Dipole Approximation for Electrically Small Loop Antennas

Measurement Techniques - An elementary magnetic emitter is considered as an approximation for the variable magnetic field of an electrically small radiating loop antenna. The frequency limits for...     

Improved Hybrid Precoding Technique with Low-Resolution for MIMO-OFDM System

This paper proposes an improved hybrid beamforming system based on multiple-input multiple-output orthogonal frequency division multiplexing (MIMO-OFDM) system. The proposed beamforming system improves energy efficiency compared to the conventional hybrid beamforming system. Both sub-connected and full-connected structure are considered to apply the proposed algorithm. In the conventional hybrid beamforming, the usage of radio frequency (RF) chains and phase shifter (PS) gives high power and hardware complexity. In this paper, the phase over sampling (POS) with switches (SW) is used in hybrid beamforming system to improve the energy efficiency. The POS-SW structure samples the value of analog beamformer to make lower resolution than conventional system. The number of output data in POS is decided by the resolution of POS system. The limited number of POS decides the resolution of antenna array and the values of POSs are designed from maximum and minimum phase angle antenna array. Energy efficiency without the phase shifter is high although channel capacity is nearly similar with conventional system. Also, the amplifier with POS-SW system is proposed to improve the BER performance. According to the data bits, the output signals of POS are decided. The system with 2, 3 and 4 bits is simulated to prove the proposed algorithm. In order to overcome the loss of low-resolution system, the amplifier with POS-SW system using channel information is proposed. The average sum-rate of 4 bits system shows the similar performance with the conventional hybrid beamforming system. This structure can play an important role by increasing the energy efficiency of the wireless communication system that many antennas are used. It is shown that the BER, average sum rate and energy efficiency of the proposed scheme are more improved than the conventional hybrid beamforming system.     

HF and VHF Source Stabilization Technique Incorporating Q-Multiplied Quartz Crystal Resonator

A new approach is described for the desiga of HF/VHF crystal-controlled frequency sources exhibiting theoretical short-term stability unattainable through the use of conventional quartz oscillator design. The signal generator design uses the concept of AFC stabilization of a conventional quartz oscillator (VCXO) by means of a crystal-controlled highly selective active frequency reference. The AFC reference is a phase-shift type frequency discriminator that employs a product detector and an active Q-multiplied quartz crystal resonator. The extremely selective transmission response, large group delay, and power gain exhibited by the resonator, together with resonator phase noise levels comparable to that exhibited by the oscillator-maintaining circuit, provide the principal means for prediction of superior output signal spectral purity. Models of the resonators have been designed and constructed at 30 and 80 MHz, exhibiting 3-dB bandwidths of 30 and 160 Hz, respectively. Based on actual measurement of VHF Q-multiplied crystal resonator performance characteristics, approximately 16 dB improvement in VHF crystal-controlled frequency source spectral purity at low and moderate modulation rates is possible, compared to that attainable using the best available VHF quartz oscillator circuit designs.     

Frequency-Impulse Modulation as a Means of Attaining Accuracy in Cesium Atomic Clocks

An electronic system has been developed which corrects for cavity-detuning errors in cesium atomic-beam frequency standards. The RF signal applied to the beam tube is square-wave phase modulated, i.e., frequency-impulse modulated. The transient response of the beam tube to these phase steps is used as a control signal. When the positive and negative transients are both equal in area and identical in shape, it will be shown that the applied RF signal must be exactly at cesium frequency (for the given magnetic field) and the RF cavities must be exactly in tune. Two feedback loops are needed; one for correcting the crystal oscillator, the other for adjusting the relative phase of the RF cavities. With cavity detuning error greatly reduced by this system, the remaining source of inaccuracy is the uncertainty of the magnetic field in the drift region of the beam tube. An experiment is described which may permit setting a given cesium-beam standard to a frequency that differs by a precisely known amount from the zero-field cesium-resonance frequency.     

Absolute Measurements for Small Loop Antennas for RF Magnetic Field Standards

A method to determine accurately the strength of magnetic fields produced by a transmitting small loop antenna for the RF field standards is presented. The field strength can be determined by the magnetic dipole moment of the loop. A loop antenna factor is introduced to express the magnetic dipole moment of a transmitting small loop antenna in terms of the incident power to the antenna input port. The emergent power from the output port of a receiving small loop antenna can also be expressed in terms of the magnetic field to be detected and of the loop antenna factor. The insertion loss method (or the three antenna method) is applied to measuring the loop antenna factor. Small loop antennas with diameters 10 cm were designed, and loop antenna factors were measured by the insertion loss method over frequencies up to 30 MHz with a systematic error of 0.08 dB.     

RF Energy Transmission for a Low-Power Wireless Impedance Sensor Node

The proper management of energy resources is essential for any wireless sensing system. With applications that span industrial, civil, and aerospace infrastructure, it is necessary for sensors and sensor nodes to be physically robust and power efficient. In many applications, a sensor network must operate in locations that are difficult to access, and often these systems have a desired operational lifespan which exceeds that of conventional battery technologies. In the present study, the use of microwave energy is examined as an alternate method for powering compact, deployable wireless sensor nodes. A prototype microstrip patch antenna has been designed to operate in the 2.4 GHz ISM band and is used to collect directed radio frequency (RF) energy to power a wireless impedance device that provides active sensing capabilities for structural health monitoring applications. The system has been demonstrated in the laboratory, and was deployed in field experiments on the Alamosa Canyon Bridge in New Mexico in August 2007. The transmitted power was limited to 1 W in field tests, and was able to charge the sensor node to 3.6 V in 27 s. This power level was sufficient to measure two piezoelectric sensors and transmit data back to a base station on the bridge.      

Design and evaluation of a feedback based phased array system for ultrasound surgery

A driving system has been designed for phased array ultrasound applicators. The system is designed to-operate in the bandwidth 1.2 to 1.8 MHz, with independent channel power control up to 60 W (8 bit resolution) for each array element. To reduce power variation between elements, the system utilizes switching regulators in a feedback loop to automatically adjust the DC supply of a class D/E power converter. This feedback reduces the RF electrical power variation from 20% to 1% into a 16 element array. DC-to-RF efficiencies close to 70% for all power levels eliminates the need for large heat sinks. In addition to power control, each channel may be phase shifted 360 degrees with a minimum of 8 bit resolution. To ensure proper operation while driving ultrasound arrays with varying element sizes, each RF driving channel implements phase feedback such that proper phase of the driving signal is produced either at the amplifier output before the matching circuitry or after the matching circuitry at the transducer face. This feedback has been experimentally shown to increase the focal intensities by 20 to 25% of two tested phased arrays without array calibration using a hydrophone.     

An RF-powered, wireless CMOS temperature sensor

We present a wireless, fully integrated CMOS temperature sensor that recovers power from a radio frequency (RF) signal, and returns data as a frequency-modulated 2.3-GHz signal to a base station. Power is recovered from a 450-MHz incident signal with the help of a low-threshold, high-efficiency, voltage rectifier-multiplier circuit. This technique decreases the minimum incident RF power required, compared to state-of-the-art wirelessly powered telemetry systems. The rectifier-multiplier can collect energy from a base station placed up to 18 m away. To further increase the range from the base, the device collects energy in a low power standby/charging mode. A mode selector circuit monitors the amount stored energy and decides if the system is transmitting data or is in the standby/charging mode. A bootstrapped reference generates a complementary to absolute temperature (CTAT) voltage with an R-squared regression of 0.9995 to a linear fit. This reference is used as the temperature sensor of the system, controlling a low-power, integrated, voltage-controlled LC oscillator (VCO). The oscillation frequency of the VCO is modulated by ambient temperature changes. The modulated carrier is transmitted by a fully integrated power amplifier. A temperature sensitivity of 126 ppm//spl deg/C is achieved and the entire sensor consumes 1.1 mA while transmitting data.     

Amplification and calibration for miniature E-field probes

The amplification and signal-conditioning system for a miniature nonperturbing RF E-field probe is described. A simple calibration procedure using an X-band slotted waveguide is presented. It requires less power and space than conventional techniques requiring antenna measurements in an anechoic chamber. Measurement results demonstrate relatively small field perturbation due to the probe; and a region in the waveguide where the field is relatively uniform and suitable for calibration. Calibration accuracy is further established by comparing results with those obtained from antenna measurements in an anechoic chamber. To establish the calibration technique, the probe-amplification system is analyzed, leading to a gain equation relating the output voltage and measured field intensity, which is experimentally verified     

Design of X-band complementary metal-oxide semiconductor-based frequency-modulation continuous-wave sensor

This study presents an X-band complementary metal-oxide semiconductor-based frequency-modulation continuous-wave (FMCW) sensor system for transportation management. The proposed sensor system has two antennas, one to transmit signals and the other to receive them. The complete radio frequency (RF) transceiver is based on standard 0.18 μm one-poly six-metal (1P6M) CMOS technology with a chip area of 1.68 mm X 1.6 mm. Two planar leaky-mode antenna arrays with a gain of 18 dB are also designed. Experimental results indicate that the isolation between two antenna arrays that are 5.0 mm apart exceeds 42.0 dB at 10.5 GHz. The prototype of the FMCW sensor system is used in the range measurement of multiple lanes for the transportation management system (TMS). The major contribution of this study is that it integrates a 0.18 μm CMOS transceiver and antenna arrays into an FMCW RF front end, and employs an IF amplifier and a digital signal processor to demonstrate that the beat frequencies are linear. Measurements made in field tests agree closely with the simulation results.     

Peculiarity of frequency balancing of quadrature circuits

Conclusions The input impedance of an amplifier does not affect the transformation of the measured quantity to frequency or period. A change of Z_in involves only a change of sensitivity of transformation.Condition (3) can be realized not only for complete identity of the switches, if one selects small resistances in series with the switches and large resistances parallel to the switches. In this case the effect on the function of transformation of the instability of the resistances of the switches is minimized and the effect of the resistance of the wires connecting the quadrature circuit with the switches and amplifier is eliminated.An experimental check of the effect of the resistances of the switches confirmed the conclusion obtained.Translated from Izmeritel'naya Tekhnika, No. 9, p. 104, September, 1971.     

Implementation of wireless power transfer and communications for an implantable ocular drug delivery system

A wireless power transfer and communication system based on near-field inductive coupling has been designed and implemented. The feasibility of using such a system to remotely control drug release from an implantable drug delivery system is addressed. The architecture of the wireless system is described and the signal attenuation over distance in both water and phosphate buffered saline is studied. Additionally, the health risk due to exposure to radio frequency (RF) radiation is examined using a biological model. The experimental results demonstrate that the system can trigger the release of drug within 5 s, and that such short exposure to RF radiation does not produce any significant (_1 8C) heating in the biological model. The conclusion of the work is that this system could replace a chemical battery in an implantable system, eliminating the risks associated with battery failure and leakage and also allowing more compact designs for applications such as drug delivery.     

Phase noise performance of analog frequency dividers

The phase noise performance obtainable using silicon and GaAs-based TTL (transistor-transistor logic) and ECL (emitter-coupled logic) logic level digital frequency dividers is discussed. Measurement of the spectral performance of two types of analog dividers is reported: a parametric divider using varactor diodes and a regenerative-type divider incorporating a double-balanced mixer in the oscillator feedback circuit. Both dividers were configured for divide-by-two operation at VHF. Evaluation indicates the regenerative divider is capable of providing much lower phase noise than conventional digital logic level devices. The regenerative divider can be successfully operated over bandwidths in excess of an octave, and the design lends itself to small (i.e. TO-8) modular package implementation. Operating frequencies are bounded only by the range of the mixer and RF amplifier utilized and, as such, should extend from HF through microwave.     

The RF-powered surface wave sensor oscillator--a successful alternative to passive wireless sensing

A novel, passive wireless surface acoustic wave (SAW) sensor providing a highly coherent measurand proportional frequency, frequency modulated (FM) with identification (ID) data and immune to interference with multiple-path signals is described. The sensor is appropriate for bandwidth-limited applications requiring high-frequency accuracy. It comprises a low-power oscillator, stabilized with the sensing SAW resonator and powered by the rectified radio frequency (RF) power of the interrogating signal received by an antenna on the sensor part. A few hundred microwatts of direct current (DC) power are enough to power the sensor oscillator and ID modulation circuit and achieve stable operation at 1.0 and 2.49 GHz. Reliable sensor interrogation was achieved over a distance of 0.45 m from a SAW-based interrogation unit providing 50 mW of continuous RF power at 915 MHz. The -30 to -35 dBm of returned sensor power was enough to receive the sensor signal over a long distance and through several walls with a simple superheterodyne FM receiver converting the sensor signal to a low measurand proportional intermediate frequency and retrieving the ID data through FM detection. Different sensor implementations, including continuous and pulsed power versions and the possibility of transmitting data from several measurands with a single sensor, are discussed.     

A K-Band Phase-Locked Transmitter-Receiver System for CW Balanced-Bridge Measurements

A major sensitivity limitation common to most microwave CW balanced-bridge systems must be attributed to bridge distortion noise, caused by residual FM in the transmitted signal. The described superheterodyne transceiver minimizes this limit for a narrowband K-band bridge system by utilizing precise, coherent IF detection. A phase-locked local oscillator provides means for an arbitrary, stable preselection of the in-phase or quadrature component with the help of a calibrated IF delay line. The transmitter is either 1) frequency-stabilized to a sample cavity by means of a high-gain AFC loop, or 2) phase-locked simultaneously to a K-band harmonic of a VHF quartz oscillator and to a tunable VHF oscillator (VFO). This yields flexibility in a wide range of applications, such as measuring small reflection coefficients, dielectric constants, or magnetic tensor susceptibilities (e. g., in ESR spectroscopy). Analytical expressions for phase and amplitude distortions are derived for a bridge containing one high-Q element. In the systems theory of operation, analytical formulas for the noise spectral densities and the loop errors are given, together with numerical examples. The additional receiver noise, due to residual FM and increased bridge power, is demonstrated by means of measured IF-noise spectra. A cavity-Q measurement with ±1 percent accuracy, using 5-?W incident bridge power, proves the system's capability for measurements of small reflection coefficients.     

Enhanced radiation from an electrically small radiator using an array of sub-wavelength holes

An experimental demonstration of radiation enhancement from an electrically small antenna (ESA) using an array of sub-wavelength holes engraved on a metallic plate is presented in this paper. A weakly radiating, chip inductor loaded open coplanar waveguide transmission line is used as the reference ESA. We show that an array of sub-wavelength hole loaded metallic aperture, placed near the antenna, can significantly enhance radiated power from the source. The hole array converts the high spatial reactive spectrum existing in the near-field of the antenna into a far-field propagating spectrum. The theory is validated by experiments and simulations in the microwave frequency regime. This novel radiation enhancement scheme is seen to enhance the gain of the antenna from ?8.5 to ?2.5?dBi and radiation efficiency from 13 to 33% around resonance.     

Gain-Enhanced Metamaterial Based Antenna for 5G Communication Standards

Metamaterial surfaces play a vital role to achieve the surface waves suppression and in-phase reflection, in order to improve the antenna performance. In this paper, the performance comparison of a fifth generation (5G) antenna design is analyzed and compared with a metamaterial-based antenna for 5G communication system applications. Metamaterial surface is utilized as a reflector due to its in-phase reflection characteristic and high-impedance nature to improve the gain of an antenna. As conventional conducting ground plane does not give enough surface waves suppression which affects the antenna performance in terms of efficiency and gain etc. These factors are well considered in this work and improved by using the metamaterial surface. The radiating element of the proposed metamaterial based antenna is made up of copper material which is backed by the substrate, i.e., Rogers-4003 with a standard thickness, loss tangent and a relative permittivity of 1.524 mm, 0.0027 and 3.55, correspondingly. The proposed antenna with and without metamaterial surface operates at the central frequency of 3.32 GHz and 3.60 GHz, correspondingly. The traditional antenna yields a boresight gain of 2.76 dB which is further improved to 6.26 dB, using the metamaterial surface. The radiation efficiency of the proposed metamaterial-based 5G antenna is above 85% at the desired central frequency.     

平衡式E/F类功率放大器的设计与实现

为了解决功率放大器设计过程中存在的效率低和输入/输出端回波损耗较大的问题,设计了一种工作频率为1.5 GHz的平衡式功率放大器。通过采用3 dB定向耦合器对射频信号进行分配及合成,大大降低了输入/输出端的驻波系数,并将逆F类功率放大器的谐波控制网络引入E类功率放大器的匹配电路中。使用ADS对晶体管进行负载牵引和源牵引,得到晶体管的输入/输出阻抗,同时结合晶体管的寄生参数,在输出匹配电路中对二次谐波、三次谐波分别进行开路和短路处理,且为了进一步提高功率放大器的工作性能,在输入电路结构中抑制了二次谐波。选用GaN HEMT器件CGH40010F晶体管,利用ADS软件进行电路仿真,并采用Rogers4350b高频板材制作该功率放大器的实际测试电路板。仿真优化和实测表明：在输入功率为28 dBm时,该功率放大器的输出功率为41.54 dBm,漏极效率为76.99%,功率附加效率（power additional efficiency,PAE）达到73.59%,输入/输出端驻波系数小于2,同时具有160 MHz的高效率带宽,且最大输出功率较单管功率放大器提高了3 dB。实测结果与仿真数据有一定的误差,但仍有较好的一致性,满足设计指标要求,验证了设计方法的可行性。该设计方法具有效率高和回波损耗低的优势,提高了功率放大器的设计效率,使它在当今高效绿色节能的射频微波通信系统中具有广阔的应用前景。     

On chip plasmonic monopole nano-antennas and circuits

Analogues of many radio frequency (RF) antenna designs such as the half-wave dipole and Yagi-Uda have been successfully adapted to the optical frequency regime, opening the door for important advances in biosensing, photodetection, and emitter control. Examples of monopole antennas, however, are conspicuously rare given the element's extensive use in RF applications. Monopole antennas are attractive as they represent an easy to engineer, compact geometry and are well isolated from interference due the ground plane. Typically, however, the need to orient the antenna element perpendicular to a semi-infinite ground plane requires a three-dimensional structure and is incompatible with chip-based fabrication techniques. We propose and demonstrate here for the first time that monopole antenna elements can be fashioned out of single element nanoparticles fabricated in conventional planar geometries by using a small nanorod as a wire reflector. The structure offers a compact geometry and the reflector element provides a measure of isolation analogous to the RF counterpart. This isolation persists in the conductive coupling regime, allowing multiple monopoles to be combined into a single nanoparticle, yet still operate independently. This contrasts with several previous studies that observed dramatic variations in the spectral response of conductively coupled particles. We are able to account for these effects by modeling the system using circuit equations from standard RF antenna theory. Our model accurately describes this behavior as well as the detailed resonance tuning of the structure. As a specific practical application, the monopole resonances are precisely tuned to desired protein absorption bands, thereby enhancing their spectroscopic signatures. Furthermore, the accurate modeling of conductive coupling and demonstrated electronic isolation should be of general interest to the design of complex plasmonic circuits incorporating multiple antennas and other current carrying elements.     

General formulas for calculating magnetic field at a receiving loopplaced at an arbitrary position from a radiating loop in a shielded room

The standard magnetic field radiated by a small loop antenna is used for sensitivity testing of radio receivers. To avoid interference from external sources, the test is often carried out in a shielded room. A formula to obtain the intensity of the magnetic field at the receiving loop, when a small radiating loop antenna and the receiving loop antenna are placed at random locations in a shielded room, is derived. The convergence of the formula is analyzed. Using the formula, an equation is derived which expresses the shielded room error or the effect of the room on the standard magnetic field. The correlations between the calculated values of the error and the measured results are reported