Estimating the size of a radio quiet zone for the radio astronomy service

Abstract

The size of a radio quiet zone (RQZ) is largely determined by transmission losses of interfering signals, which can be divided into free space loss and diffraction loss. The free space loss is dominant. The diffraction loss presented in this paper is described as unified smooth spherical and knife edge diffractions, which is a function of minimum path clearance. We present a complete method to calculate the minimum path clearance. The cumulative distribution of the lapse rate of refractivity (g n ), between the earth surface and 1 km above, is studied by using Chinese radio climate data. Because the size of an RQZ is proportional to g n , the cumulative distribution of g n can be used as an approximation for the size of the RQZ. When interference originates from mobile communication or television transmissions at a frequency of 408 MHz, and $\overline {g_n } $ is 40 N/km, where the refractivity $N=\left( {n-1} \right) \times 10^6$ , the size of the RQZ would be 180 km for a mobile source or 210 km for a television source, with a probability in the range of 15–100% in different months and for different stations. When speaking of the size of an RQZ, the radius in the case of a circular zone is implied. It results that a size of an RQZ is mainly influenced by transmission loss rather than effective radiated power. In the case where the distance between an interfering source and a radio astronomical observatory is about 100 km, at a frequency of 408 MHz, the allowable effective radiated power of the interfering source should be less than −30 dBW with a probability of about 85% for $\overline {g_n } $ equals 40 N/km, or −42 dBW with a probability less than 1 % for $\overline {g_n } $ equals 80 N/km.