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FREQUENCY BEHAVIOR OF A SHIELDED LOOP ANTENNA WITH A VARIABLE GAP POSITION.
Abstract not provided.
Wearable Wireless Telemetry System for Implantable BioMEMS Sensors
Telemetry systems of a type that have been proposed for the monitoring of physiological functions in humans would include the following subsystems: Surgically implanted or ingested units that would comprise combinations of microelectromechanical systems (MEMS)- based sensors [bioMEMS sensors] and passive radio-frequency (RF) readout circuits that would include miniature loop antennas. Compact radio transceiver units integrated into external garments for wirelessly powering and interrogating the implanted or ingested units. The basic principles of operation of these systems are the same as those of the bioMEMS-sensor-unit/external-RFpowering- and-interrogating-unit systems described in "Printed Multi-Turn Loop Antennas for Biotelemetry" (LEW-17879-1) NASA Tech Briefs, Vol. 31, No. 6 (June 2007), page 48, and in the immediately preceding article, "Hand-Held Units for Short-Range Wireless Biotelemetry" (LEW-17483-1). The differences between what is reported here and what was reported in the cited prior articles lie in proposed design features and a proposed mode of operation. In a specific system of the type now proposed, the sensor unit would comprise mainly a capacitive MEMS pressure sensor located in the annular region of a loop antenna (more specifically, a square spiral inductor/ antenna), all fabricated as an integral unit on a high-resistivity silicon chip. The capacitor electrodes, the spiral inductor/antenna, and the conductor lines interconnecting them would all be made of gold. The dimensions of the sensor unit have been estimated to be about 110.4 mm. The external garment-mounted powering/ interrogating unit would include a multi-turn loop antenna and signal-processing circuits. During operation, this external unit would be positioned in proximity to the implanted or ingested unit to provide for near-field, inductive coupling between the loop antennas, which we have as the primary and secondary windings of an electrical transformer.
Studies of whistler propagation along a plasma density gradient that is parallel to the magnetic field
Low frequency plasma wave generation in space is important for both scientific and practical applications. One of the most promising techniques for doing this is to directly inject whistler waves into the space environment from an antenna onboard one or more satellites. This technique has been discussed for years, but there are still open questions about the best way to generate plasma waves. So far, most theoretical [Kondrat92], lab based [Pribyl2010, Stenzel2016] and space-based experiments [DSX] have focused on studying the generation of whistler waves from an electric dipole antenna. However, a dipole antenna is very inefficient because it puts a lot of energy in waves that are not effective for most applications. Theoretical [Kondrat92] and lab experimental [Stenzel2016] results indicate that a loop antenna is much more efficient at generating whistler waves than a dipole antenna. A satellite experiment will need to be developed to demonstrate that whistler waves can be generated from a loop antenna in the space environment. The challenge is that to efficiently transmit whistler modes in the natural plasma environment of space, the loop antenna will have to be very large. For example, at L=2 (one earth radius away from the surface of the earth) a loop antenna would need a radius on the order of ~200 m to radiate efficiently, as shown in fig. 1, left. The antenna size and complexity would require a prohibitively large and expensive satellite mission. Our proposed innovation is to exploit the fact that the characteristic wavelength of whistler waves decreases in more dense plasma, which reduces the size needed for an antenna to radiate efficiently. Fortunately, a technique already exists for enhancing the local plasma density in space, called a plasma contactor [Kovaleski2001]. A plasma contactor can be used to create a local environment where the plasma density is enhanced around the satellite, which in turn reduces the size of an antenna that is needed to radiate efficiently (Fig. 1, right).
A new linear quadratic optimal controller for the 34-meter high efficiency antenna position loop
The design of a new position loop controller for the 34-meter High Efficiency Deep Space antennas using linear quadratic (LQ) optimal control techniques is discussed. The LQ optimal control theory is reviewed, and model development and verification are discussed. Families of optimal gain vectors are generated by varying weight parameters. Performance specifications were used to select a final gain vector. Estimator dynamics were selected and the corresponding gain vectors were computed. Final estimator selection was based on position, commanded rate, and estimator error responses.
Generation of waves in magnetized plasma [PowerPoint]
The presentation goes through assumptions made for the study, sections on plasma dispersion, whistler modes for RBR, excitation of plane waves with antenna, dipole vs loop antenna, verification of results, excitation of whistler waves with electron beams, assumptions for the model, distribution of current in space in general case, Fourier transform of current distribution, resonant excitation of waves, and ending with a discussion of generic debunching.
Efficiency of electrically small antennas combined with matching networks
In applications where electrically small antennas are necessary and transmitter power is limited, the system efficiency may be a critical parameter in determining the feasibility of the system. The principle of conservation of energy is used to determine the efficiency of an antenna combined with a matching network composed of elements with finite quality factors. It is shown that conditions for maximum efficiency can be met by using a simple L section as the matching network. Illustrative examples concern a cryogenic loop antenna and a ferrite loaded loop antenna.
Broadband antenna systems for lightning magnetic fields
Broadband magnetic antenna systems suitable for recording submicrosecond field changes are described, and typical data from distant lightning are presented. Two types of systems are described, one with a high-impedance antenna loop connected to the integrator by a twisted pair of coaxial cables and another with the antenna loop and twisted signal loops formed from a single piece of coaxial cable. Data for correlated magnetic and electric field waveforms from lightning at a distance of 50 to 100 km are presented and are shown to be almost identical.
DE 1 VLF observations during Activny wave injection experiments
We report on coordinated high-altitude satellite observations in support of one of the first space-based very low frequency (VLF) wave injection experiments, namely the USSR Aktivny mission. The Aktivny satellite (A) was designed to carry a VLF transmitter (nominal frequency approximately 10 kHz, transmitter power approximately 10 kW) coupled to a 20-m-diameter loop antenna in a nearly polar orbit (83 deg inclination, apogee approximately 2500 km, perigee approximately 500 km). We focus our attention on conjunction experiments between the Aktivny and DE 1 satellites. Because of problems in the deployment of the loop antenna, the radiated power capability of the antenna was significantly reduced. Although this substantially reduced the expectation of receiving detectable signal levels on the satellite, the DE 1/Aktivny conjunction experiments were nevertheless carried out as a means of possibly placing an upper limit on the radiated power. During the period November 1989 through April 1990, a total of 10 DE 1/Aktivny wave injection sessions were conducted. During each session the Aktivny transmitter operated at 10.537 kHz with 1 s On - 1 s Off format, for a period of 6 min centered around the conjunction time. During three conjunction periods (December 12, 26, and 27, 1989) both DE 1 and Aktivny were in the southern hemisphere, and DE 1 was at relatively low altitudes (ranging from 6211 to 14,810 km), thus providing the best conjunction possibilities according to the ray tracing criteria developed above. On most days, Omega transmitter signals as well as commonly occuring natural wave phenomena such as whistlers (0(+)) and hiss were clearly seen well above the background level, but there was no evidence of the Aktivny 1 s On/ 1 s Off pattern. Though no Aktivny signals were detected by the LWR on the DE 1 satellite, the experimental constraints allow us to place an upper limit on the total power radiated by the Aktivny transmitter in the whistler-mode. Using experimental parameters, and the minimum detectable signal level of 0.05 muV/m for LWR, we find the upper limit on the total power radiated by the Aktivny satellite in the whistler-mode to be approximately 10 mW. Several recommendations for future space-based wave injection experiments are presented.
Measurements of Siple transmitter signals on the DE 1 satellite - Wave normal direction and antenna effective length
A new experimental technique is developed to simultaneously measure the wave propagation direction and the effective length of a small (L value much smaller than wavelength) electric dipole antenna on a spin-stabilized satellite in the magnetosphere. The technique relies on the near simultaneous measurement of single components of the electric and magnetic fields of a coherent VLF signal injected into the medium from a ground-based source. The spin fading characteristics of the signal received by the electric dipole and the magnetic loop antenna permit the measurement of the wave normal direction assuming whistler-mode propagation. In-situ and remote measurements of the local cold plasma density are used to determine the refractive index. The wave electric field is then inferred from the wave magnetic field as measured on the loop antenna, the refractive index and the direction of propagation. Comparing this electric field with the measured voltage across the dipole antenna leads to the determination of the effective length of the receiving electric dipole. The technique is applied to data from the Dynamics Explorer 1 satellite observations of whistler mode signals injected into the magnetosphere from the Siple, Antarctica, VLF transmitter. In one case, with the measured background cold plasma density being 15 el/cu cm, the effective length of the 200 m-long electric dipole antenna is found to be 222 + or - 56 m, i.e., about twice the conventional value.
Studying NASA's Transition to Ka-Band Communications for Low Earth Orbit
As the S-band spectrum becomes crowded, future space missions will need to consider moving command and telemetry services to Ka-band. NASA's Space Communications and Navigation (SCaN) Testbed provides a software-defined radio (SDR) platform that is capable of supporting investigation of this service transition. The testbed contains two S-band SDRs and one Ka-band SDR. Over the past year, SCaN Testbed has demonstrated Ka-band communications capabilities with NASAs Tracking and Data Relay Satellite System (TDRSS) using both open- and closed-loop antenna tracking profiles. A number of technical areas need to be addressed for successful transition to Ka-band. The smaller antenna beamwidth at Ka-band increases the criticality of antenna pointing, necessitating closed loop tracking algorithms and new techniques for received power estimation. Additionally, the antenna pointing routines require enhanced knowledge of spacecraft position and attitude for initial acquisition, versus an S-band antenna. Ka-band provides a number of technical advantages for bulk data transfer. Unlike at S-band, a larger bandwidth may be available for space missions, allowing increased data rates. The potential for high rate data transfer can also be extended for direct-to-ground links through use of variable or adaptive coding and modulation. Specific examples of Ka-band research from SCaN Testbeds first year of operation will be cited, such as communications link performance with TDRSS, and the effects of truss flexure on antenna pointing.
Studying NASA's Transition to Ka-Band Communications for Low Earth Orbit
As the S-band spectrum becomes crowded, future space missions will need to consider moving command and telemetry services to Ka-band. NASAs Space Communications and Navigation (SCaN) Testbed provides a software-defined radio (SDR) platform that is capable of supporting investigation of this service transition. The testbed contains two S-band SDRs and one Ka-band SDR. Over the past year, SCaN Testbed has demonstrated Ka-band communications capabilities with NASAs Tracking and Data Relay Satellite System (TDRSS) using both open- and closed-loop antenna tracking profiles. A number of technical areas need to be addressed for successful transition to Ka-band. The smaller antenna beamwidth at Ka-band increases the criticality of antenna pointing, necessitating closed loop tracking algorithms and new techniques for received power estimation. Additionally, the antenna pointing routines require enhanced knowledge of spacecraft position and attitude for initial acquisition, versus an S-band antenna. Ka-band provides a number of technical advantages for bulk data transfer. Unlike at S-band, a larger bandwidth may be available for space missions, allowing increased data rates. The potential for high rate data transfer can also be extended for direct-to-ground links through use of variable or adaptive coding and modulation. Specific examples of Ka-band research from SCaN Testbeds first year of operation will be cited, such as communications link performance with TDRSS, and the effects of truss flexure on antenna pointing.
Spiral Chip Implantable Radiator and Printed Loop External Receptor for RF Telemetry in Bio-Sensor Systems
The paper describes the operation of a patented wireless RF telemetry system, consisting of a bio-MEMS implantable sensor and an external hand held unit, operating over the frequency range of few hundreds of MHz. A MEMS capacitive pressure sensor integrated with a miniature inductor/antenna together constitute the implantable sensor. Signal processing circuits collocated with a printed loop antenna together form the hand held unit, capable of inductively powering and also receiving the telemetry signals from the sensor. The paper in addition, demonstrates a technique to enhance the quality factor and inductance of the inductor in the presence of a lower ground plane and also presents the radiation characteristics of the loop antenna.
Improved manual radio frequency direction finder
Antenna loop direction finder, consisting of two separate loops mounted rigidly together at right angles to each other, requires single rotation for operation. The loops, one for bearing, the other for sensing, are rotated manually. Sensing loop, in conjunction with nondirectional antenna, produces signal with directional pattern.
Space Radio Patrol
Radio research in United States satellite program and whip and/or loop antennas for use in space radio patrol
Antennas in matter: Fundamentals, theory, and applications
The volume provides an introduction to antennas and probes embedded within or near material bodies such as the earth, the ocean, or a living organism. After a fundamental analysis of insulated and bare antennas, an advanced treatment of antennas in various media is presented, including a detailed study of the electromagnetic equations in homogeneous isotropic media, the complete theory of the bare dipole in a general medium, and a rigorous analysis of the insulated antenna as well as bare and insulated loop antennas. Finally, experimental models and measuring techniques related to antennas and probes in a general dissipative or dielectric medium are examined.
Wearable Wireless Telemetry System for Implantable Bio-MEMS Sensors
In this paper, a telemetry and contact-less powering system consisting of an implantable bio-MEMS sensor with a miniature printed square spiral chip antenna and an external wearable garment with printed loop antenna is investigated. The wearable garment pick-up antenna and the implantable chip antenna are in close proximity to each other and hence couple inductively through their near-fields and behave as the primary and the secondary circuits of a transformer, respectively. The numerical and experimental results are graphically presented, and include the design parameter values as a function of the geometry, the relative RF magnetic near-field intensity as a function of the distance and angle, and the current density on the strip conductors, for the implantable chip antenna.
Antenna radiation patterns in the whistler wave regime measured in a large laboratory plasma
Antenna radiation patterns of balanced electric dipoles and shielded magnetic loop antennas are obtained by measuring the relative wave amplitude with a small receiver antenna scanned around the exciter in a large uniform collisionless magnetized laboratory plasma in the whistler wave regime. The boundary effects are assumed to be negligible even for many farfield patterns. Characteristic differences are observed between electrically short and long antennas, the former exhibiting resonance cones and the latter showing dipole-like antenna patterns along the magnetic field. Resonance cones due to small electric dipoles and magnetic loops are observed in both the near zone and the far zone. A self-focusing process is revealed which produces a pencil-shaped field-aligned radiation pattern.