HF Noise Research Articles

Active HF and microwave noise spectroscopy for characterization of superconducting materials

Most superconducting materials under a suitable DC current bias emanate excess noise which may extend from low frequencies up to the terahertz region. The microwave noise in some granular high-T/sub c/ films at 12 GHz can be as high as 10 pW in a 100-MHz bandwidth. The authors studied this noise in Y-Ba-Cu-O and Bi-Sr-Ca-Cu-O thin films grown on different substrates in the temperature range from 4.2 to 100 K in the frequency bands 0-2 GHz and 11-12.5 GHz, using current bias and magnetic field as experimental variables. Analysis of the measurements provides valuable information on the superconductor crystalline microstructure, the parameters of the intrinsic Josephson clusters, the synchronization of the radiation from clusters, and the properties of the intergranular inclusions. In particular, the degree of granularity, the strength of the pinning centers, and the size and the geometry of weak links and of weak-link clusters can be deduced from the noise amplitude and frequency dependence measured as a function of the bias current and externally applied magnetic field. Noise emission tests can be easily performed along with standard superconducting transition measurements, providing instant microstructural information without the need for a sophisticated electron microscope.

Day-to-day variation of HF signal strength and noise

Measurements of the day to day variations of signal and noise on 120 and 970km paths at 9.3 and 15.7MHz are compared with CCIR reports. Appreciable correlation is found between signal and noise, indicating a need for modification to existing fading allowances for HF service planning.

Ionospheric mapping using satellite data of natural HF noise

Natural HF noise measured by the radiospectrometer IRS-1 aboard satellite InterKosmos-19 was used for ionospheric mapping. The topside sounder aboard the satellite KOSMOS-18o9 could also be used to the same end because its AGC voltage corresponds to the natural noise level near the satellite. From the cut-off frequency of the noise spectrum can the distribution of the local plasma density be derived. Global maps could be deduced that can be helpful when modelling the ionosphere.

Study and development of HF noise generators using vacuum diodes with improved metrological characteristics

Study and development of HF noise generators using vacuum diodes with improved metrological characteristics

Bandwidth expansion effects on the voltage deviation parameter (Vd) of MF and HF atmospheric radio noise

Medium frequency atmospheric radio noise measured during periods in 1980 and 1981, and recorded on digital tape in 100‐kHz bandwidth has been analyzed to model effects of bandwidth on the noise envelope voltage deviation parameter (Vd). It has been found that the magnitude of this impulsiveness parameter Vd for a given noise sample increases with increasing bandwidth at a slower rate than predicted by CCIR report 322 (International Radio Consultative Committee, Geneva, 1964). Moreover, the rate (slope) is not constant, but rather becomes steeper as the initial (narrower bandwidth) Vd increases due to the impulsive character of the noise at the time of measurement. A simple model based on multiple regression analysis of the MF data allows estimation of Vd in a new bandwidth from a measured Vde using the equation Vd = Vde + (0.4679 + 0.2111 Vde) log BWR, where BWR is the ratio of the new bandwidth to the measured bandwidth. The reason for the unexpectedly small slope lies in the fact that the average envelope voltage, formerly assumed to be independent of bandwidth, increases with increasing bandwidth, but at a slower rate than the rms envelope voltage. Preliminary analysis of a few HF (4–8 MHz) noise recordings indicates a similar behavior in bandwidth effects on Vd.

Some properties of em radiation from lightning

Simultaneous records of electrostatic fields and optical and HF (10 MHz) radiation from ground and cloud flashes occurring in Suva, Fiji, have been made, and gaps (brief cessations) in the HF radiation from lightning discharges have been analyzed. Gaps in the HF noise following return strokes appear to be terminated by K changes, the gap duration distribution following closely that of previously observed K change interval distributions. These observations support conjecture that in‐cloud junction streamers, rather than ceasing immediately following a return stroke, proceed from the top of the return stroke channel until a pocket of charge is tapped to produce the K change which terminates the noise gap. The streamer itself is presumed to radiate HF only weakly. Earlier proposals that HF noise gaps of duration greater than a certain length are associated with return strokes and therefore can be used to discriminate, for lightning counting purposes, between cloud‐ground discharges and intracloud discharges are not supported.

On the usefulness of topside HF noise measurements in determining foF2

A study was undertaken to assess the operational usefulness of employing topside HF noise measurements to determine the F region critical frequency, foF2. Detailed comparisons have been made between observed values of foF2, foF2 inferred from topside noise measurements and predicted values of foF2. It was found that during daylight hours foF2 inferred from noise measurements yields values that are on the average comparable to those determined from predictions. During nighttime hours, however, it is found that foF2 values determined from noise measurements are a significant improvement compared to predicted values, and that noise measurements can under certain conditions provide a means of usefully estimating ƒoF2 for operational purposes.

Determining the F-region critical frequency from satellite-borne noise measurements

A means of determining the F2-region critical frequency using noise measurements made on-board satellites orbiting above the F2-peak is investigated. Values of foF2 observed on iomograms recorded by the ISIS 2 satellite were compared against corresponding records of the ISIS AGC voltage trace superimposed on the ionograms. The difference between foF2 and the frequency at which the AGC trace displayed a continuous enhancement above the cosmic background level, was taken as a measure of how well noise measurements can be used to infer the sub-satellite value of foF2. It is seen that above regions of the globe where ground-based HF noise is high and the ionospheric structure does not display severe horizontal gradients, measurements of noise breaking through the ionosphere can be used to determine foF2 generally to within one MHz. Comparison of daytime and nighttime measurements over the ocean shows a considerably smaller difference between foF2 and noise breakthrough at night. It is postulated that this feature is due to multihop propagation modes that are attentuated less at night than during the day.

Radio detection of thunderstorm activity with an Earth-orbiting satellite

A study was made to determine the feasibility of using artificial earth satellites to monitor thunderstorm activity. The nighttime noise-temperature measurements made with the earth-oriented vee antenna of the Radio Astronomy Explorer (RAE 1) satellite in the frequency range 0.2-9.2 MHz were correlated with reported surface thunderstorm activity. Analysis shows that the minimum nighttime HF noise level (in the absence of surface thunderstorms) at an altitude of 5850 km over the United States is fixed by man-made noise. When thunderstorms are active below the satellite, the noise level is increased by about 6-12 dB. The highest level is associated with the most intense storms. It is concluded that thunderstorm regions can be detected by an orbiting satellite using HF radio techniques, but ionospheric effects must be taken into account.

Azimuthal Variation of Measured HF Noise

HF noise (2-8 MHz) as a function of azimuth, frequency, and time of day was measured in England during the winter of 1966-1967. For a given frequency interval eight unequal azimuthal sectors (with five sectors between 45°-180°) were sampled each four hours in a standardized fashion. Power levels for the tenth, fiftieth, and ninetieth percentiles were derived and plotted. The results provide the integrated total external noise (both local and propagated). In addition to the well-known distinctive diurnal and frequency changes, a marked variation with azimuth is present. A separate examination of noise intensities measured simultaneously from several azimuths reveals strong differences with azimuth. The investigation indicates that noise levels as a function of azimuth are needed and are the ones to be applied to practical HF systems.

Distribution and Frequency Dependence of Incidental Man-Made HF/VHF Noise in Metropolitan Areas

Evaluation and consolidation have been performed on HF and VHF man-made incidental radio noise data obtained from journal publications and corporate reports released during the past seventeen years by several investigators. This summary represents an extention to 20 MHz of a compilation and analysis of metropolitan area incidental radio noise data initially begun for the 200- to 500-MHz range. The data sources included in the present summary represent the noise environments of metropolitan areas located in Australia, Asia, Europe, and North, Central, and South America. The noise regions, Urban, Suburban I, and Suburban II, previously identified for the 200- to 500-MHz frequency range and representing concentric zones in urbanized areas have been found appropriate for classifying the HF/VHF noise data. Prior to graphically consolidating the information, all data were converted into units of noise power in dB relative to one mW/kHz of receiver bandwidth. To the consolidated data of each noise region has been fitted a power series representation. It has been found necessary, because of differences in magnitude, to segregate the noise region data into two group: one set recorded with quasi-peak field intensity meters, and the other obtained with detectors calibrated to measure available power.

Abnormal Enhancements of HF Noise Intensity at Syowa Station, Antarctica

Abnormal Enhancements of HF Noise Intensity at Syowa Station, Antarctica

Spring thunderstorms over Bangalore

A study of the flashing characteristics of thunderstorms can provide useful information of scientific interest and application value if all types of flashes could be recorded with equal satisfaction. One way of realizing this requirement is by utilizing the now known experimental result that all types of flashes are equally effective in radiating HF noise. This paper describes a technique based on this criterion for investigating the flashing characteristics of thunderstorms and deducing therefrom values of flash densities and thunderstorm parameters such as storm life, number of cells developed during the lifetime of a storm, cell life, rates of flashing, etc. The results of such studies of spring thunderstorms over Bangalore are discussed.

Riometer observations of HF noise at Samoa following high-altitude nuclear test

Abstract Long-term enhancement of 42 Mc/s cosmic noise following the 9 July 1962 high-altitude nuclear test was observed on a riometer in Western Samoa. The enhancement was small, less than 1 dB above cosmic background, but remained readable, though decreasing in magnitude, for 10 days after the event.

Design of multiple-loop feedback control systems

A single-loop design is basic for a two-degree-of-freedom plant, and it is theoretically able to achieve any desired insensitivity to plant variations or rejection of disturbances, if the plant is minimum-phase. In exacting feedback problems where the parameter variations or disturbances are large, the resulting single-loop transmission may require a larger gain and bandwidth than that of the plant. The added feedback compensation networks then have rising frequency characteristics and make the system very sensitive to HF noise in the feedback path. It is shown how a multiple-loop design permits the attainment of the same benefits of feedback, but with considerably less sensitivity to the HF noise. The basic problem is how to divide up the feedback burden most efficiently among the various loop transmission functions. Detailed procedures for this purpose are presented in the paper. The treatment is restricted to cascade-type plants.