Band Interferometer Research Articles

VHF-UHF帯電波干渉計システムによる部分放電源特定のための基礎研究

The electromagnetic waves emitted from partial discharge (PD) due to material defects in high-voltage electric power apparatus and facilities are broadband signals including VHF, UHF (Very High Frequency, Ultra High Frequency). A VHF-UHF radio interferometer system (VURIS) for locating PD source has been originally designed, constructed and tested. The system is based on the idea extracting the phase differences at different frequencies between a pair of the received signals by the Fourier transform and computing the direction of the electromagnetic waves emitted from PD source. It can be small-sized, more suitable for compact and portable system, because UHF half-wavelength dipole antennas are used to receive the electromagnetic waves and the baseline between the antennas is very short. And it requires fewer antennas to achieve equivalent angular resolution than a narrow band interferometer system, which requires multiple baselines corresponding to a few times of half-wavelength of the observation frequency. Therefore, it's able to reduce the operating time at the measurement site because it's not needed to arrange many antennas at the many points. The indoor test results indicate that the system is able to estimate the arrival angle of the electromagnetic waves emitted from PD source.

Dependency of VHF broad band lightning source mapping on Fourier spectra

In the VHF broad band interferometer, the Fourier transform is applied to analyze the whole frequency range of lightning radiation sources. The position of a radiation source can be determined by resolving the incident angle for each frequency component against at least three‐antenna sensors, and then by averaging the incident angles of all frequency components the angular position can be located. This arithmetic‐averaging method as proposed by previous authors has a disadvantage, especially for cloud‐to‐ground flashes, that is positions of lightning leader channels near ground level can not be mapped well. It is found that Fourier spectral amplitudes of broad band pulses decrease with increasing frequency, and frequency components having low spectral amplitudes give less contribution to the lightning mapping. To improve the mapping, a weighted‐averaging method is proposed. The method provides a way to minimize the influence of frequency components, which contribute less to the lightning mapping.

雷観測のための広帯域及び狭帯域干渉計の比較

The location of lightning radiation sources observed by our narrow band interferometer and the lightning channel image by an all sky camera are compared with that of our broad band interferometer. The broad band interferometer system is described by Ushio and Kawasaki et al. [1997] and measures the two dimen-sional direction of the radiation sources from 25 to 250 MHz with sub-microsecond time resolution, using 500 MHz digitizer which has 8 Megabyte memory. These three observations show the excellent correspondence, and we can conclude the potential capability of the newly designed wide band interferometer. By analyzing simultaneously observed data by narrow band and wide band interferometers for of one flash, the advantages and disadvantages of the both type of interferometers are discussed. The analysis shows that the broad band system can also locate two radiation sources, which propagate simultaneously like branching, in time and space. And we conclude this is a new technology.

Broad band interferometric measurement of rocket triggered lightning in Japan

A broad band interferometer to investigate the location of radio sources due to lightning flashes in two dimension has been manufactured and evaluated during the Rocket Triggered Lightning Experiment in Japan. Having high digitization rate, a broad band interferometer is not able to record the entire radiation process emitted by a lightning discharge. Therefore, we introduced a method of sequential triggering for each electromagnetic pulse that allows to record as long periods of radiation as we need. We obtained several data sets of upward stepped leaders for both negative polarity and positive one during the experiment. The apparent speed of an upward negative leader is estimated at 6 × 105 m/sec. The time intervals between successive electromagnetic pulses are from 11 to 93 microseconds with a mean of 22 microseconds. A triggered lightning to an experimental power transmission tower, normally it is called an altitude triggered lightning, is also conducted. One of the altitude triggered lightning lowered negative charges to the tower. In this case both the negative downward leader and the positive upward one can be generated. Some isolated pulses emitted by the downward negative stepped leader which attaches to the transmission tower are observed and located. Pulses emitted by the positive upward leader penetrated into a negatively charged region of a thundercloud are also observed. The location of radiation sources are distributed within a few degrees of azimuth. The time intervals between successive pulses are from 14 to 188 microseconds with a mean of 78 microseconds.

Broad band radio interferometry for lightning observations

A broad band radio interferometer for locating lightning emissions has been designed, constructed and tested. For a broad band interferometer, a single fixed pair of antennas is equivalent to having many baselines in a narrow band interferometer. So, a broad band system requires fewer antennas than a narrow band system to achieve equivalent angular resolution. In addition, frequency dependent locations of the radio emissions can be extracted for a more detailed look at the lightning breakdown processes. Such a system has been tested by a computer simulation and by measuring a man‐made broad band radiation source. The system consists of two antennas which are separated vertically. Bandwidth of the system is from 40 to 350 MHz. Measurements of the man‐made source indicate that with a signal‐to‐noise ratio (SNR) above 10 dB, the system is able to locate the source with an accuracy of about 2° over the detectable frequencies. Preliminary observations of lightning discharges with this new technique appear to indicate that breakdown processes of a dart leader to ground radiate solely at its descending tip.

Synthetic aperture radar interferometry applied to ship‐generated internal waves in the 1989 Loch Linnhe experiment

Interferometer synthetic aperture radar images collected during the 1989 Loch Linnhe experiment showed mean Doppler variations across the phase of ship‐generated internal waves that corresponded to “velocity” variations of the order of 50 to 100 cm/s. The in situ current data, however, showed surface currents associated with the internal wave features of the order of 5 to 10 cm/s and virtually ruled out the existence of surface currents as large as the interferometer‐inferred values. In this paper we show how the pixel‐to‐pixel phase difference measured by the Jet Propulsion Laboratory interferometer is related to the mean Doppler frequency of the backscattered field. Model calculations are used to show how this frequency can sometimes change by a large amount, even when rather small surface currents are present. In particular, for winds blowing roughly across the internal wave features, as was the case for the interferometer runs in Loch Linnhe, computations based on our wave‐current interaction and time dependent scattering models show that changes in the mean Doppler frequency corresponding to large velocities can, in fact, be produced from the much smaller measured surface currents. We show that the larger interferometer velocity estimates are essentially due to the different modulation strengths of the surface Bragg waves advancing toward and receding from the radar. Thus for these crosswind conditions, care must be taken in converting the phase differences measured by the interferometer to a surface current image. When the wind is aligned more nearly along the internal wave propagation direction, the mean Doppler shifts (and the phase differences) are dominated mostly by advection, and interferometer current estimates are more accurate. C band computations predict that if the antenna spacing is small enough so that the fields from the two antennas remain correlated, then the C band interferometer current estimates will be better than those at L band.

A multi-frequency study of symbiotic stars - I. Near-simultaneous optical and radio observations

The relationship between optical line flux and 5 GHz radio flux is investigated for a sample of 17 northern sky symbiotic stars. Data were obtained near-simultaneously with the Manchester Echelle Spectrograph mounted on the Isaac Newton Telescope, La Palma and the Broad Band Interferometer at Jodrell Bank. Color excesses, calculated from Balmer hydrogen lines fluxes, are compared with other reddening estimates and also combined with extinction maps to provide improved distance estimates. Optical line fluxes are used in combination with redio fluxes to estimate physical parameters of these objects, including mass-loss rates. The suggestion that the ionized regions of D-type symbiotics are much more extensive than those on S-type is comfirmed.

Co-ordinated Optical and Radio Observations of Symbiotic Stars

AbstractWe present preliminary results from absolutely flux-calibrated optical spectroscopy, together with 5 GHz radio flux measures of 17 symbiotic stars. The data were obtained quasi-simultaneously using the Manchester Echelle Spectrograph on the Isaac Newton Telescope, La Palma between 1988 September 20 and 24, and the Broad Band Interferometer at Jodrell Bank during 1988 October. This represents the largest sample of these stars observed in this way to date. Distances are calculated using visual extinctions and are compared with other quoted values. In general, the agreement is surprisingly good. Plots of individual line luminosities vs. radio luminosity indicate that the D-type (dusty) symbiotics (both also proto-planetary nebulae) have far higher radio luminosities than S-types (stellar) for comparable recombination and forbidden line luminosities. This may however be due to underestimation of optical line luminosities for the D-types as the circumstellar extinction contribution is uncertain. The single D’-type (cool dust, yellow secondary) lies at the low luminosity end of the S-type region. ‘Very slow novae’ in our sample lie in both groups.

Q band interferometer for high temperature measurements

A Q band interferometer for determining the complex permittivity of solids and liquids between 20°C and 1300°C has been developed. The Roberts and Von Hippel standing-wave method was used. The technique involves the utilization of a computer-assisted digital voltmeter for data acquisition and treatment. Results for a 67SiO2−33Na2O within the temperature range 20–400°C and in the liqid state (950°C) are presented.

Precision enhancement in 8 mm interferometry

A Q band interferometer for determining the complex permittivity of materials having dielectric loss factor within 10−3 to 1, has been developed. The technique involves the use of a computer-assisted digital voltmeter for data acquisition and treatment. The method of computing the dielectric parameters from the deduced VSWR and minima displacement is outlined and precision increase with this type of measurement is discussed. Results for polar solutes in non polar and in polar solvants are presented. Measurements on solids within the temperature range 20–400°C are also discussed in conclusion.

Digital image reconstruction of microwave holograms

A system capable of recording the Fourier hologram of a reflecting target at various microwave frequencies is described. The assembly consists of a swept frequency X -band interferometer circuit that illuminates a given target and a detector that scans a large square receiving area. The system is supervised by a microprocessor which controls the scanning operation, data acquisition, and storage of the resulting holograms. Image reconstruction is accomplished by discrete Fourier inversion of the data, and the output format uses computer graphics. The system's performance is evaluated by the analysis of various metallic objects. The lateral resolution is determined by the operating wavelength, linear dimensions of the recording area, and the range of the target, and for the conditions of the experiment is approximately one wavelength. Adequate depth resolution is obtained by combining multifrequency holograms measured in a range from 8 to 12 GHz, thereby achieving a depth resolution of 4 cm. Some potential applications are demonstrated. These include the measurement of target shape and displacement using convolution and subtraction techniques.