X-Band Squintless Horn Antenna Array (96 elements)

Reutech Radar Systems (Pty) Ltd recently required a new antenna for an X-Band radar system and the design of a full plank of the receive antenna was performed using CST MICROWAVE STUDIO® (CST MWS). Typical design requirements have been vertical polarisation, 70º elevation beamwidth, 1º azimuth beamwidth with 30dB sidelobe levels, no squint and 20% operating bandwidth. The first two requirements are met by using a small E-plane sectoral horn shown in figure 1.




Figure 1: E-Plane horn


The azimuth properties of the antenna must be met by the design of the antenna feed. A 12-way squintless wideband waveguide E-plane divider section (see figure 2), based on a feed described by A Rogers [1], has been used as building block for the feed.



Figure 2: E-plane divider building block


An impedance transformer and 90º waveguide twist section are required to match the feed to the E-plane sectoral horn radiator. A machineable waveguide twist section consisting of L-shaped waveguide, similar to that published by Lenzing and Gans [2], is used to change the polarization from horizontal to vertical.



Figure 3: Twisted waveguide section


To evaluate the design approach, a prototype sub-antenna consisting of a 12-way feed, twist sections and E-plane horns was manufactured from Aluminium using CNC-machining and wire-cutting. The gain patterns were measured in the anechoic chamber at Stellenbosch University and are in good agreement with the expected results given by CST MICROWAVE STUDIO®.



Figure 4: Manufactured 12-element antenna


The design was then extended to a 2.4 meter full antenna plank with 96 radiating elements. For the excitation distribution the 96 elements were divided into 16 banks with a Villeneuve distribution. Each of the banks contains 6 elements with a linear excitation distribution. Such a distribution allows for the reuse of 6-way E-plane dividers throughout the antenna feed.



Figure 5: Full antenna blank with 96 radiating elements


The final feed consists of four levels of 3-port E-plane dividers connected with standard X-band waveguide. Since the feed structure is symmetrical, in a first step only half of the feed was analysed in CST MICROWAVE STUDIO®.



Figure 6: CST MWS model of one-half of full antenna bank




Figure 7: Magnitude of frequency response (half design)




Figure 8: Phase variation between channels (half design)


The complete antenna (with 96 ports excited at the same time) was also analysed in CST MICROWAVE STUDIO®, giving the following azimuth gain patterns.



Figure 9: Excitation and phase pattern used for the 96 elements




Figure 10: Azimuth patterns from full plank CST MWS analysis


The full antenna plank was manufactured and the radiation patterns were measured at the compact range at the University of Pretoria.



Figure 11: Compact range measurement setup at University of Pretoria




Figure 12: Measured azimuth pattern




Figure 13: Measured azimuth pattern


The azimuth radiation patterns show that no side-lobes higher than –30dB are measured outside ±20º from the main lobe. Some larger close-in side-lobes exist, which can be seen more clearly in figure 13.  The highest side-lobe level is measured as –23dB at 10GHz. The 3dB beam-widths of the azimuth patterns vary from 1.2 to 0.8º over the frequency range. This corresponds very well with the expected results.
References
[1] A Rogers, “Wideband Squintless Linear Arrays”, The Marconi Review, Volume XXXV, Number 187, Fourth Quarter 1972, pp 221 – 243.
[2] H. F. Lenzing, M. J. Gans, “Machined Waveguide Twist”, IEEE Trans. MTT, Vol. 38, no. 7, July 1990, pp. 942 – 944.