Radiating coaxial cable having groups of spaced apertures...

Wave transmission lines and networks – Long lines – Leaky lines

Reexamination Certificate

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Details

C343S770000

Reexamination Certificate

active

06292072

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates generally to a radiating coaxial cable, and more particularly to a radiating coaxial cable having groups of spaced apertures for generating a surface wave at low frequencies and a combination of surface and radiated waves at higher frequencies.
BACKGROUND OF THE INVENTION
Radiating and leaky coaxial cables are employed as longitudinal antennas in confined spaces like tunnels, mines, buildings and in other stretched-out applications involving a narrow lateral corridor needed for one or two way communication, such as railroads and highways.
Leaky coaxial cables support surface waves. The coupling loss between cable and antenna increases proportionally as 1/r
2
, where r is the distance between the cable and the antenna. The coupling loss also increases with increasing frequency. Leaky coaxial cables are known to employ, for example, outer conductors defining equally spaced groups of apertures, longitudinal apertures, corrugated outer conductors having milled-off corrugation tops, loosely braided outer conductors. Such prior art leaky coaxial cable designs are typically broad-banded.
Radiating coaxial cables radiate a free space wave. The coupling loss increases with 1/r, and is fairly constant over a relatively narrow design bandwidth. Prior art radiating coaxial cables are known to employ groups of apertures defined in the outer conductor. About half of the apertures within a group are tilted forward, the other half backward. The spaced groups of apertures must be designed about the center operational wavelength and to be configured for the specific bandwidth requirement.
U.S. Pat. No. 4,366,457 and corresponding German Pat. DE 30 04 882 C2 employ groups of coupling apertures which are so designed as to primarily support surface waves in comparison with radiating waves. The spacing between adjacent groups of coupling apertures is substantially larger than the operational wavelength, and more specifically is larger than 10 meters.
U.S. Pat. No. 5,276,413 describes a coaxial cable defining equally-spaced groups of apertures which excite a surface wave. There is one aperture per group in the first section at the cable input. The number of apertures within a group progressively increases with each subsequent cable section in order to maintain the coupling loss to be approximately constant along the length of the cable while the internal insertion loss increases. The group spacing must be smaller than half the smallest operational wavelength to avoid resonance return loss spikes within the operational frequency band. The number of apertures per cable length is therefore high, thus increasing the insertion loss and increasing the potential of moisture migrating through the apertures and into the cable.
U.S. Pat. No. 5,291,164 shows a coaxial cable which radiates a free space wave within the operational frequency band. The cable provides groups of apertures designed with respect to the center operational frequency. This design generates a surface wave below the radiating operational frequency band, but a high coupling loss limits its use at those frequencies. The bandwidth of the radiating operational frequency band is relatively narrow.
OBJECTS OF THE INVENTION
In response to the foregoing, an object of the present invention is to provide a radiating coaxial cable having groups of spaced apertures for generating a surface wave at low frequencies and a combination of surface and radiated waves at higher frequencies.
Another object of the present invention is to provide a radiating coaxial cable exhibiting low coupling loss over a wide frequency band.
A further object of the present invention is to provide a radiating coaxial cable which minimizes the number of apertures defined in its outer conductor in order to minimize the potential of water migrating into the inside of the cable and to obtain a low insertion loss for internal TEM waves propagating therethrough.
SUMMARY OF THE INVENTION
According to a first aspect of the present invention, a radiating coaxial cable includes an inner conductor, an electrical insulator located about a periphery of the inner conductor, and an outer conductor located about a periphery of the electrical insulator. The outer conductor defines groups of apertures spaced from each other a respective predetermined distance in the range from about eight meters to less than ten meters. The spacing is varied for return loss reasons.
According to a second aspect of the present invention, a radiating coaxial cable having a longitudinally extending center axis includes an inner conductor, an electrical insulator located about a periphery of the inner conductor, and an outer conductor located about a periphery of the electrical insulator. The outer conductor defines groups of apertures spaced from each other a predetermined distance in the range from about eight meters to less than ten meters. Each aperture includes at least one opening extending circumaxially of the center axis about a portion of the outer conductor. A respective height of each aperture in a direction along a circumference of the outer conductor is about 20% to about 40% of the circumference of the outer conductor. A respective width of each aperture in a direction along a length of the outer conductor is in a range from about half to less than a respective height of the aperture, and a spacing of at least a first and last aperture within an aperture group has a half wavelength resonance in a range from within to slightly above an operational frequency band of the cable, whereby a surface wave is generated at low frequencies and a combination of surface and radiated waves is generated at higher frequencies.
An advantage of the present invention is that the radiating coaxial cable exhibits low coupling loss over a wide frequency band.
Another advantage of the present invention is that the radiating coaxial cable minimizes the number of apertures defined in its outer conductor in order to minimize the potential of water migrating into the inside of the cable and to obtain an acceptable insertion loss for internal TEM waves.


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patent: 3004882 (1989-10-01), None

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