211 research outputs found
Space-qualified submillimeter radiometer
The purpose of this research was to develop a reliable submillimeter wave spectrometer for space-borne high frequency spectral line work. The emphasis was on improving the efficiency of frequency multipliers to limit the system components to rugged, low power consumption solid-state devices. This research has allowed Millitech to develop increased efficiency and performance in Millitech's existing line of submillimeter components and systems. Millitech has fabricated and tested a complete solid-state spectrometer front end for use at 560 GHz (the 1(sub 10) to 1(sub 01) transition of water vapor). The spectrometer was designed with the rigors of flight conditions in mind. The spectrometer uses a phase-locked, solid-state Gunn diode oscillator as the local oscillator, employing a tripler to produce about 3 mW of power at 285 GHz, and a low noise second harmonic waveguide mixer which requires less than 2 mW of LO power. The LO (and the signal) is injected into the mixer by means of a quasioptical diplexer. The measured system noise temperature is 2800 K (DSB) over 400 MHz. The whole spectrometer front end is compact (21 cm by 21 cm by 24 cm), light (7.4 kg), and has a power consumption of less than 8 W. Other topics explored in this work include compact frequency agile phase lock loops, optical filters, and InP Gunn oscillators for low noise applications. As a result of this research, the improvement in the design of multipliers and harmonic mixers will allow their use as the LO power for a variety of satellite-borne receivers operating in the 200 to 600 GHz frequency range
Ground feasibility tests of unity-gain, high directivity antenna Final report
Four dipole, high directivity antenna for radio astronom
Microwave Gaseous Disharges
Contains reports on seven research projects.Atomic Energy Commission under Contract AT(30-1)184
Simultaneous Single-Pulse Observations of Radio Pulsars: V. On the Broadband Nature of The Pulse Nulling Phenomenon in PSR B1133+16
We revisit the phenomenon of pulse nulling using high-quality single-pulse
data of PSR B1133+16 from simultaneous multifrequency observations.
Observations were made at 325, 610, 1400 and 4850 MHz as part of a joint
program between the European Pulsar Network and the Giant Metrewave Radio
Telescope. The pulse energy time series are analysed to derive improved
statistics of nulling pulses as well as to investigate the frequency dependence
of the phenomenon. The pulsar is observed to be in null state for approximately
15% of the time; however, we find that nulling does not always occur
simultaneously at all four frequencies of observation. We characterise this
"selective nulling'' as a function of frequency, separation in frequency, and
combination of frequencies. The most remarkable case is a significantly large
number of nulls (6%) at lower frequencies, that are marked by the
presence of a fairly narrow emission feature at the highest frequency of 4850
MHz. We refer to these as "low frequency (LF) nulls." Our analysis shows that
this high frequency emission tends to occur preferentially over a narrow range
in longitude and with pulse widths typically of the order of a few
milliseconds. We discuss the implications of our results for the pulsar
emission mechanism in general and for the broadbandness of nulling phenomenon
in particular. Our results signify the presence of an additional process of
emission which does not turn off when the pulsar nulls at low frequencies, and
becomes more prominent at higher frequencies. Our analysis also hints at a
possible outer gap origin for this new population of pulses, and thus a likely
connection to some high-energy emission processes that occur in the outer parts
of the pulsar magnetosphere.Comment: 12 pages, 13 figures, Accepted for publication in A&
Interstellar Plasma Turbulence Spectrum Toward the Pulsars PSR B0809+74 and B0950+08
Interstellar scintillations of pulsars PSR B0809+74 and B0950+08 have been
studied using observations at low frequencies (41, 62, 89, and 112 MHz).
Characteristic temporal and frequency scales of diffractive scintillations at
these frequencies have been determined. The comprehensive analysis of the
frequency and temporal structure functions reduced to the same frequency has
shown that the spectrum of interstellar plasma inhomogeneities toward both
pulsars is described by a power law. The exponent of the spectrum of
fluctuations of interstellar plasma inhomogeneities toward PSR B0950+08 (n =
3.00 +- 0.05) appreciably differs from the Kolmogorov exponent. Toward PSR
B0809+74 the spectrum is a power law with an exponent n = 3.7 +- 0.1. A strong
angular refraction has been detected toward PSR B0950+08. The distribution of
inhomogeneities along the line of sight has been analyzed; it has been shown
that the scintillations of PSR B0950+08 take place on a turbulent layer with
enhanced electron density, which is localized at approximately 10 pc from the
observer. For PSR B0809+74 the distribution of inhomogeneities is
quasi-uniform. Mean-square fluctuations of electron density on inhomogeneities
with a characteristic scale rho_0 = 10^7 m toward four pulsars have been
estimated. On this scale the local turbulence level in the 10-pc layer is 20
times higher than in an extended region responsible for the scintillations of
PSR B0809+74.Comment: 13 pages, 11 figure
Plant diversity greatly enhances weed suppression in intensively managed grasslands
Weed suppression was investigated in a field experiment across 31 international sites. The study included 15 plant communities at each site, based on two grasses and two legumes, each sown in monoculture and 11 four-species mixtures varying in the relative proportions of the four species. At each site, one grass and one legume species was selected as fast establishing and the other two species were selected for persistence. Average weed biomass in mixtures over the whole experiment was 52% less (95% confidence interval, 30 to 75%) than in the most suppressive monoculture (transgressive suppression). Transgressive suppression of weed biomass persisted over each year for each mixture. Weed biomass was consistently low and relatively similar across all mixtures and years. Average sown species biomass was greater in all mixtures than in any monoculture. The suppressive effect of sown forage species on weeds in mixtures was achieved without any herbicide use. At each site, weed biomass for almost every mixture was lower than the average across the four monocultures. The average proportion of weed biomass in mixtures was less than in the most suppressive monoculture in two thirds of sites. Mixtures outyielded monocultures, and mixture yield comprised far lower weed biomass
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