Coding metamaterials are the new technology where the aperture coding provides the hardware foundation for the miniaturization of the devices. As a synthetic 2-D plane, metamaterial antennas are composed of subwavelength resonant particles. It can realize real-time control of electromagnetic wave and build multifunction radar array system. We make a detailed explanation of its array structure, working principle, and hardware system. However, it is usually difficult to synthesize flexible beams because the phase value is very limited in this antenna. Two methods are proposed in this paper to demonstrate the beam synthesis based on repetitive coding and convolution, and the distribution of beam pointings is analysed on the basis of this mechanism. Experiments that measure the radiation pattern of this antenna are carried out to verify the simulated results using only 1-bit coding metasurface in a radar system, whose phase value is controlled by pin diode on the surface.
Phased array radar is extensively adopted into detection in virtue of its flexible properties in beam scanning. Its phase shift properties are constrained by the properties of the phase shifter, which can be easily affected by the temperature and the nature of the device itself. Conventional phased array radar is generally bulky and complex in structure. Although it overcomes the limitations of mechanical scanning for radar performance, there are some new techniques such as printed antenna, conformal array, solid T/R components, time delay device, optical and digital beamforming being applied these days. The huge cost of T/R components, large size, weight, and manufacturability always push the innovation and progress of the whole technology. The metamaterial particles and digital codes are incorporated with each other in the wake of the proposed coding metamaterial, which have aroused wide concern recently [
In 2013, Lipworth et al. in Duke University proposed that aperture made by metamaterial could be used to realize compressive microwave imaging, and the whole system would require only a microwave detector [
The research of artificial electromagnetic material was carried out in the National Key Laboratory of Millimetre Wave in the Southeast University in China several years ago. Their team has designed a series of metasurfaces with ultralow profiles and used them in the fields of beam synthesis, microwave imaging, and holographic surfaces [
In this paper, a radar system with a coding metamaterial antenna is introduced. Its performance in beam synthesis and target detection where the number of coded bits for each element in the coding metasurface may not be substantial subject to hardware conditions. The wavefronts of EM waves can be steered via the discontinued phase introduced at a subwavelength scale superficially on substrates in virtue of this system. The focus of this paper is mainly to analyse how to synthesize flexible beams and to verify it with experiment when a very confined phase value is employed in a metamaterial antenna, which is called “low-bit coding metasurface”. For conventional phased array antennas, it can achieve continuous beam synthesis because of its flexible and continuous phase shifters, where the phase difference between the adjacent elements is related to the beam deviation. Another difference is that the phase difference between every adjacent element is always a fixed value in order to obtain the maximum coherence stack of the distant field in the direction of beam synthesis. However, in metamaterial antenna, the phase difference which can be selected is very limited. So, the coding rules can no longer be simply set according to the mapping relationship between the beam pointing and the phase difference. We also set up an experimental system to prove our theory.
The rest of the paper is organized as follows: Section
The coding metasurface consists of different subwavelength resonant particles where the phase shift is represented by discrete codes like “0” and “1”, which can be employed to represent the phase value
The feed in this antenna is not realized by a feed network but by a horn antenna (shown in Figure
The experimental setup in a microwave anechoic chamber.
A schematic diagram of this detection system is presented in Figure
A schematic diagram of the metamaterial antenna detection system.
For a uniform line array antenna, in the case of all the elements equally coupled with amplitude
Arising from the confined phase values in the foregoing system, each code shall be
The simulation shows that this method is feasible (see Figure
Simulated results when
To prove that the repetition of the code is indeed equivalent to increase the “equivalent spacing” of the element, the analytic and approximate radiation pattern are both given for comparison. The analytic radiation expression refers to the actual pattern by repetitive coding, and the approximate expression refers to the result got by replacing
For the analytic radiation expression, the codes are generally arranged in the following order (shown in Table
Coding arrangement in a uniform line array.
Uniform line array | |||||||||||
---|---|---|---|---|---|---|---|---|---|---|---|
Sum | 2 | ||||||||||
Sum | 2 |
||||||||||
Code | 0 | 0 | 1 | 1 | 2 |
2 |
|||||
Element number | 0 | 2( |
2 |
To facilitate the calculation, the element numbers who have the same phase are put into the same matrix.
Assuming that the amount of elements is exactly
For approximate expression, simply use
The results are exhibited in Figure
Simulated results when
It can be seen from the simulated results that the approximate pattern is highly similar to the actual pattern in the beam pointing although there is a difference between their main lobe width and side lobe level. It provides an important theoretical support for repetitive coding, where it can achieve the similar results of increasing the physical array spacing. Although this method is very straightforward, its synthetic beam pointing is very confined. If the appropriate wavelength, code bits, and other parameters are selected, it may produce
Beam pointing distribution with different parameters.
Arising from the properties of the inverse trigonometric function itself, the beam pointing is centrally distributed at small angles (
When the repetitive coding method is adopted, the far field radiation pattern can be decomposed from (
It can be seen from (
Without loss of generality, the following sequence of time domains shall be written as follows:
The approximation adopted above merely denotes the approximation of the beam pointing. When two “time arrays,” namely,
In that the angle domain is projected to the frequency domain (
When it comes to 3-D beam synthesis, the metasurface contains
The total coding pattern is the modular addition of sub coding patterns.
To explain this process more vividly, the results of the simulation experiment are given in Figure
Simulated results of modular addition when
To better demonstrate the distribution of the beam pointing synthesized by this method, the following simulation experiment is carried out: assuming that
Statistical result and distribution models of the beam pointing when
As can be seen from the statistical distribution of the beam, the beam pointings are intensive between
In terms of a fixed combination of designed elevation and azimuth, there may be more than one coding matrix. Different performances are indicated in beam pointing accuracy, main lobe width, and side lobe level. To seek out the optimal coding matrix, the normalized errors or costs of them shall be overall reckoned with to comprise the following cost function:
The distribution of cost function for (a) beam pointing error, (b) main lobe width cost, and (c) side lobe level cost.
The experiment was carried out in a microwave anechoic chamber using the metamaterial antenna in the radar system described in Figure
By using vector network analyzer, we got the radiation pattern and compared it with the simulated results. The simulated and experimental results are compared in Figure
The comparison (section view and normalized magnitude) of simulated and experimental results when elevation is
In this paper, we propose a metamaterial antenna with 400 elements on the surface and the phase values are controlled by the pin diodes. It is made without a very large feed network but by only a horn antenna, which illuminates the coding surface and makes the antenna more miniaturized and flexible. We present the beam synthesis methods with regard to this kind of antenna, where the code bits are limited to 1 or 2 bit. Through employing repetitive coding and convolution properties, the number of beam pointing that can be synthesized on the metasurface is greatly increased. We analyze the distributions of the predesigned beam pointings produced by these two methods. This not only achieves beam synthesis using metamaterial antennas but also provides an approach to realize beam synthesis under special conditions where phase values are confined in the conventional phased array antenna. We also use the experiment to verify this theory of beam synthesis by taking two azimuths for the test. As the result shows, although the amplitude is different at every point, the position of the main and side lobe is basically accurate. There are two main problems to be solved in the future work: First, beam synthesis under about
The authors declare that there are no conflicts of interest regarding the publication of this paper.
The paper is supported by the National Natural Science Foundation of China under Grant 61571011.