Near-field measurement of continuously modulated fields employing the time-harmonic near- to far-field transformation

Abstract. Near-field far-field transformations (NFFFTs) are commonly performed for time-harmonic fields. Considering arbitrary in-situ measurement scenarios with given transmission signals, time-varying aspects of modulated signals have to be taken into consideration. We investigate and characterize two methods for the measurement of modulated fields, which work with a time-domain representation of the radiated fields and, at the same time, allow to employ the standard time-harmonic NFFFT. One method is based on the fact that the modulation signal can be assumed to be constant in a short enough measurement interval under the condition that the modulation and carrier frequencies are several decades apart. The second method performs long-time measurements in order to obtain the complete frequency spectrum in every single measurement. Both methods are verified by the NFFFT of synthetic field data.

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Near-field to far-field transformation applied to UHF antennas over lossy ground

International Journal of Microwave and Wireless Technologies ◽

10.1017/s1759078719001272 ◽

2019 ◽

Vol 12 (3) ◽

pp. 227-232

Author(s):

Nicolas Bourey ◽

Muriel Darces ◽

Yves Chatelon ◽

Marc Hélier

Keyword(s):

Surface Wave ◽

Near Field ◽

Field Measurements ◽

Directional Antennas ◽

Far Field ◽

Biconical Antenna ◽

Field Transformation ◽

Set Up ◽

Dipole Sources

AbstractThis paper deals with a near-field to far-field transformation able to predict the radiation of UHF antennas located over a lossy ground. From in-situ near-field measurements, an equivalent set of dipole sources is obtained as a model of the characterized antenna. The paper details the main steps of the transformation and describes the specific experimental set-up designed for the application. Simple directional antennas (monopoles array) as well as more complex omnidirectional antennas (like a biconical antenna as a scaled-down model of a HF antenna) have been tested in realistic environments. This approach is very efficient for separating the contributions of the radiated waves: the sky wave and the surface wave.

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Far field modeling of the Mamala Bay outfalls

Water Science & Technology ◽

10.2166/wst.1998.0418 ◽

1998 ◽

Vol 38 (10) ◽

pp. 323-330

Author(s):

Philip J. W. Roberts

Keyword(s):

Field Model ◽

Near Field ◽

Far Field ◽

Field Modeling ◽

Visitation Frequency ◽

Statistical Variation ◽

Long Time ◽

Harmonic Average ◽

Ocean Outfall ◽

Acoustic Doppler Current Profilers

The results of far field modeling of the wastefield formed by the Sand Island, Honolulu, ocean outfall are presented. A far field model, FRFIELD, was coupled to a near field model, NRFIELD. The input data for the models were long time series of oceanographic observations over the whole water column including currents measured by Acoustic Doppler Current Profilers and density stratification measured by thermistor strings. Thousands of simulations were made to predict the statistical variation of wastefield properties around the diffuser. It was shown that the visitation frequency of the wastefield decreases rapidly with distance from the diffuser. The spatial variation of minimum and harmonic average dilutions was also predicted. Average dilution increases rapidly with distance. It is concluded that any impact of the discharge will be confined to a relatively small area around the diffuser and beach impacts are not likely to be significant.

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