The identification, friend or foe (IFF) system [1] is a well-established standard that was developed during World War II to identify friendly aircrafts from the foe’s on the battlefield via radio communication. It consists of two parts, the interrogator and the transponder, where the interrogator is the asking part transmitting at 1030 MHz and receiving at 1090 MHz and the transponder is the replying part receiving at 1030 MHz and transmitting at 1090MHz. The interrogator is often a ground based station and the transponder is often located on the aircraft. Nowadays, these identification systems are not only used for identification of targets’ characteristics, but also for the management of air traffic control [2].
Although the antenna utilized for the interrogator used to be a linear antenna array piggybacked on the primary search radar to be co-rotated mechanically, it can be a cylindrical antenna array as well with its unique scan-invariant pattern characteristics and electronic rotation. Note that cylindrical antenna arrays are popular because of their capability to scan the radiation beam in all angles of the plane of array without any change in their radiation patterns and compared to linear and rectangular arrays they are less sensitive to mutual coupling between the elements. Moreover, the wraparound configuration of the cylindrical antenna array makes it suitable for installations where the antenna should wrap around a tower or a ship mast [3] [4].
In a cylindrical antenna array, the antenna elements remain stationary or non-rotating while the beam is steered electronically to scan the full 360 degrees around the array. Also the beam can be positioned selectively in any direction within microseconds. There are several feeding systems enabling the electronic rotation of a prescribed illumination over a subset of active radiating elements distributed around a cylindrical antenna array (i.e. the commutation of the active sector), however, the most robust antenna architectures satisfying the requirements of reliability are the ones based on the Butler matrix [5].
The Butler matrix [6] is one of the most important parts of a cylindrical antenna array, specifically for the electronic rotation of the main beam and the permutation of the amplitudes and phases applied to the radiating elements during the commutation of the active sector around the array [7]. But as is well-known, the increase in the number of input and output ports of the Butler matrix, which is inevitably accompanied by the increase in the number of internal hybrid couplers and crossovers, makes the design and realization of this type of structure cumbersome, especially in the dimensions of 16x16 and beyond. So a simple search in the available sources reveals that almost all the available articles and reports are limited to the design of 4x4 and 8x8 Butler matrices.
If in the conceived structure for the IFF radar’s interrogator, a 64-element cylindrical antenna array is assumed and if the active sector is assumed to consist of 16 elements, then the electronic rotation of the radiation pattern around the array can be realized with the help of a 16x16 Butler matrix plus sixteen SP4T switches [3].
Regarding the design of the 16x16 Butler matrix, in [8] it has been tried to overcome the problem of many crossovers of this type of structure with the help of two back-to-back microstrip boards and thus a two-layer 16x16 Butler matrix is presented to be used in the frequency range of 25GHz to 30 GHz. It is worth mentioning that in the proposed structure in [8], the number of existing crossovers has been substantially reduced from 85 to 12.
In [9] and in order to avoid the problematic crossovers, the 16x16 Butler matrix structure is divided into several sub-sections and by separately designing and connecting these sub-sections through coaxial cables, a somehow 3D structure for use in the frequency range of 1.65 GHz to 2.17 GHz is presented.
In [10] and similar to what is sometimes suggested for 4x4 and 8x8 Butler matrices (as in [11]), it has been tried to change the topology of the structure and relocate the positions of the input and output ports around the PCB in order to avoid the occurrence of the problematic crossovers as much as possible. Thus the number of the crossovers has been reduced to only four and finally a structure for use in the frequency range of 9 GHz to 11 GHz is presented.
In this paper the design and simulation of the 16x16 Butler matrix is dealt with as well; and with a combination of the above methods, that is the relocation of the input and output ports of the Butler matrix and the use of two-layer microstrip boards we will try to avoid the occurrence of the problematic crossovers as much as possible. Thus, in Section II the new topology based on the relocation of input/output ports is presented. Then in Section III a new miniaturized wideband hybrid coupler is introduced in order to save as much space on the PCB as possible while obtaining a bandwidth as wide as possible. Then in Section IV the design and simulation of a 16x16 Butler matrix is discussed, and the simulation results related to the amplitudes and phases of the scattering parameters in all the input and output ports are presented. Finally a conclusion is drawn in Section V and the references are listed in Section VI.
Note must be made that since the desired Butler matrix is intended to be used in the interrogator part of an IFF radar, the design frequency is set to be 1060 MHz and Rogers 4003 substrate with a dielectric constant of 3.55 and a thickness of 60 mil is used. It should also be noted that due to the mentioned design frequency, which leads to relatively large dimensions for different parts of the structure, we are also facing limitations in choosing the dimensions of the PCB board, and therefore, in addition to the above methods, we will also use the common miniaturization techniques such as meandering to fit the entire structure in the largest PCB board available in the market, which is 610x457 mm2.