequencies below 30 MHz.