Introduction to antenna gain improvement method based on phase compensation method
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Introduction to antenna gain improvement method based on phase compensation method

Posted Date: 2024-01-24

Said it in front

Research on electromagnetic metamaterials has continued for decades. It is still a hot spot in academic research and is gradually being implemented in engineering applications. Electromagnetic metamaterials can be divided into many categories according to their functions: increasing gain, expanding bandwidth, achieving stealth, improving isolation, etc...

This article will introduce the application of using electromagnetic metamaterials to improve antenna gain. Gain is one of the most important indicators of an antenna, which determines the maximum directional radiation distance of the antenna. Improving antenna gain has become an important direction for antenna design optimization. There are many specific implementation methods: 1) based on array antenna; 2) based on electromagnetic band gap structure; 3) based on resonant cavity structure; 4) based on phase compensation Metasurfaces; 5) Based on dielectric lenses….

Based on array antenna

The gain improvement based on the array antenna has been explained in detail in the previous article "Basics of Array Antenna Analysis and Comprehensive Analysis (Theory)": The radiation mechanism of the array antenna is that the electromagnetic waves radiated by a large number of antenna units produce an "interference" phenomenon. Different antenna arrays When the electromagnetic waves radiated by the element are superimposed in phase, they produce wave lobes, and zero points are formed where they are in phase and cancel each other. The figure shows the near-field and far-field distribution images of an array antenna containing 5 units. It can be seen from the figure that: on the one hand, the electric field propagates forward from near to far like undulating waves. On the other hand, unlike a single antenna, The near-field distribution of the array antenna is not uniform along the circumferential direction. It can be found that it is brighter in some sectors (the field distributions are superimposed in phase), while in some sectors it is darker (the field distributions are out of phase). cancellation), resulting in the far-field pattern of the antenna array showing obvious directivity.

Based on resonant cavity structure

Open resonant cavity, also known as open cavity resonator, is an extension of the optical resonant structure Fabry-Perot resonator in the microwave and millimeter wave frequency bands. The two parallel reflection plates of the open resonant cavity have extremely high reflectivity. They will reflect the incident electromagnetic wave in a specific frequency band multiple times. Each reflection will be superimposed in phase with the previous incident wave after transmission. The multiple reflections are multiple. Sub-in-phase superposition, thereby achieving energy convergence of transmitted electromagnetic waves. This resonant cavity is open and exchanges energy with the outside world through input and output coupling during resonance.

Based on dielectric lens

Dielectric lenses can well meet the antenna array's pursuit of performance such as miniaturization, high gain, low side lobes, and multi-beams. Its basic principles can be explained by referring to the relevant theories of optical lenses. Millimeter-wave lens antennas have similar effects to reflective surface antennas such as rotating paraboloids or hyperboloids, that is, they can converge low-gain, wide-beam antenna feed radiation into high-gain, pencil-shaped beam radiation, thereby greatly increasing the gain of the antenna and reducing array size, reducing side lobe levels.

Based on phase compensation metasurface

The mechanism of using phase compensation metasurfaces to achieve high gain is similar to that of dielectric lenses, except that dielectric lenses adjust the insertion phase shift by changing the thickness of the medium, while phase compensation metasurfaces use changes in the size of periodic structural units to achieve insertion. For the adjustment of phase shift, phase compensation metasurfaces are generally flat-plate structures, which have better low-profile characteristics compared to traditional dielectric lenses.

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Mechanism analysis

Far and near field analysis of horn antenna

Based on the speaker modeling module in FEKO's component library, by inputting the operating frequency, the horn antenna can be quickly modeled with one click, and the feed port can be excited by one wave port.

The far-field pattern of the horn antenna and the electric field distribution in the longitudinal section are shown in the figure. The antenna gain is 20dB. Compared with the waveguide without an opening horn (gain 8dB), the gain is significantly improved. By comparing the near-field amplitudes of the horn and the waveguide The phase distribution can provide a glimpse of the reason.

Comparing the near-field distribution of the two antennas, it can be found that after the electromagnetic wave is emitted from the waveguide port of the horn antenna, due to the "constraint" and "guidance" of the open horn, the energy is limited to a sector-shaped area at a certain angle and propagates forward. The far-field pattern shows high gain and good directivity; however, for waveguide antennas, due to the lack of "constraint" and "guidance" of the open horn, the electromagnetic energy radiates from the waveguide port and immediately spreads around in all directions. All have strong propagation, which is manifested in low gain and poor directivity in the far-field pattern.

The distribution of the Poynting vector (representing the flow of energy) clearly shows the impact of the open speaker on the internal electromagnetic waves. After the electromagnetic energy is radiated from the waveguide port, it spreads around, but due to the obstruction of the metal wall of the speaker, it "rolls" "After that, it is transported to the central area of ​​the speaker. After several cycles of "tossing", the energy in the central area of ​​the speaker is significantly higher than the energy in the edge area, while the energy in the central area basically radiates forward, and the energy in the edge It will appear to radiate to all directions.

The propagation direction of electromagnetic energy can be most intuitively felt by observing the distribution of phase planes such as electric fields. The equal phase plane, also known as the wave front, is always perpendicular to the propagation direction of the electromagnetic wave. Comparing the electric field phase distribution in the longitudinal section of the horn and the waveguide, it can be seen that: 1) the equal-phase surface in the horn is an arc, and it gradually forms a closed ellipsoid surface until the horn mouth; 2) the equal-phase surface in the waveguide is approximately an arc In a straight line until the waveguide mouth, the electromagnetic waves diverge in all directions, and the equal phase planes accordingly form a closed spherical surface.

Huygens-Fresnel principle

The wavefront (equal phase plane) distribution characteristics of horns and waveguides can be explained using the Huygens-Finier principle. In 1678, Huygens completed his book "On Light", which was released to the public in 1690. In this book he proposed the "Huygens' Principle":Each point of the wave front can be considered as a point wave source that generates spherical secondary waves, and the wave front at any subsequent time can be regarded as the envelope of these secondary waves.That is to say, the propagation of electromagnetic waves is based on the way that the "wavelet source" on the "wave front" propagates in the circumferential direction and forms a new envelope surface as a new "wave front", and continuously propagates forward in a cycle. As shown in the figure below: the envelope formed by the wavelet source on the omnidirectional spherical "wavefront" is still a spherical wave, while the envelope formed by the directional planar "wavefront" wavelet source is still a plane wave.

Based on this principle, let’s analyze the differences between horn antennas and waveguide antennas. The figure shows:

Waveguide: When the wavelet source radiates outward from the waveguide, the outer envelope of the wavelet source located in the middle of the radiation port is approximately flat, while the envelope of the wavelet source at the edge is still spherical, and the energy distribution on the aperture is relatively uniform. After synthesis, the total envelope is approximately an ellipse, which radiates around, and gradually approximates a sphere, thus showing strong omnidirectionality;

Speaker: For speakers, the situation is different. From the above analysis, it can be seen that the impact of the open speaker on the internal electromagnetic energy causes the energy of the wavelet source located at the edge of the horn mouth to be suppressed, which is significantly lower than the energy and directivity of the wavelet source in the central area. It mainly depends on the radiation of the wavelet source in the central region. The phase plane in the central region is flat and approximately plane, thus showing strong directivity.

Back to Horn Antenna Aperture Field

The horn antenna has strong directivity, but by observing the electric field phase distribution in the longitudinal section of the horn, it can be seen that when the electromagnetic wave radiates out from the horn radiation port, the equal phase surface (wave front) is only approximately a plane, but it is still a curved surface. The phase of the aperture field is not completely equal-phase distribution, which also affects the directional propagation of the horn antenna.

Further adjust the phase distribution of the electromagnetic field on the horn diameter, so that when the electromagnetic wave is radiated from the horn radiation port surface, its equal phase surface (wave front) is a plane as much as possible, which can further improve the directional performance of the horn antenna. Phase compensation metasurfaces can accomplish this task very well.

Implementation process

Step1: Extract the horn aperture field

Complete the calculation of the near field distribution on the radiation port surface of the horn antenna in the electromagnetic simulation software FEKO, as shown in the figure below. Export the electric field/magnetic field distribution on the radiation port surface in Postfeko and generate the near field files .efe and .hfe.

The near-field data file consists of three parts: 1) File description, which introduces the format, operating frequency, number of sampling points and other information of the near-field file; 2) Data header, which is the coordinates of each sampling point and the actual values ​​of the field components in three directions. Part and imaginary part; 3) Data corresponding to the header.

The object of phase compensation is the main polarization component of the electric field on the aperture surface, usingEuler's formulaBy converting the expression of the real part + imaginary part into the expression of amplitude + phase, the phase distribution data of the main polarization component of the electric field on the radiation port surface can be extracted, which provides a basis for the subsequent design of the phase control metasurface. Use the following program to process the extracted near-field data to generate phase distribution data of the main polarization component of the aperture field.

step2: Phase compensation metasurface unit design

Phase control unit mechanism

The most important step in the implementation process is to design a phase compensation metasurface unit with excellent performance. There are two standards: 1) By adjusting the size of the unit structure, the phase adjustment range should be as large as possible, preferably covering 360°; 2 ) Unit structure size adjustment process, the transmission coefficient of the unit is as large as possible to reduce transmission loss.

This article chooses a phase-controlled metasurface with a three-layer ring structure, which has good polarization symmetry and angular stability.

The resonant frequency of the unit is most closely related to the circumference of the ring. Generally, the resonant wavelength = the unit perimeter. The insertion phase shift of the electromagnetic wave passing through the unit depends on the electrical parameters (capacitance, inductance) of the unit's equivalent circuit model. The inductance comes from the obstruction of the current by the magnetic field induced around the metal when the current flows through the metal patch between units; the capacitance comes from the potential difference generated by the accumulation of charges on the inner and outer diameter of the ring. Changes in the inner diameter of the ring have a relatively small impact on the inductance value in the equivalent circuit, while the capacitance has obvious differences due to the different widths of the ring. The difference in capacitance leads to obvious differences in the insertion phase shift of the unit. Thus, the phase of the transmitted electromagnetic wave is adjusted, and at the same time, the amplitude of the transmitted electromagnetic wave is less affected. Observing the wave image of the cross-section electric field passing through the FSS, it can be seen that when the parameters are appropriately selected, the electric field passes through the unit in an approximate traveling wave state without reflection.

Regarding the design of periodic structures, the three mainstream simulation software FEKO/HFSS/CST can be used for simulation. In terms of calculation efficiency, CST and HFSS are better. In terms of the setting process, CST and FEKO are simpler. , the modeling methods of the three software are as follows:

HFSS: It is necessary to set the master-slave boundary conditions and Floquet model. The calculation speed is fast, the accuracy is high, and the parameter optimization is also more convenient;

CST: Use templates to directly model and simulate, calculate speed blocks, and the operation is simple;

FEKO: It is relatively simple to set the periodic boundary and plane wave incidence, but the calculation speed is relatively slow, and parameter optimization is also inconvenient.

This article chooses CST for the simulation and design of the phase control unit. Taking the transmission coefficient of the target frequency point as the optimization term, the unit size parameters were optimized, and the unit size p, medium thickness d, and ring outer circle size r1 were determined. The inner circle size r2 of the ring was scanned and different results were obtained. Transmission curve at ring size.

Parameter Description Value (mm) Unit period p 7.65 Medium thickness d 1.91 Outer circle size r1 3.78 Inner circle size r2 0.25~3.12

The transmission curves of the parameter sweep are messy when put together. Right-click the curve and select "0D from 1D" to view the changes in the amplitude and phase of the transmission characteristics at the target frequency point (16GHz) with the parameter sweep.

It can be seen from the figure: 1) When r2=3.12, the wave transmittance is 78%, and the wave transmittance in other sizes is greater than 80% (generally speaking standards); 2) The phase decreases monotonically from 150° to -150°, and the phase control range is 300°. Although it does not meet the requirement of 360°, it is basically sufficient without affecting the transmission characteristics of the unit.

The curve is exported in the curve post-processing module Post-Processing for subsequent call in phase-controlled metasurface design.

Step3: Design phase compensation metasurface

After the unit design is completed, the design features of the phase control metasurface come. The basic working mechanism of the phase compensation metasurface is to use discrete phase control units with different phase shift parameters to accurately adjust the near-field phase distribution of the aperture to achieve The in-phase distribution of the near-field phase of the aperture improves the antenna gain. Therefore, what needs to be done is to convert the aperture near-field phase distribution into a phase control unit size distribution according to the corresponding curve of phase control unit size-phase shift parameter.



Among them: 1) The phase extraction result is the phase distribution data calculated based on FEKO simulation. The interpolation algorithm is used to extract the phase distribution at the control unit according to the size of the phase control surface and the size of the unit; 2) The design has been completed based on the phase correction result. The actual phase adjustment range of the phase control unit, adjust the phase distribution to be controlled to the controllable phase range; 3) The unit size calculation result is based on the phase correction result, referring to the unit phase-size relationship curve, and based on the interpolation fitting algorithm, Calculate the corresponding unit size distribution and save it as .xls to provide a basis for subsequent modeling.

Step4 Automatic modeling of phase-controlled metasurface

After the phase-controlled metasurface size distribution is designed, the next step is to carry out modeling work. The modeling ideas are based on the previous issues."Array antenna automatic modeling"The idea is the same, that is: if the inner diameter of the ring is unevenly distributed, script modeling can be used; if the outer diameter of the ring, the medium layer, etc. are evenly distributed or the structure is simple, it can be directly modeled manually.

The ring phase control unit is relatively simple, and the API interface functions used only require "circle surface" modeling, feko software running and a few auxiliary API functions for unit settings.


After all preparations are done, the main program is very simple. On the one hand, it calls the file "isunitflg.xls" to obtain the size data of the inner diameter of the phase control unit ring. On the other hand, it calls the interface program (API) to calculate the size distribution of the phase control unit. Data is used to generate an automatic modeling script for phase-controlled metasurfaces.

Copy the modeling script .lua file to the Script editor and run it to complete the automatic modeling of the inner circle of the phase control unit.

After the inner circle is modeled, the dimensions of the outer circle will be the same. Cycle operations can be directly used, and media, metal substrates, etc. can be modeled manually.

Step5: Add antenna integrated simulation

According to the position of the aperture field, the modeled phase control metasurface is placed above the horn antenna, and an integrated simulation is performed to compare the simulation results of the far field pattern of the horn antenna and the horn-phase control metasurface: Load the phase control metasurface Finally, the gain of the horn antenna increased from 17.1dB to 19.4dB, an increase of 2.3dB. Correspondingly, the main lobe beam width also decreased significantly, that is, the phase control metasurface achieved the purpose of beam focusing.

near field analysis

The near-field distribution characteristics determine the far-field radiation characteristics. The author hopes to conduct a more in-depth analysis of the near-field distribution of the antenna before and after loading the phase-controlled metasurface from a near-field perspective. This should be something interesting too.

Comparing the near-field phase distribution from the cut surface of the horn before and after loading the phase-controlled metasurface, the "wavefront" of the electromagnetic wave radiated from the horn mouth becomes flatter after being "adjusted" by the phase-controlled metasurface.

After loading the phase control metasurface, the near-field phase distribution difference on the oral surface is significantly reduced, especially the phase distribution in the center area of ​​the oral surface, which changes from the original arc-top distribution to a flat-top distribution (of course there are some fluctuations). At the same time, it also It can be noted that due to the periodic destruction of edge units, the closer to the edge of the oral surface, the worse the phase control effect is.


Loading the phase-controlled metasurface does not always bring beneficial aspects. Comparing the distribution of the Poynting vector (energy flow) along the z direction in the longitudinal section before and after loading the phase-controlled metasurface, it can be seen that: the unloaded phase-controlled metasurface The Poynting vector in the center area of ​​the horn antenna always points in the +z direction, indicating energy phase radiation without reflection. After loading the phase control metasurface, the situation is different. The Poynting vector in the center area exists along the -z direction. The directional component indicates that energy has been reflected at the phase control metasurface. The disadvantage is that it will cause a certain amount of energy loss.

extension

As mentioned above, the use of phase-controlled metasurfaces to improve the gain of the horn antenna is introduced. Through simulation practice, it can be seen that the effect is indeed significant. However, in-depth research on the near field of the horn antenna before and after loading the phase-controlled metasurface can be found. There are still some Places need further optimization:

Miniaturized unit design improves control accuracy: Observing the phase distribution after control, it is found that the phase fluctuates, and the fluctuation period is roughly similar to the unit period. The miniaturized design of the unit can further improve the phase control accuracy, and may further increase the antenna gain;

For further optimization of the edge unit size, improve the aperture edge phase control effect: observing the phase control results above, it can be seen that the phase control effect in the central area is better, the isophase surface is relatively flat, and there is a large gap between the edge area phase and the central area , the analysis reason should be that the edge unit is periodically destroyed, and the phase control has a certain deviation from the target. Secondary optimization for edge units may further provide antenna gain;

Summarize

This article focuses on the purpose of improving antenna gain and based on phase control metasurface technology, giving a detailed introduction to its working mechanism, design process, and optimization effects. The attached references, simulation models, and modeling scripts are helpful for those who want to Students who conducted in-depth research immediately started relevant work.






Review Editor: Liu Qing


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