Radars

The most challenging of these applications has been the location and characterisation of anomalies in thick concrete structures using seismic methods and the detection of reinforcing steel and pre-stressing cables in congested structures using radar. 

In the example shown a covermeter is used to detennine the distance to a known single bar, and this information is in turn used to establish a reference for the radar measurements. A wave speed of 110 m/ps is calculated for this concrete. 

A number of individual radar scans have been joined together Variations in cover depth give rise to variations in arrival time for the wave reflected from the reinforcing bars... 

Combining High Energy Radiography with the Covermeter and Radar to determine reinforcing details... 

By determining the depth of the bars using a eovermeter for the near surface bars (to 50 mm) and radar for up to 3 layers of bars (to 200 mm), it is only necessary to make one radiograph at a given loeation. In addition the radar will give usefiil information about the number and spacing of the bars. As described above, the eovermeter is used as a reference in establishing a baseline for radar interpretation. 

LIDAR is an acronym for light detection and ranging . The technique uses the same principles as RADAR, an acronym for radiowave detection and ranging . RADAR is used for measuring the distance of an object from an observer. The time taken for radiowave radiation to travel to and from the object, which reflects or backscatters the radiation, is readily translated into a measurement of distance. 

Fibrillated Fibers. Instead of extmding cellulose acetate into a continuous fiber, discrete, pulp-like agglomerates of fine, individual fibrils, called fibrets or fibrids, can be produced by rapid precipitation with an attenuating coagulation fluid. The individual fibers have diameters of 0.5 to 5.0 ]lni and lengths of 20 to 200 )Jm (Fig. 10). The surface area of the fibrillated fibers are about 20 m /g, about 60—80 times that of standard textile fibers. These materials are very hydrophilic an 85% moisture content has the appearance of a dry soHd (72). One appHcation is in a paper stmcture where their fine fiber size and branched stmcture allows mechanical entrapment of small particles. The fibers can also be loaded with particles to enhance some desired performance such as enhanced opacity for papers. When filled with metal particles it was suggested they be used as a radar screen in aerial warfare (73). 

High Frequency Dielectric Strength. Dielectric strength at high frequency is important in microwave power uses such as radar (see Microwave technology). Because SF has zero dipole moment, its dielectric strength is substantially constant as frequency increases. At 1.2 MHz, SF has... 

Weathering. Articles fabricated from FEP are unaffected by weather, and thek resistance to extreme heat, cold, and uv kradiation suits them for apphcations in radar and other electronic components. For example, after 15 years of solar exposure in Florida, the tensile strength (73) and light transmission (96%) of a 25-p.m thick film was unchanged and the film remained crystal clear. Elongation increased slightly for the first 5 to 7 years of outdoor exposure, probably as a result of stress relaxation. Beyond 10 years, a small decrease was observed. 

Electrical Properties. CeUular polymers have two important electrical appHcations (22). One takes advantage of the combination of inherent toughness and moisture resistance of polymers along with the decreased dielectric constant and dissipation factor of the foamed state to use ceUular polymers as electrical-wire insulation (97). The other combines the low dissipation factor and the rigidity of plastic foams in the constmction of radar domes. Polyurethane foams have been used as high voltage electrical insulation (213). 

Corning 9606 cordierite, 2 MgO-2 A12 03-5 Si02 spinel, Mg0Al203 MgO-stuffed P-quart2 quart2, Si02 low expansion, high transparency to radar missile radomes... 

A. V. Jelahan, Laser Radar Systems, Artech House, Boston, Mass., 1992. 

The Calcium Halophosphate Phosphors. Early fluorescent lamps used various combinations of naturally occurring fluorescent minerals. The development of the calcium halophosphate phosphor, Ca (P0 2(Cl, F) Sb ", Mn, in the 1940s was a significant breakthrough in fluorescent lighting (7). As is often the case in new phosphor discoveries, this phosphor was found accidentally while searching for phosphors for radar screens. 

Frequency Allocations. Under ideal conditions, an optimum frequency or frequency band should be selected for each appHcation of microwave power. Historically, however, development of the radio spectmm has been predominantly for communications and information processing purposes, eg, radar or radio location. Thus within each country and to some degree through international agreements, a complex Hst of frequency allocations and regulations on permitted radiated or conducted signals has been generated. Frequency allocations developed later on a much smaller scale for industrial, scientific, and medical (ISM) appHcations. 

Most, if not all, microwave biological effects and potential medical appHcations are beheved to be the result of heating, ie, thermal effects. The phenomenon of microwave hearing, ie, the hearing of clicking sounds when exposed to an intense radar-like pulse, is generally beheved to be a thermoelastic effect (161). Excellent reviews of the field of microwave bioeffects are available (162,163). 

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