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This paper presents a dual-band shared-aperture antenna array using a filtering magnetoelectric (ME) dipole antenna for the application of mobile-communication base station. It is realized by interleaving a lower-band (LB) filtering ME dipole antenna operating from 1.7 to 2.8 GHz and four higher-band (HB) patch antennas in the band of 3.3-3.9 GHz. The filtering performance of the proposed ME dipole antenna is realized by U-shaped slots etched on the radiating arms and a cross-shaped microstrip line inside the aperture of the magnetic dipole, and used to realize a high cross-band isolation. The loss caused by the filtering structure is quite low. The gain suppression of the LB filtering ME dipole antenna reaches about 18 dB in the HB. Results demonstrate that both the HB and LB antennas can maintain stable broadside radiation patterns within their operating bandwidths. The magnitude of the cross-band isolation reaches more than 25 dB.

Abstract A novel windmill‐shaped metasurface (MTS) structure is firstly presented and applied to realise broadband antennas with stable radiation patterns. Different from conventional square MTS structure, the proposed windmill‐shaped MTS structure is formed by etching several triangular sawtooth slots instead of rectangular slots on the radiating square patch. Through the characteristic mode analysis, it can be found that the proposed MTS structure exhibits a much stable broadside radiation within a large bandwidth of 45.9%. Besides, the tapered edges resulted from the sawtooth slots can be regarded as smoother transitions, which would bring a better impedance matching. Based on above analysis, two antennas with different aperture sizes using the proposed windmill‐shaped MTS are well designed accordingly. Both antennas demonstrate the capability to achieve large impedance bandwidths over 40% and stable radiations (2‐dB gain bandwidth >33%). Compared with the conventional MTS‐based antenna, these proposed antennas improve the bandwidth and radiation performances effectively, showing great promises for modern wireless communication systems.

Abstract A broadband dual‐polarised antenna with L‐shaped strips for long‐term evolution (LTE)/5th generation mobile networks (5G) is proposed in this article. Based on the principle of multi‐mode resonance, wide bandwidth is achieved with L‐shaped strips and H‐shaped strips. The metal strips and dipoles are printed on the same substrate, which also contributes to antenna miniaturisation. The short pins are designed for impedance matching and improving isolation. The metal reflector is designed to control beamwidth and increase gain. A prototype was fabricated using an easy Printed Circuit Board technique to verify the proposed design. Low‐profile of 0.2λ0 (λ0 is the free‐space wavelength at the centre frequency) is achieved. The measured results show that the relative bandwidth is 74% (2.3–5.0 GHz) at Voltage Standing Wave Ratio

In this paper, we hybridize the Least Square Method (LSM) and Genetic Algorithm (GA) to efficiently synthesize the linear and planar array Plane Wave Generator (PWG) to create a 3-D Quiet Zone (QZ) with high performance for the antenna pattern measurement in a chamber with limited space. Based on the PWG with an 8-element linear array, the amplitude and phase deviations of 0.89 dB and 14.50° are obtained in a circular QZ with a diameter of $7\lambda $ , and that with an $8\times 8$ -element plane array achieves 1.09 dB and 14.89° deviations in a 3-D sphere QZ. The synthesized results using other existing algorithms based on different configurations of PWG are compared and the synthesis errors are discussed. Simulation results show the robustness and high efficiency of the proposed algorithm on 2-D and 3-D PWG synthesis.

Abstract A wideband dual‐polarized planar antenna with triple radiators is proposed for the LTE600/LTE700/GSM850/GSM900 base station in this paper. A novel concept of multi‐radiator for multi‐resonance by close coupling feed is presented and analysed . The proposed antenna is mainly composed of two pairs of loop dipole, L‐shaped radiator, step dipole, two L‐shaped feed lines, balun and reflector. The reflector is designed with a centrosymmetric partible baffle for directional radiations, which contribute to stable radiations as well. Owing to the novel closely coupling scheme for multi‐radiators, three resonances are simultaneously excited within the desired band, producing a wide bandwidth of 47.7% (from 0.59 to 0.96 GHz) for |S11|

In the present study, an immersion experiment was carried out to examine how N80 steel corrodes when exposed to formation water containing dissolved CO2 and supercritical CO2 (Sc-CO2) along with water vapor. We employed electrochemical and surface analysis methods to examine the influence of various factors, including the temperature and duration of immersion, on the extent of corrosion. The results show that the corrosion patterns of N80 steel in a supercritical CO2 environment and CO2-saturated formation water differed significantly. The presence of similar corrosion features was suggested by the constant structure of the corrosion products identified in the formation water. However, the morphology of the corrosion product was complex in the supercritical CO2 environment, exhibiting features of pitting and localized corrosion. Furthermore, a non-linear trend in the corrosion rate was observed between 40 °C and 120 °C. Specifically, the rate of corrosion declined from 40 °C to 80 °C, but it then resumed its growth from 80 °C to 120 °C. These findings suggest that very high temperatures could lead to the destruction of corrosion products and subsequently enhance the corrosion process.

Electromagnetic metasurfaces have been intensively used as ultra-compact and easy-to-integrate platforms for versatile wave manipulations from optical to terahertz (THz) and millimeter wave (MMW) ranges. In this paper, the less investigated effects of the interlayer coupling of multiple metasurfaces cascaded in parallel are intensively exploited and leveraged for scalable broadband spectral regulations. The hybridized resonant modes of cascaded metasurfaces with interlayer couplings are well interpreted and simply modeled by the transmission line lumped equivalent circuits, which are used in return to guide the design of the tunable spectral response. In particular, the interlayer gaps and other parameters of double or triple metasurfaces are deliberately leveraged to tune the inter-couplings for as-required spectral properties, i.e., the bandwidth scaling and central frequency shift. As a proof of concept, the scalable broadband transmissive spectra are demonstrated in the millimeter wave (MMW) range by cascading multilayers of metasurfaces sandwiched together in parallel with low-loss dielectrics (Rogers 3003). Finally, both the numerical and experimental results confirm the effectiveness of our cascaded model of multiple metasurfaces for broadband spectral tuning from a narrow band centered at 50 GHz to a broadened range of 40~55 GHz with ideal side steepness, respectively.

The precision and reliability of test findings are closely related to the uniformity of remolded loess samples. Although many methods to evaluate the uniformity of remolded loess have been proposed, they have not been widely accepted, due to the many defects. Therefore, it is necessary to explore the evaluation method and index of the uniformity of remolded loess samples in laboratory tests. In this paper, variations in shear strength and microscopic properties along the sample height were examined using the direct shear test and scanning electron microscopy (SEM) test. The feasibility and reliability of employing shear strength as an indicator to assess the uniformity of remolded loess samples were confirmed based on the test results. The results showed that the shear strength and shear strength parameters decreased from top to bottom along the sample height, and that the difference in the shear strength and shear strength parameters between the top and the bottom of remolded loess samples increased with the increase in the sample height. The variation tendency of the shear strength and shear strength parameters altered when the height–diameter (H/D) ratio exceeded 0.97. The SEM test results revealed that as the sample height increased, the disparity in microstructure and pore distribution in various portions of remolded loess samples became more apparent. The effect of sample height on the uniformity of remolded loess samples was mostly attributable to the variation in coating thickness and the content of clay particles on the surface of coarse particles, induced by a reduction in compactness from the top to the bottom of samples. The coefficient for evaluating the uniformity of the remolded loess sample was defined as the ratio of the shear strength at the top to that at the bottom of the remolded sample, and 0.8 was chosen as the threshold value. The shear strength reflected the uniformity of remolded loess samples as accurately as the microscopic test parameters. Furthermore, the direct shear test is the most classic and widely used laboratory test, with a low cost, easy accessibility, short test cycle, simple data processing and low environmental requirements, which provides another feasible method to evaluate the uniformity of remolded loess.

The confined compression test is the most widely used test to study the compressive deformation of soil in a laboratory, and the height–diameter (H/D) ratio has a significant effect on the confined compression test results. However, there has been limited research on the effect of the H/D ratio on loess compression deformation. In this work, a series of confined compression tests were carried out on the remolded loess samples using a modified oedometer to explore the effect of the H/D ratio on the compression deformation. Scanning electron microscopy (SEM) tests were used to investigate the variation of the microstructure in soils with different H/D ratios along the sample height. The results showed that the axial strain of the remolded loess decreases with the increase in the H/D ratio. The influence of the H/D ratio on the compression deformation is not obvious at lower pressures and compaction degrees, but it is significantly enhanced when the H/D ratio is beyond 0.65. The compression curves of the remolded loess samples with different H/D ratios fit the Gompertz equation well, and the transition point of the compression curves becomes difficult to identify with the increase in the H/D ratio. The compression index (Cc) decreases exponentially as a function of the H/D ratio, while the compressive yield stress (Pc) increases linearly with the increase in the H/D ratio. SEM analysis showed that, with the increase in the H/D ratio, the compression deformation of the sample changed from the whole sample involved in compression to the part of the soil in contact with the pressure. The compression of loess is mainly dominated by the compression of macropores and mesopores. The development of the soil–ring wall friction on the compression deformation induced by the H/D ratio was analyzed based on the change in microstructure, and a three-stage development of the soil–ring wall friction was established. These results and analysis established the connection between the specification and the literature, providing strong support for the rationality of the confined compression test results.

The uniformity of remolded loess samples prepared in the laboratory directly determines the reliability of various test results. While many tests have been designed to reflect the properties of undisturbed loess, there are obvious structural differences between remolded and undisturbed loess. Therefore, it is of great theoretical and practical significance to explore a non-destructive evaluation index of the uniformity and structure of loess samples for indoor geotechnical tests. In this study, the loess from the Malan loess section of the Chanhe River in Xi’an, China, was used as the experimental material to analyze the uniformity of remolded loess samples in terms of the stress distribution, stratification range, and sample variance. Based on the test results, the feasibility of using the air permeability coefficient as an index by which to evaluate the uniformity and structure of loess samples is investigated for the first time. The results demonstrate that when the overall height of the sample is fixed, the higher the designed dry density of the sample and the lower the air permeability of the sample. Moreover, when the designed dry density of the sample is constant, with the increase in the overall height of the sample, the range of the stratified dry density of the sample gradually decreases, as does the sample variance. The more uniform the sample, the greater the air permeability, and under the same conditions, the air permeability of remolded soil is lower than that of undisturbed loess. SEM images reveal that there is a considerable difference between the structures of undisturbed and remolded loess samples. Remolded loess is characterized by the greatly reduced effectiveness of cementation between skeleton particles, which causes its structure to be weaker than that of undisturbed loess. In this paper, a quantitative index of loess structure based on air permeability is proposed, and the test results show that the index can adequately reflect the loess structure. Due to its obvious advantages of non-destructiveness, high efficiency, economy, convenience, and high speed, it has a unique advantage for the evaluation of the uniformity and structure of remolded soil samples.

This paper presents a universal design approach of multiport filtering power divider with the arbitrary power-division ratio, which is applicable for the designs of the single-ended/balanced circuit or hybrid of them. The power allocation in a specified passband is fulfilled by relying on two key factors in building a filter, i.e., coupling coefficient and external Q -factor (Qe). After synthesis for the bandpass response, the total input/output Qe and the total coupling coefficient at each power dividing level can be determined. Subsequently, according to the desired power-dividing ratio, the coupling coefficient between the two coupled resonators and Qe of each output can be easily determined. Once the parameters for filtering response are fixed, the power allocation can be determined by Qe without effect on the filtering response. Thus, the two functions of bandpass and power dividing can be designed independently, which provides tremendous freedom for the overall design with the arbitrary order of filtering and number of output ports. The proposed topology has several advantages, such as flexibility, cost efficiency, and simple design procedure, which would be a good candidate for multiple-path operation in the current and future MIMO systems.

A fully-integrated 24/77 GHz dual-band receiver is presented for automotive radar applications. The proposed receiver consists of a dual-band LNA, a dual-band sub-sampling PLL, and a wideband mixer, operating at 24 and 77 GHz alternatively. Implemented in 65 nm CMOS technology, the receiver achieves 760 and 1800 MHz bandwidth with 52 and 60 mW power consumption in 24 and 77 GHz operation modes, respectively. To the best of authors' knowledge, this is the first fully-integrated CMOS 24/77 GHz dual-band receiver for automotive radar applications.

A compact rectifier with high efficiency at wide input power dynamic range is proposed in this paper. The circuit consists of a matching network, two shunt connected diodes with an integrated impedance compression network (IICN) and a dc-pass filter. The IICN significantly reduces the diode impedance variation when input power changes, hence improves matching as well as efficiency of the rectifier at wide input power dynamic range. Different from the conventional resistance/impedance compression networks, which can only work under certain impedance condition with dual-branch topology, the IICN can directly operate two diodes in parallel in a single-branch with no impedance limitation. Thus, less components and branch are needed, resulting in size reduction and lower circuit complexity. The design flexibility is also enhanced. Mechanism of the IICN is analyzed. A prototype operating at 2.45 GHz is optimized, fabricated and measured. The total size of the prototype is 0.18λ2. The efficiency higher than 70% of the peak value is realized at input power ranging from 0.8 dBm to 18.0 dBm.

A polarization-reconfigurable and frequency-tunable mechanism based on an active artificial magnetic conductor (AMC) structure is proposed. The active AMC unit cell consists of a four-corner-cut inner patch and four triangles loaded with four varactors, and it can realize polarization-reconfigurable and frequency-tunable reflection characteristics by adjusting the capacitance of the varactors. By employing it as a ground plane of a pair of linearly-polarized dipole antennas, a multi-functional antenna can be well designed with polarization reconfigurability and frequency-tuning capability. By controlling the DC bias of the varactors accordingly, the three polarization states of right-/left-handed circular polarization (R/LHCP) and linear polarization (LP) can be switched, respectively, while the operating frequency of each polarization can be flexibly tuned. The measured tuning frequency ratio for the LP state (|S11|

A novel class of low-temperature cofired ceramic (LTCC) bandpass filters (BPFs) is proposed in this paper, featuring of compact size and good filtering responses in both microwave and millimeter-wave (mm-wave) bands. By effectively merging the lumped elements and distributed effects, a modified LTCC-based hybrid lumped and distributed resonator (HLDR) is introduced. In detail, the lumped part of a LTCC interdigital capacitor and a high-impedance line is used to reduce circuit size, loss, and cross-talks. The distributed part, saying, the mutual couplings between adjacent HLDRs and to the ground, is used to improve the design flexibility and filtering performance for HLDR filters. With mutual couplings, the coupling matrix method can be applied. Furthermore, the frequency-variant mixed couplings between HLDRs are investigated, and coupling matrix method is improved by introducing a new coupling mechanism. Multiple transmission zeroes (TZs) are firstly realized with second-order in-line topology, but without adding cross-couplings or bandstop structures. To validate above design strategies, one S-band and one Ka-band second-order HLDR BPFs are simulated and measured, respectively. Very compact size, low loss, high-selectivity, and wide upper stopband are observed, especially in mm-wave range.

This paper presents a novel four ports double Y-shaped photonic crystals circulator with ultra-wide bandwidth in 5G millimeter wave band. Its insertion loss can be designed as low as 0.38 dB at 26.10 GHz while its optimal isolation is up to 74.86 dB at 25.00 GHz. The circulator keeps its isolation above 20 dB from 21.80 to 29.04 GHz. Our simulations also show the relative bandwidth can be effectively broadened to be 28.48% through adding an impedance matching transformer composed of four triangular metal pedestals. We confirmed our theoretical results with a circulator sample made of a triangular-lattice Al2O3 ceramic rod array and ferrite posts. The minimal measured isolation and insertion loss have reach 47.61 dB and 0.80 dB at 25.55 GHz and 26.10 GHz, respectively. The operating spectral region of our sample covers from 24.08 to 28.23 GHz (the relative bandwidth is 15.87%), which is about seven times higher than the best simulated result of 2.5% and fifteen times higher than 1.07%, the published experimental value. Our experimental results are well matched with the numerical simulations. The low loss, high isolation and ultra-wideband characteristics demonstrate that our photonic crystal circulator has potential applications in 5G communication systems.

A novel three-port Y-shaped circulator based on magneto-photonic crystals is first experimentally investigated. The band gaps of TE mode for the photonic crystals are calculated by plane wave expansion method. The transmission characteristics of the circulator are simulated by finite element method and the experiments are performed in the microwave regime. At the central frequency of 9.83 GHz, the optimal experimental isolation of -61.11 dB is better than the numerical results.

This paper presents a wideband circularly polarized (CP) antenna based on two Vivaldi antenna elements. The two elements are orthogonally placed for compact consideration, and are driven by a specially designed wideband feeding network that splits the input signal into two-way signals with equal magnitude and orthogonal phase. In order to avoid crossing of the feeding lines of two Vivaldi elements, a small shift between them is introduced. This, however, will result in the undesired phase difference of two Vivaldi elements inevitably. To compensate the phase difference and ensure a wide axial ratio bandwidth, the feeding network is specially designed. Besides, to make full use of the antenna space, the feeding network is placed on the surface of two Vivaldi elements. For demonstration purpose, a prototype is fabricated. The measured results show a -10 dB impedance bandwidth of 101.3% (2.35 to 7.18 GHz) and 3 dB axial-ratio bandwidth of 118.4% (2.05 to 8 GHz), and the maximum realized gain is 7.5 dBic. Meanwhile, stable radiation patterns are achieved with the cross-polarization lower than -20 dB across the wide operating bandwidth at boresight direction.

This paper presents a V-band Doherty power amplifier (PA) which is implemented in a standard 65-nm CMOS technology. The voltage combination technique is used to realize the millimeter-wave Doherty PA without the λ/4 transmission lines. A Marchand balun with balance compensation is designed to combine the output power with reduced power loss. Moreover, a nonlinear driver is used to drive the peaking amplifier to enhance the output power and the turn-on speed of it. The power-added efficiency (PAE) of this PA at the 6-dB power back-off point is 8.7% and the peak PAE reaches 16.8%. The small signal power gain is 18 dB, and the maximum output power is 14.9 dBm. The core circuit only costs 0.195 mm2 chip area as no λ/4 transmission lines.

A dual-layer patch antenna using multiple parasitic patches for the bandwidth enhancement is presented. Two additional resonances can be obtained with the help of the parasitic patches of the antenna. Consequently, a wide impedance bandwidth of 17% (|S11|

Patch antennas with multiple feeds are proposed in this paper. The antennas consist of a traditional square patch and feeding probes that are set in a line orthogonal to the polarization direction. In addition to radiating EM energy just like the conventional antennas, the proposed multi-port antennas can also realize on-antenna power combining without power combiner. Thanks to this, high-line losses and large system size caused by extra power combiner can be avoided. Moreover, since the location and the number of ports have a limited effect on the antenna performances, flexible feeding strategies can be chosen in different applications. For demonstration, four patch antennas with a different number of ports (one port, two ports, three ports, and four ports) are designed at the center frequency of 1.5 GHz. Compared with the traditional one-port patch antenna, the proposed three multi-port patch antennas can achieve nearly the same antenna performance, including bandwidth, polarization, efficiency, and radiation pattern. Besides, one-port antenna and two-port antenna integrated with a two-way power amplifier are designed and measured, respectively. Then, they are compared to further prove the proposed power combining method in multi-port antennas.

Based on the concept of conventional Gysel power divider, a novel concept of half-mode substrate integrated waveguide (HMSIW) Gysel power divider was reviewed and developed, by simply combining a traditional HMSIW power divider and a microstrip isolation network. The proposed concept was reviewed, investigated, and analyzed thoroughly, after that, Y-type, T-type, and Parallel-type HMSIW Gysel power divider were designed and analyzed, respectively. Furthermore, based on Parallel-type design, a developed version using two isolation networks was proposed for frequency bandwidth enhancement. All the four prototypes were fabricated and measured for verification and comparison. The good electrical performances and compact configuration indicate the advantages of the proposed concept and an excellent candidate for high power-handling HMSIW/SIW circuits or systems.

Abstract In this paper, a new spoof surface plasmon polaritons (SPPs) based transmission line (TL) with semi-open structure is proposed, which is implemented on a single-layer substrate with metallized via holes planted on a ground plane. The electromagnetic (EM) power propagates along it in the form of controlled slow surface wave, which is the same with its existing counterparts. The proposed TL can adjust the degree of EM energy confinement, and thus balance its performance in every characteristic, in particular attenuation and interference. As the TL is semi-open (i.e., EM energy distributes in the half space above the ground plane), it is less vulnerable to the nearby interference compared with its former counterparts, which are fully-open structure (i.e., EM energy distributes in the full space). Prototypes working at Ka band are fabricated and measured. Bianco-Parodi (BP) method is used to derive the attenuation of the proposed TL from the measured S-parameters. The proposed structure can be easily scaled for THz applications, which opens the door for future high performance THz components and systems.

Abstract Condensation is a common physical process which widely exists in natural phenomena and thermal energy systems. In a condensation process, cluster is considered as the important bridge between vapor body and condensates. However, limited by the minimum imaging dimension of traditional measurements, early experimental studies about initial stages of condensation process are not sufficient. This paper provides a powerful optical platform for the study of dynamic clusters process. Based on the Rayleigh law, optical experiments were firstly introduced to investigate the clusters spatial distribution close to and far from condensation surface. The results show that clusters are mainly generated in the vicinity of the condensation surface within the thickness of 200 μm. When they move away from the condensation surface, clusters progressively vanish and they have a life cycle of a fraction of a millisecond. Though scattering intensity is proportional to the 6th power of cluster radius r and cluster number density N c theoretically, the scattering intensity does not increase sharply with the increase of subcooling degree from the experimental results, so we can infer that the cluster number density plays a dominate role in this process and the effect of cluster radius almost can be ignored.Zhong Lan and Di Wang contributed equally to this work.

This paper presents experimental results showing reduced phase noise in a very high frequency band microelectromechanical systems (MEMS) oscillator. This has been achieved by engineering the embedded MEMS resonator with phononic crystal structures. The aluminum nitride on silicon MEMS resonator with phononic crystal tethers achieves an unloaded quality factor (Qu) 2.3 times of the same resonator with a simple tether design. The increase in Qu leads to a measured 6dB close-to-carrier phase noise reduction in an MEMS-based oscillator that sustained by a transimpedance amplifier implemented in 65nm CMOS process. The 141 MHz MEMS-CMOS oscillator with phononic crystal tethers has a phase noise better than -83dBc/Hz at 1 kHz offset and -134dBc/Hz at the far-from-carrier range while consuming less than 3mW.

Bilobalide, the only sesquiterpene compound from Ginkgo biloba leaf, exhibits various beneficial pharmaceutical activities, such as antioxidant, anti-inflammation, and protective effects for the central nervous system. Several bioactive components extracted from Ginkgo biloba extract reportedly have the potential to attenuate lipid metabolism. However, the effect of bilobalide on lipid metabolism remains unclear. In this study, we used 3T3-L1 cells as the cell model to investigate the effect of bilobalide on adipogenesis. The results showed that bilobalide inhibited 3T3-L1 preadipocyte differentiation and intracellular lipid accumulation. Quantitative real-time PCR and western blotting results indicated that several specific adipogenic transcription factors and a few important adipogenesis-related genes were significantly down regulated on both mRNA and protein levels in bilobalide treatment groups. By contrast, the expression of some lipolytic genes, such as adipose triglyceride lipase, hormone-sensitive lipase (HSL), and carnitine palmitoyltransferase-1α, were all up-regulated by bilobalide treatment, and the phosphorylation of AMP-activated protein kinase (AMPK), acetyl-CoA carboxylase 1, and HSL were stimulated. Furthermore, bilobalide treatment partially restored AMPK activity following its blockade by compound C (dorsomorphin). These results suggested that bilobalide inhibited adipogenesis and promoted lipolysis in 3T3-L1 cells by activating the AMPK signaling pathway.

A waveguide 10 dB directional coupler operating from 325 GHz to 400 GHz is designed based on the short-slot Riblet-type coupling configuration and fabricated using the deep reactive ion etching (DRIE) silicon micromachining technique. The skin depth and the conductivity of the gold film with the roughness of 0.2 μm are investigated at 300~1000 GHz frequency band for the higher accuracy. In order to measure the small-size four-port coupler using the two-port VNA with big-size flanges, three testing topologies are designed, in which the terahertz (THz) wedged-type absorbing material terminals are adopted as the waveguide matching loads. The measured average insertion loss is 0.5 dB after deducting the intrinsic loss and the measured average isolation is better than 25 dB, which are in good agreement with simulations. The analysis and the design are verified to be accurate and valuable for the high-performance sub-millimeter-wave waveguide components.

A novel vertical meandering technique to reduce the lateral size of a planar printed antenna is presented. It is implemented by dividing a conventional spiral patch into a different number of segments and placing them on different sides of the microwave substrate with vias as the connections. To confirm the validity of this technique, measured electrical performance and radiation characteristics of five antennas with different numbers of segments are compared. The smallest antenna is reduced in size by 84% when compared with the conventional printed spiral antenna.