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Time-Average Radiated Acoustic Power
Published in Lawrence J. Ziomek, An Introduction to Sonar Systems Engineering, 2023
The directivity of an aperture, when used in the active mode as a transmitter, is a measure of its ability to concentrate the available acoustic power into a preferred direction. When used in the passive mode as a receiver, the directivity of an aperture is a measure of its ability to distinguish between several sound-sources located at different positions in the fluid medium. Therefore, directivity is basically a measure of the beamwidth and sidelobe levels of a far-field beam pattern. In the analysis that follows, the aperture is being used as a transmitter. Note that if the transducer or transducers that make up an aperture are reversible, that is, if they can be used as either transmitters or receivers, then the transmit and receive far-field beam patterns of the aperture are identical.
Design and Measurements Process of Wearable Communication, Medical and IOT Systems
Published in Albert Sabban, Wearable Systems and Antennas Technologies for 5G, IOT and Medical Systems, 2020
Antenna measurements will be discussed in this chapter in Sections 17.15 and 17.16. It is more convenient to measure antennas in the receiving mode. If the measured antenna is reciprocal, the antenna’s radiation characteristics are identical for the receiving and transmitting modes. Active antennas are not reciprocal. The radiation characteristics of antennas are usually measured in the far field. Far-field antenna measurements suffer from some disadvantages. A long free space area is needed. Reflection from the ground from walls affect measured results and add errors to measured results. It is difficult and almost impossible to measure an antenna in the antenna’s operating environment, such as an airplane or satellite. Facilities for antenna measurements are expensive. Some of these drawbacks may be solved by near-field and indoor measurements. Near-field measurements are presented in [1]. Small communication companies do not own antenna measurement facilities. However, there are several companies around the world that provide antenna measurements services, including near-field and far-field measurements. One day near-field measurements may cost around 5000 USD. One day far-field measurements may cost around 3000 USD.
Foundations of electromagnetism
Published in Riadh Habash, BioElectroMagnetics, 2020
The near-field region represents electric and magnetic fields that exist at the radio frequency (RF) source and extends to a distance of about one wavelength from the antenna. In this region, electric and magnetic fields are decoupled where the phase differences between waves emitted at different points of the antenna are relatively large, and the correlation between the electric field and the magnetic fields is not clear. In general, the near-field region is divided into two sub-regions. First is the reactive near-field where the strength decreases rapidly with distance from the antenna. Second is the radiative near-field where the average PD remains quite constant at different distances from the antenna, with some localized variations (at an approximate distance of less than λ/2π to 0.5 d²/λ from the antenna, where d is the largest linear aperture dimension of the antenna). In the near-field case of RFR, the distance is generally a few centimeters. For example, a frequency of 900 MHz corresponds to a near-field of about 5 cm. In this case, the most appropriate physical quantity for characterizing exposure is the specific absorption rate (SAR). Since the distance between a local source, like a cell phone antenna, and the user is only a few centimeters, the user is within the emitter’s near field with a maximum power of 0.1 to 2 W.
A review of microwave testing of glass fibre-reinforced polymer composites
Published in Nondestructive Testing and Evaluation, 2019
Zhen Li, Arthur Haigh, Constantinos Soutis, Andrew Gibson, Ping Wang
The far field is the region of operation for most antennae, where the radiation pattern does not change with distance and absorption of the radiation in the far-field region does not feed back to the transmitting antenna. The far-field (Fraunhofer) distance must satisfy three conditions: