Design considerations for high-power VHF radar transceivers: Phase matching long coaxial cables using a cable radar

The Poker Flat 49.92-MHz MST radar uses 64 phase-controlled transmitters in individual shelters distributed throughout the antenna array. Phase control is accomplished by sampling the transmitted pulse at the directional coupler of each transmitter and sending the sample pulse back to a phase-control unit. This method requires phase matching 64 long (256 meter) coaxial cables (RG-213) to within several electrical degrees. Tests with a time domain reflectometer showed that attenuation of high frequency components in the long RG-213 cable rounded the leading edge of the reflected pulse so that the cables could only be measured to within 50 cm (about 45 deg at 49.92 MHz). Another measurement technique using a vector voltmeter to compare forward and reflected phase required a directional coupler with unattainable directivity. Several other techniques were also found lacking, primarily because of loss in the long RG-213 cables. At this point it was realized that what was needed was a simple version of the phase-coherent clear-air radar, i.e., a cable radar. The design and operation of this cable are described

Design considerations for high-power VHF radar transceivers: The Poker Flat MST radar phase control system

Sixty-four separate 50-kW peak-power transmitters are distributed throughout the 200 x 200 meter Poker Flat MST radar antenna array. The relative phase of each transmitter is automatically controlled by a 64-channel unit located in the main building at the edge of the antenna. The phase control unit is described. In operation the RF pulse from a transmitter coupler is power divided and compared with the phase reference in a mixer. The mixer output is low-pass filtered and sampled near the center of the resulting video pulse by an amplifying sample-and-hold integrated circuit. Phase control is effected by maintaining the mixer output pulse near zero volts by amplifying the sample-and-hold output which then drives the voltage-controlled phase shifter in the direction to null the mixer output. The voltage-controlled shifter achieves over 360 deg phase shift in the range from 0.7 to 24 volts. When the voltage into the shifter tracks to either voltage limit the wrap-around control resets the voltage so that the shifter is always operating within its control range

Collapse of the random phase approximation: examples and counter-examples from the shell model

The Hartree-Fock approximation to the many-fermion problem can break exact symmetries, and in some cases by changing a parameter in the interaction one can drive the Hartree-Fock minimum from a symmetry-breaking state to a symmetry-conserving state (also referred to as a ``phase transition'' in the literature). The order of the transition is important when one applies the random phase approximation (RPA) to the of the Hartree-Fock wavefunction: if first order, RPA is stable through the transition, but if second-order, then the RPA amplitudes become large and lead to unphysical results. The latter is known as ``collapse'' of the RPA. While the difference between first- and second-order transitions in the RPA was first pointed out by Thouless, we present for the first time non-trivial examples of both first- and second-order transitions in a uniform model, the interacting shell-model, where we can compare to exact numerical results.Comment: 8 pages, 7 figure

Formation of nanocrystals based on decomposition in the amorphous Zr41.2Ti13.8Cu12.5Ni10Be22.5 alloy

Primary crystallization and decomposition in the bulk amorphous alloy Ar41.2Ti13.8Cu12.5Ni10Be22.5 have been studied by small angle neutron scattering (SANS), transmission electron microscopy (TEM), and differential scanning calorimetry (DSC). SANS data of samples annealed isothermally at 623 K exhibit an interference peak centered at q=0.46 nm(^-1) after an incubation time of about 100 min. TEM and DSC investigations confirm that the respective periodic variation in the scattering length density is due to the formation of nanocrystals embedded in the amorphous matrix. These observations suggest that during the incubation time a chemical decomposition process occurs and the related periodic composition fluctuations give rise to the observed periodic arrangement of the nanocrystals

Symmetrization and enhancement of the continuous Morlet transform

The forward and inverse wavelet transform using the continuous Morlet basis may be symmetrized by using an appropriate normalization factor. The loss of response due to wavelet truncation is addressed through a renormalization of the wavelet based on power. The spectral density has physical units which may be related to the squared amplitude of the signal, as do its margins the mean wavelet power and the integrated instant power, giving a quantitative estimate of the power density with temporal resolution. Deconvolution with the wavelet response matrix reduces the spectral leakage and produces an enhanced wavelet spectrum providing maximum resolution of the harmonic content of a signal. Applications to data analysis are discussed.Comment: 12 pages, 8 figures, 2 tables, minor revision, final versio

Investigation of fiber bridging in double cantilever beam specimens

The possibility to eliminate fiber bridging or at least to reduce it, and to evaluate an alternative approach for determination of in situ mode 7 fracture toughness values of composite matrix materials were investigated. Double cantilever beam (DCB) specimens were made using unidirectional lay-ups of T6C/Hx205 composite material in which the delaminating halves were placed at angles of 0, 1.5, and 3 degrees to each other. The small angles between the delaminating plies were used to avoid fiber nesting without significantly affecting mode I teflon insert. The DCB specimens were fabricated and it was found that: (1) the extent which fiber bridging and interlaminar toughness increase with crack length can be reduced by slight cross ply at the delamination plane to reduce fiber nesting; (2) some fiber bridging may occur even in the absence of fiber nesting; (3) the first values of toughness measured ahead of the thin teflon insert are very close to the toughness of the matrix material with no fiber bridging; (4) thin adhesive bondline of matrix material appears to give toughness values equal to the interlaminar toughness of the composite matrix without fiber bridging

Required technologies for a lunar optical UV-IR synthesis array

A Lunar Optical UV-IR Synthesis Array (LOUISA) proposed to take advantage of the characteristics of the lunar environment requires appropriate advances in technology. These technologies are in the areas of contamination/interference control, test and evaluation, manufacturing, construction, autonomous operations and maintenance, power and heating/cooling, stable precision structures, optics, parabolic antennas, and communications/control. LOUISA needs to be engineered to operate for long periods with minimal intervention by humans or robots. What is essential for LOUISA operation is enforcement of a systems engineering approach that makes compatible all lunar operations associated with habitation, resource development, and science

Epitaxial Thin Films of the Giant-Dielectric-Constant Material CaCu_3Ti_4O_{12} Grown by Pulsed-laser Deposition

Pulsed-laser deposition has been used to grow epitaxial thin films of the giant-dielectric-constant material CaCu_3Ti_4O_{12} on LaAlO_3 and SrTiO_3 substrates with or without various conducting buffer layers. The latter include YBa_2Cu_3O_7, La_{1.85}Sr_{0.15}CuO_{4+\delta} and LaNiO_3. Above 100K - 150K the thin films have a temperature independent dielectric constant as do single crystals. The value of the dielectric constant is of the order of 1500 over a wide temperature region, potentially making it a good candidate for many applications. The frequency dependence of its dielectric properties below 100K - 150K indicates an activated relaxation process.Comment: 11 pages, 4 figure

Frequency Shifts Inherent in the 6328 a Helium-neon Laser

Frequency shifts inherent in helium neon lase

Influence of the resin on interlaminar mixed-mode fracture

Both literature review data and new data on toughness behavior of seven matrix and adhesive systems in four types of tests were studied in order to assess the influence of the resin on interlaminar fracture. Mixed mode (i.e., various combinations of opening mode 1, G sub 1, and shearing mode 2; G sub 2) fracture toughness data showed that the mixed mode relationship for failure appears to be linear in terms of G sub 1 and G sub 2. The study further indicates that fracture of brittle resins is controlled by the G sub 1 component, and that fracture of many tough resins is controlled by total strain-energy release rate, G sub T. Regarding the relation of polymer structure and the mixed mode fracture: high mode 1 toughness requires resin dilatation; dilatation is low in unmodified epoxies at room temperature/dry conditions; dilatation is higher in plasticized epoxies, heated epoxies, and in modified epoxies; modification improves mode 2 toughness only slightly compared with mode 1 improvements. Analytical aspects of the cracked lap shear test specimen were explored