Printed log-periodic feeds for radar applications

Radar systems are well known since the end of world war II and are widespread. The basic concept of radar is relatively simple: the system uses the propagation characteristics of electromagnetic waves and works by radiating electromagnetic energy in the space and detecting the echo reflected by the objects (named “target”). The information related to the target is available in the echo signal: the range, or distance, to the target is found from the time it takes for the radiated energy to travel to the target and back; the angular location is detected using narrow beam width directive antennas (i.e. reflector antennas). The ability to discern about the nature and size of the target depends on the radar resolution which is related to the bandwidth and to the electrical size of the antenna. The physical principles of radar are suitable also to the weather radars, used to detect the precipitations at long distance, where the "object of interest” (the target) are the particles of rain and clouds. The detection of these objects involves radar operating at multiple frequencies located in S and C bands, where the wavelength (from 15 to 3.75 centimeters) is comparable to the size of hydrometeors. Radar systems use directive antennas, such as mechanically steered parabolic reflector antennas and planar phased arrays. Aim of these antennas is to concentrate the energy in a narrow beam width of about 1° or 2°. This characteristic not only concentrates the energy on the target itself, but also permits a measure of its direction. In this work we have studied the substitution of reflector horn feeds with printed log-periodic planar feeds in order to obtain the same performance of the horn antennas, but with considerable advantages in terms of weight and cost. The design of log-periodic feeds has been focused on the feed antenna network, and their possible improvements with respect to the state-of-the-art. In the first chapter, a general description of radar systems is given, introducing the Probert-Jones equation for meteorological target. The second and third chapter provide a brief description of antennas concepts and an introduction to reflector antennas. The chapter four is fully dedicated to reflector horn antennas and printed microstrip feed with a brief description of the state-of-art. Finally, in chapter five, three printed log-periodic feed for reflector antennas are presented. The first structure is a printed log-periodic array (LPDA) operating over the C, X and Ku bands. The antenna feeding structure consist of two coaxial cables, in order to realize an infinite balun which provides the required broadband input matching. The second coaxial cable mirrors the first one, connected to the antenna input, and is capable of both stabilizing the antenna phase center and improving the radiation pattern. Both the simulated and measured results have show that the LPDA can be successfully user as a wide band feed for reflector antennas. The second feed consists of a LPDA operating between 3 and 6 GHz, and fed with a coplanar waveguide. The structure has been designed starting from Carrel’s theory, optimized using CST MICROWAVE STUDIO 2012 and then realized. The comparison between simulated and measured results shows that the proposed antenna can be used as feed for reflector broadband applications in the whole operating frequency band, with a very good input matching and a satisfactory endfire radiation pattern. The results obtained with these two printed LPDA provided the starting point for the design of a high gain “V-shaped” log-periodic feed for weather radar applications, operating in S and C band and fed by the dual coaxial cable configuration. The developed feed provides very good input matching and a symmetric radiation pattern both in E-plane and H-plane, with a reasonable gain over the whole operating bandwidth. This feed may be recommended for usage in weather radar systems having a transmitting power less than 400 KW, allowing a discrete operational range.