阅读说明：本技术 石墨烯贴片型太赫兹法布里-珀罗谐振天线及其实现方法 (Graphene patch type terahertz Fabry-Perot resonant antenna and implementation method thereof ) 是由 吴林晟 陈谢鹏 冯金龙 黄一 毛军发 于 2021-06-10 设计创作，主要内容包括：一种石墨烯贴片型太赫兹法布里-珀罗谐振天线及其实现方法,包括：由上而下依次设置的固定夹具、法布里-珀罗谐振结构以及加脊馈源喇叭,其中：固定夹具与加脊馈源喇叭相连,法布里-珀罗谐振结构固定设置于加脊馈源喇叭上方并与加脊馈源喇叭的金属上表面共同构成法布里-珀罗谐振腔,加脊馈源喇叭辐射出的电磁波在法布里-珀罗谐振结构中振荡并经调控后在主方向形成所需要的窄波束电磁辐射。本发明将图形化的石墨烯贴片结构应用于太赫兹天线可以提供优良的性能,引入了丰富的设计自由度,该天线工作于太赫兹频段,具有波束宽度易于设计与调控的优点。(A graphene patch type terahertz Fabry-Perot resonant antenna and an implementation method thereof are disclosed, and the method comprises the following steps: from top to bottom set gradually mounting fixture, fabry-perot resonant structure and add spine feed loudspeaker, wherein: the fixed clamp is connected with the ridge feed source loudspeaker, the Fabry-Perot resonant structure is fixedly arranged above the ridge feed source loudspeaker and forms a Fabry-Perot resonant cavity together with the metal upper surface of the ridge feed source loudspeaker, and electromagnetic waves radiated by the ridge feed source loudspeaker oscillate in the Fabry-Perot resonant structure and form required narrow-beam electromagnetic radiation in the main direction after being regulated and controlled. According to the terahertz wave antenna, the patterned graphene patch structure is applied to the terahertz wave antenna, so that excellent performance can be provided, rich design freedom is introduced, the terahertz wave antenna works in a terahertz frequency band, and the terahertz wave antenna has the advantage that the wave beam width is easy to design and regulate.)

1. A terahertz narrow-beam Fabry-Perot resonant antenna based on a metal-graphene mixed structure is characterized by comprising: from top to bottom set gradually mounting fixture, fabry-perot resonant structure and add spine feed loudspeaker, wherein: the fixed clamp is connected with the ridge feed source loudspeaker, the Fabry-Perot resonant structure is fixedly arranged above the ridge feed source loudspeaker and forms a Fabry-Perot resonant cavity together with the metal upper surface of the ridge feed source loudspeaker, and electromagnetic waves radiated by the ridge feed source loudspeaker oscillate in the Fabry-Perot resonant structure and form required narrow-beam electromagnetic radiation in the main direction after being regulated and controlled.

2. The terahertz narrow-beam Fabry-Perot resonant antenna based on the metal-graphene hybrid structure as claimed in claim 1, wherein the fixing fixture is a hollow cylinder structure, a polygonal geometric cavity is formed inside the fixing fixture to accommodate the Fabry-Perot resonant structure, and the fixing fixture is made of polylactic acid by processing using a 3D printing technology.

3. The terahertz narrow-beam fabry-perot resonator antenna based on a metal-graphene hybrid structure as claimed in claim 1, wherein the fabry-perot resonator structure comprises: metal pattern and graphite alkene pattern on the dielectric layer are located in proper order, wherein: the graphene pattern and the metal pattern have conductivity and form a reflective surface to achieve partial reflection of electromagnetic waves.

4. The terahertz narrow-beam Fabry-Perot resonant antenna based on the metal-graphene mixed structure as claimed in claim 3, wherein the dielectric layer has no conductivity and is used for dividing part of the reflecting surface and the upper surface of a ridge feed source horn serving as a metal reflecting surface.

5. The terahertz narrow-beam fabry-perot resonator antenna based on the metal-graphene hybrid structure as claimed in claim 1, wherein the ridged feed horn comprises: loudspeaker main part, set up respectively in the metal spine and the waveguide mouth at loudspeaker main part both ends, wherein: the upper side of the horn main body is a metal upper surface, the metal ridge is positioned on one side of the horn main body close to the Fabry-Perot resonant structure, and the waveguide port for external connection feeding is positioned on one side of the bottom of the horn far away from the Fabry-Perot resonant structure.

6. The terahertz narrow-beam Fabry-Perot resonant antenna based on the metal-graphene hybrid structure as claimed in any one of claims 1 to 5, wherein the Fabry-Perot resonant structure is realized by a metal-graphene hybrid structure process, and comprises the following steps:

step 1, spin-coating a photoresist on a substrate which can endure a high-temperature process of 1000 ℃ and drying the photoresist to obtain a complete uniform photoresist layer;

step 2, placing the mask on a substrate covered with a complete and uniform photoresist layer, and carrying out exposure, development and etching operations to form a patterned photoresist layer;

step 3, evaporating and plating a layer of uniform nickel or copper on the substrate by using an electron beam evaporation process;

step 4, putting the substrate into a photoresist solvent, and removing the patterned photoresist to obtain a patterned metal film;

and 5, putting the substrate into a tubular reaction furnace, and growing patterned graphene on the patterned metal film by using a chemical vapor deposition method to finally obtain the graphene-loaded patch structure.

Technical Field

The invention relates to a technology in the field of microwave communication, in particular to a terahertz Fabry-Perot resonant antenna based on a graphene patch type structure and an implementation method thereof.

Background

The Fabry-Perot (Fabry-Perot) resonant structure is a common method for realizing a narrow beam antenna in the prior art, and the working principle is that when the frequency of incident electromagnetic waves meets a resonant condition, the directivity coefficient corresponding to the frequency has a high peak value, and the amplitude of the directivity coefficient corresponding to the frequency which does not meet the resonant condition is small. The Fabry-Perot resonant (FPR) antenna comprises a Fabry-Perot resonant cavity and an antenna feed source, wherein the resonant cavity comprises two reflecting surfaces, one reflecting surface is usually a metal reflecting surface with total reflection characteristic, the other reflecting surface is a partial reflecting surface with partial reflection characteristic and usually comprises a Frequency Selective Surface (FSS), and the distance between the two reflecting surfaces and a reflection coefficient surface of the partial reflecting surface jointly determine the resonant frequency. After the electromagnetic wave is emitted from the antenna feed source, partial reflection and partial transmission occur on the partial reflecting surface, the total reflection occurs on the metal reflecting surface, and the oscillation is carried out back and forth in the resonant cavity. When the resonance condition is satisfied, the forward radiation of the antenna is increased, the beam width is reduced, and the directivity is enhanced.

Most of the conventional Fabry-Perot resonant antennas work in microwave and millimeter wave low frequency bands, and FPR antennas working in terahertz frequency bands are rare. Firstly, with the increase of the working frequency, higher requirements are put forward on the precision of the processing technology. Secondly, for the FPR antenna of the terahertz frequency band, the specification of the terahertz frequency band is less due to the limitation of the thickness of the medium layer serving as the resonant cavity.

Disclosure of Invention

Aiming at the defects in the prior art, the invention provides the graphene patch type terahertz Fabry-Perot resonant antenna and the implementation method thereof, the patterned graphene patch structure is applied to the terahertz antenna, so that excellent performance can be provided, rich design freedom is introduced, the antenna works in a terahertz frequency band, and the advantage that the beam width is easy to design and regulate is achieved. The provided process for patterning the metal-graphene mixed structure can be used for processing and realizing the designed terahertz antenna.

The invention is realized by the following technical scheme:

the invention relates to a terahertz narrow-beam Fabry-Perot resonant antenna based on a metal-graphene mixed structure, which comprises: from top to bottom set gradually mounting fixture, fabry-perot resonant structure and add spine feed loudspeaker, wherein: the fixed clamp is connected with the ridge feed source loudspeaker, the Fabry-Perot resonant structure is fixedly arranged above the ridge feed source loudspeaker and forms a Fabry-Perot resonant cavity together with the metal upper surface of the ridge feed source loudspeaker, and electromagnetic waves radiated by the ridge feed source loudspeaker oscillate in the Fabry-Perot resonant structure and form required narrow-beam electromagnetic radiation in the main direction after being regulated and controlled.

The fixing clamp is made of a medium with a low dielectric constant and low loss, and the specific medium comprises: polylactic acid (PLA);

the Fabry-Perot resonant structure comprises: metal pattern and graphite alkene pattern on the dielectric layer are located in proper order, wherein: the graphene pattern and the metal pattern have conductivity and form a reflective surface to achieve partial reflection of electromagnetic waves.

The dielectric layer has no conductivity and is used for dividing part of the reflecting surface and the upper surface of the ridge feed source loudspeaker serving as the metal reflecting surface.

The ridged feed horn comprises: loudspeaker main part, set up respectively in the metal spine and the waveguide mouth at loudspeaker main part both ends, wherein: the upper side of the horn main body is a metal upper surface, the metal ridge is positioned on one side of the horn main body close to the Fabry-Perot resonant structure, and the waveguide port for external connection feeding is positioned on one side of the bottom of the horn far away from the Fabry-Perot resonant structure.

The Fabry-Perot resonant structure is realized by a metal-graphene mixed structure process, and comprises the following steps:

step 1, preparing a substrate which can resist the high temperature process of 1000 ℃.

And 2, spin-coating a photoresist on the substrate and drying to obtain a complete and uniform photoresist layer.

And 3, placing the mask on the substrate covered with the complete and uniform photoresist layer, and carrying out exposure, development and etching operations to form a patterned photoresist layer.

Step 4, using electron beam evaporation process to evaporate a uniform metal film on the substrate, wherein the metal film is usually nickel or copper.

And 5, putting the substrate into a photoresist solvent, and removing the patterned photoresist to obtain the patterned metal film.

And 6, putting the substrate into a tubular reaction furnace, and growing patterned graphene on the patterned metal film by using a Chemical Vapor Deposition (CVD) method to finally obtain the graphene-loaded patch structure.

Technical effects

Most of the existing Fabry-Perot antennas work in lower frequency bands such as microwave and millimeter wave bands, and the Fabry-Perot antennas working in terahertz frequency bands are rare. With the increase of the working frequency, the common processing technology under the low frequency can not meet the technological requirements of the antenna. In addition, due to the characteristic that the electric conductivity of the graphene is adjustable, a new degree of freedom is introduced to a partial reflecting surface in the Fabry-Perot antenna. At present, due to the fact that the existing work of the terahertz frequency band resonant frequency reconfigurable Bury-Perot antenna by utilizing the graphene conductivity adjustable characteristic is achieved, the reflection characteristic of a part of reflecting surfaces is difficult to adjust and the beam width of the antenna cannot be accurately controlled due to the fact that complete non-graphical graphene is adopted. In order to adjust the characteristics of the partial reflecting surface, the invention designs the graphene patch type frequency selective surface as the partial reflecting surface, introduces more design freedom compared with the whole graphene, and provides a metal-graphene mixed structure process to ensure the feasibility of the actual processing of the antenna.

Compared with the prior art, the method has the advantages that the patterned graphene, specifically the patch type graphene is applied to the design of a partial reflecting surface in the Fabry-Perot antenna, more design freedom degrees are introduced compared with the traditional metal material and the whole piece of graphene, the size of a patch structure is optimized, the Fabry-Perot antenna with the wave beam width meeting specific conditions is realized, and the processing technology of a metal-graphene mixed structure is provided for ensuring the feasibility of design.

Drawings

FIGS. 1(a) and 1(b) are schematic views of the overall structure of the present invention;

FIG. 2 is a schematic view of a holding fixture of the present invention;

FIG. 3 is a diagram of a Fabry-Perot resonant structure in the present invention;

fig. 4 is a schematic partial enlarged view of a graphene pattern of a fabry-perot resonant structure according to the present invention;

fig. 5 is a schematic diagram of a basic unit structure of a fabry-perot resonant structure according to the present invention;

fig. 6(a) and 6(b) are schematic equivalent circuit diagrams of a monolithic graphene and a frequency selective surface of a metal-graphene hybrid structure in the present invention;

fig. 7(a) is a reflection coefficient curve of a monolithic graphene and a metal-graphene mixed structure frequency selective surface in the present invention; fig. 7(b) is a transmission coefficient curve of a monolithic graphene and a frequency selective surface of a metal-graphene hybrid structure in the present invention;

fig. 8 is a schematic view of a process flow of the metal-graphene hybrid structure proposed in the present invention;

FIG. 9 is a schematic view of a ridged feed horn of the present invention;

FIGS. 10(a) and 10(b) show terahertz ridged feed horn and FPR antenna at 310GHz,and a normalized pattern at 90 °;

in the figure: the device comprises a fixed clamp 1, a Fabry-Perot resonant structure 2, a graphene pattern 21 of the Fabry-Perot resonant structure, a metal pattern 22 of the Fabry-Perot resonant structure, a dielectric layer 23 of the Fabry-Perot resonant structure, a graphene pattern 211 of a basic unit structure, a metal pattern 221 of the basic unit structure, a dielectric layer 231 of the basic unit structure, a ridged feed source horn 3, a metal upper surface 31 of the ridged feed source horn, a horn main body 32 of the ridged feed source horn, a ridged feed source horn metal ridge 33 and a standard rectangular waveguide port 34 of the ridged feed source horn.

Detailed Description

In this embodiment, a patterned graphene with a patch type structure is adopted, the period of the unit is p, and the size of the patch is a. The graphene patch type partial reflecting surface has a band elimination characteristic and can be equivalent to a series-parallel resonant circuit of a capacitor, an inductor and a resistor, and the graphene patch resistor introduced by the real part of the finite conductivity of graphene is R_GPThe inductance introduced by the imaginary part of the conductivity is L_GPThe inductance and the capacitance introduced by the metal patch are respectively L_MP、C_MP. In addition, the patch itself provides the inductance L_PBut because the values are small, their effect is generally negligible for analysis.

When the in-band conductivity is dominant, the graphene can be analyzed by using a Drude model, and the surface conductivity of the graphene is as follows: sigma-sigma₀(1+ j ω τ), wherein: direct current conductivity of grapheneWhen the unit period p of the graphene patch-type structure is much less than half wavelength, the surface impedance can be approximated as:wherein the equivalent capacitanceε₀Is a vacuum dielectric constant of ∈_effIs the equivalent relative dielectric constant; when the graphene patch is arranged between two layers of medium made of the same materialIn the middle, the equivalent relative dielectric constant is equal to the relative dielectric constant of the dielectric substrate, namely: epsilon_eff＝ε_r(ii) a Equivalent relative permittivity ε when graphene patch is interposed between air and medium_eff＝(ε_r+1)/2。

Substituting the conductivity expression of the graphene to obtain the surface impedance of the graphene patch type structureWherein: z_GP1Is an inductive reactance of a resistor and an inductor connected in series, Z_GP2Is an equivalent capacitance; the resistance and inductance values of graphene are:for the whole graphene, the graphene can be equivalent to a series resonance circuit of an inductor and a resistor, and the resistance of the graphene introduced by the finite real part of the conductivity of the graphene is R_GThe inductance introduced by the imaginary part of the conductivity is L_G。

The surface impedance of the whole graphene is: z_G＝1/σ_G＝(1+jωτ)/σ₀＝R_G+jωL_GThe resistance and inductance of the whole graphene are: r_G＝1/σ₀，L_G＝τ/σ₀. Comparing the equivalent impedance of the FSS and the whole graphene, the difference between the resistance and the inductance of the graphene is p/(p-a) times. In addition, because the patch structure introduces inter-chip capacitance, the graphene patch type structure has resonance characteristics.

The imaginary part of the surface impedance of the whole graphene is always larger than zero, the whole graphene is sensitive under any frequency, and the magnitude of the imaginary part and the magnitude of the frequency are in a linear relation. The patch type graphene FSS has a resonant frequency, the imaginary part of the surface impedance of the plane where the graphene is located is smaller than zero when the imaginary part is lower than the resonant frequency, and is larger than zero when the imaginary part is higher than the resonant frequency, namely, the imaginary part is lower than the resonant frequency, the plane where the graphene is located is capacitive, and the imaginary part is inductive when the imaginary part is higher than the resonant frequency. As the frequency increases, the sensitivity provided by the patch-type FSS graphene increases gradually, and exceeds the sensitivity provided by the whole graphene when the frequency is higher than a certain frequency, that is, the graphene surface of the patch-type FSS graphene can provide a higher inductance value than the whole graphene after reaching a certain frequency.

As shown in fig. 1, the terahertz fabry-perot resonant antenna based on a metal-graphene hybrid structure according to this embodiment includes: fixed clamp 1, fabry-perot resonant structure 2 and add spine feed loudspeaker 3, wherein: the Fabry-Perot resonant structure 2 is fixedly arranged between the fixed clamp 1 and the ridge feed source loudspeaker 3.

The fixed clamp 1 is of a hollow cylinder structure, a polygonal geometric cavity is arranged inside the fixed clamp 1 to accommodate the Fabry-Perot resonant structure 2, the fixed clamp 1 is processed by using a 3D printing technology, and the formed material is a polylactic acid (PLA) medium with a low dielectric constant, so that the disturbance of radiation electromagnetic waves is effectively avoided while the processing precision is ensured.

The terahertz Fabry-Perot resonant antenna based on the metal-graphene mixed structure is particularly applied to systems such as 310GHz wireless communication and radar detection.

The fabry-perot resonator structure 2 includes: graphene pattern 21, metal pattern 22 and dielectric layer 23, wherein: the graphene pattern 21 is a graphene patch which is square and periodically arranged at equal intervals, and the thickness of the graphene patch is 2.6 nm; the metal patterns 22 are metal nickel patches which are correspondingly arranged in a square shape at equal intervals, and the thickness of the metal nickel patches is 400 nm; the dielectric layer 23 is a cylinder with the radius of 10mm, the height of the dielectric layer is 1mm, the composition material is quartz with the relative dielectric constant of 3.8, when electromagnetic waves enter the ridge feed source horn 3, the electromagnetic waves can radiate outwards from a circular horn mouth at one side of the ridge feed source horn 3 close to the Fabry-Perot resonant structure 2, the radiated electromagnetic waves reach the Fabry-Perot resonant structure 2, the beam width of the radiated electromagnetic waves is wide, the gain is low, the electromagnetic waves can oscillate back and forth in a Fabry-Perot resonant cavity formed by the Fabry-Perot resonant structure and the metal upper surface 31 of the ridge feed source horn, reflection and transmission occur at the laminated structure of the Fabry-Perot resonant structure graphene pattern 21 and the metal pattern 22, and total reflection occurs at the metal upper surface 31 of the ridge feed source horn. When the resonance condition is satisfied, the forward radiation of the antenna is enhanced, the beam width is reduced, and the required narrow beam width and high directivity are obtained in the main direction.

As shown in fig. 4, the patterns of the graphene pattern 21 and the metal pattern 22 are completely overlapped, the unit period is 400 μm, and the side length of the square patch is 360 μm.

As shown in fig. 5, the fabry-perot resonator structure is formed by repeating a periodic arrangement of basic constituent units, wherein the graphene patch 211 and the metal patch 221 have the same size and are both located above the medium 231.

An equivalent circuit of the proposed stacked structure of the fabry-perot resonant structure graphene pattern 21 and the metal pattern 22 is shown in fig. 6 (a). It can be seen that the equivalent circuit can be regarded as the parallel connection of the RLC series resonant circuit of the metal patch type structure and the RLC series resonant circuit of the graphene patch type structure, and the graphene patch resistance introduced by the limited conductivity real part of the graphene is R_GPThe inductance introduced by the imaginary part of the conductivity is L_GPThe inductance and the capacitance introduced by the metal patch are respectively L_MP、C_MP. In addition, the patch itself provides the inductance L_PBut can be generally ignored due to the small values. And the equivalent circuit of the monolithic graphene structure is shown in FIG. 6(b), wherein R_GResistance of graphene patch, L_GThe inductance introduced for the limited conductivity of the graphene can be changed according to the external voltage of the graphene due to the specific conductivity adjustable characteristic of the graphene. Compared with an equivalent circuit and the content of the invention, the equivalent circuit of the graphene patch type structure provided by the invention has more elements, and provides more degrees of freedom compared with the design of the whole graphene.

The reflection coefficient amplitude and transmission coefficient amplitude curves of the proposed stacked structure of the fabry-perot resonant structure graphene pattern 21 and the metal pattern 22 and the whole graphene structure are shown in fig. 7(a) and 7 (b). According to the graph shown in fig. 7(a), since the patch type has a band-stop characteristic structure, the reflection amplitude of more than-0.5 dB can be achieved in a 280-330 GHz frequency band, and the difference between the reflection amplitude and the reflection coefficient of the whole graphene structure is not large. According to the graph shown in fig. 7(b), the transmission coefficient of the patch type structure is about-10 dB at 310GHz, while the transmission coefficient of the whole graphene structure is about-50 dB, the patch type structure has obvious advantages compared with the transmission coefficient of the whole graphene structure, and for the FPR antenna, the partial reflection surface structure has a larger transmission coefficient to provide a larger beam focusing capability, so that the gain of the antenna is improved.

Add spine feed horn 3 for the material of copper surface gilt, include: a square metal top surface 31, a horn body 32, a metal ridge 33, and a standard rectangular waveguide port 34.

The side length of the metal upper surface 31 is 20 mm.

The height of the horn main body 32 is 21.7mm, and the inner diameter of a top round horn mouth is 7 mm.

The thickness of the metal ridge 33 is 1mm, and the distance is 2.3 mm.

The waveguide port 34 is used for connecting external excitation, is a WR3 standard waveguide, and has the internal dimension of 0.86 multiplied by 0.43mm²。

The embodiment relates to a preparation method of the Fabry-Perot resonant structure 2, which comprises the following steps:

step 1, as shown in fig. 8a, a quartz substrate 23 is prepared.

And step 2, as shown in fig. 8b, spin-coating a photoresist on the quartz substrate 23 and drying to obtain a complete uniform photoresist layer.

And step 3, as shown in fig. 8c, placing the chromium mask on a quartz substrate covered with a complete and uniform photoresist coating, and performing exposure, development and etching operations to form a graphical photoresist layer.

And 4, as shown in fig. 8d, putting the quartz substrate coated with the patterned photoresist into an electron beam evaporation machine, and evaporating a uniform nickel film by using an electron beam evaporation process.

And 5, as shown in fig. 8e, putting the substrate into a photoresist solvent, and removing the patterned photoresist to obtain the patterned nickel film 22.

And 6, as shown in fig. 8f, putting the substrate into a tubular reaction furnace, and growing patterned graphene 21 on the patterned nickel film by using a chemical vapor deposition method to finally obtain the fabry-perot resonant structure 2.

As shown in fig. 10(a), the terahertz ridged feed horn of the antenna of the present embodiment is at 310GHz,andnormalized directional pattern of time. It can be seen that the feed source with ridge is arranged atThe-10 dB beam width is 17.90 DEG whenThe-10 dB beamwidth is 16.68 deg.. The beam widths of the E-plane and the H-plane differ by 1.22 °.

As shown in fig. 10(b), for the FPR antenna of this embodiment at 310GHz,andnormalized directional pattern of time. It can be seen that the FPR antenna is atThe-10 dB beam width is 14.97 DEG whenThe time-10 dB beamwidth is 14.62 °. The beamwidths of the E-plane and the H-plane differ by only 0.35 °.

As shown in the figure, after the graphene fabry-perot resonant structure is loaded, the-10 dB beam widths of the main polarization plane of the antenna are respectively reduced from 17.90 ° and 16.68 ° to 14.97 ° and 14.62 °, which indicates that the graphene fabry-perot resonant structure effectively narrows the beam and enhances the directivity of the antenna; and the graphene Fabry-Perot resonant structure is loaded to reduce the-10 dB beam width difference of the two main polarization surfaces from 1.22 degrees to 0.35 degrees, so that the beam equalization degree of the main polarization surfaces is effectively improved.

Compared with the prior art, the terahertz Fabry-Perot resonant antenna based on the metal-graphene mixed structure is realized based on the provided metal-graphene mixed structure processing technology, the radiation characteristic that the designed beam width is 14.62-14.97 degrees is realized at 310GHz, and the feasibility of the graphene for the terahertz frequency band antenna is verified. Through simulation experiments, after the graphene Fabry-Perot resonant structure is loaded, the-10 dB beam widths of two main polarization surfaces of the antenna are respectively reduced from 17.90 degrees and 16.68 degrees to 14.97 degrees and 14.62 degrees, the specific requirements of 15 degrees +/-10 percent are met, the-10 dB beam width difference value is reduced from 1.22 degrees to 0.35 degrees, and the beam equalization degree is obviously improved.

Compared with the prior art, the method can strictly control the beam width of the FPR antenna within a specific range (taking 15 degrees +/-10 percent as an example), and simultaneously improve the beam equalization degree of the antenna.

The foregoing embodiments may be modified in many different ways by those skilled in the art without departing from the spirit and scope of the invention, which is defined by the appended claims and all changes that come within the meaning and range of equivalency of the claims are therefore intended to be embraced therein.