The objective of the proposed design is to reduce the height of an FP cavity resonator in a MIMO antenna configuration and still achieve a reduced ρ. Note that ρ is a function of both the radiation patterns of the MIMO antenna elements and the propagation environment. However, in most cases, a fixed isotropic propagation environment with a balanced polarization is assumed when evaluating the performance of MIMO antennas. In such a scenario, ρ is only a function of the radiation patterns of the MIMO antenna elements. The expression for ρ in such a condition is given as1:
where F1 and F2 represent the complex field patterns of elements 1 and 2 of the MIMO antenna, respectively. Eq. (1) shows that ρ can be reduced in three different ways. First of these is to manipulate the relative phase of the antenna elements such that the numerator becomes small. This is mainly achieved by spatially separating the antenna elements. Some isolation techniques also affect the relative phase and helps to reduce ρ. The second way to achieve a low ρ is to place the elements orthogonal to each other to render their polarization also orthogonal with respect to each other, and thereby reduce ρ. Lastly, the main beam directions of individual antenna elements can be so tilted relative to each other that the numerator in Eq. (1) becomes small, thereby, decreasing ρ.
The method of beam tilting to reduce ρ has been reported in9 for closely placed antenna elements. Radiated beams of each element are tilted away from each other thus reducing their spatial correlation. This beam tilt in each element is achieved by designing an FP cavity-based MIMO antenna wherein a phase-gradient PRS is used. This phase gradient PRS is placed at an appropriate height over the primary radiating antenna elements which essentially forms a resonance FP cavity. This technique has shown to reduce the ρ by almost 95%.
The design technique of9 requires the antenna to be in the FP cavity configuration, thus making the antenna non-planar.
From10, the height of the FP cavity is given by:
$$h=\left({\phi }_{PRS}+{\phi }_{G}\right)\frac{\lambda }{4\pi }+N\frac{\lambda }{2} , N\in Z \left(2\right)$$
where φPRS is the reflection phase of the PRS; φG is the reflection phase of the ground plane; λ represents the free space wavelength and N represents the excitation mode of the FP cavity. Usually, the cavity is excited at the dominant mode for which N = 0. Therefore, the reflection phases φPRS and φG essentially dictate the choice for height of the cavity. In a standard FP cavity both surfaces are nearly Perfect Electric Conductors (PECs), essentially each providing a reflection phase of π. According to Eq. 2 this makes the height of cavity λ /2. Following the design approach presented in9, a 4-element MIMO antenna with PRS has been presented in11 while dielectric resonator-based MIMO antennas have been described in12 and13. In all of these designs, a cavity height of λ /4 was achieved by designing the phase-gradient PRS to be an Artificial Magnetic Conductor (AMC) which offers 00 φPRS instead of π.
Theoretically, the height of the cavity can be tailored to any desired value by manipulating the reflection phases φPRS and φG, albeit over a narrow band. Comprehensive research work has been carried out to investigate ways by which the height of the FP resonator can be reduced for a single antenna element. The techniques employed rely on altering the reflection phase φPRS14, φG15, or both, in a complementary fashion16 to design the ultrathin FP cavities. Technique of altering the φPRS, requires it to present reflection phase closer to -π so it can complement the φG having π value. Similarly, φG can also be designed to present reflection phase to complement φPRS. More closely these two surface complement each other in term of reflection phase, more compact cavities can be realized. Ultrathin cavities are formed by simultaneous tailoring of φG and φPRS so that they almost cancel each other at resonance frequency. Most of the previous works only attempt to change the φPRS because it does not affect the antenna operation. Once a PRS is placed at the correct height which satisfies the reflection phase condition in accordance with Eq. (2), a resonant FP cavity is formed from which waves escape in a coherent manner thus making radiated fields more directive10.
Antenna designs with tilted beams using the FP cavity-based configuration can also be found in the literature, as for instance in17-19, where phase-gradient PRSs are utilized. Such PRSs are made up of non-uniform unit cells which offer gradual decrease in the reflection coefficient magnitude in a particular axis, thus enabling the beam to tilt. To achieve higher degree of tilt, change between unit cells of maximum and minimum reflection unit cell has to be gradual which increases the lateral size of the structure. The reduced FP cavity height based work is also limited to a single radiating source.
The design proposed herein utilizes a phase-gradient PRS to tilt the beams of two radiating elements in a MIMO antenna in opposite directions to reduce ρ. However, it is designed to be placed at an ultra-low height for which the reflection phase of the
PRS is altered. The PRS is so designed that its reflection phase approaches − π, which is the complement of the reflection phase of the ground plane. At the same time, an important design consideration is to maintain a low coupling between the closely placed antenna elements even as the PRS is introduced.