Electricity: electrical systems and devices – Housing or mounting assemblies with diverse electrical... – For electronic systems and devices
Reexamination Certificate
2000-08-07
2002-03-19
Gandhi, Jayprakash N. (Department: 2841)
Electricity: electrical systems and devices
Housing or mounting assemblies with diverse electrical...
For electronic systems and devices
C361S752000, C361S800000
Reexamination Certificate
active
06359792
ABSTRACT:
BACKGROUND OF THE INVENTION
In compact microwave structures the printed transmission lines are widely used. They are broadband in frequency and generally economical to produce. The most commonly used is a microstrip line. This is a transmission-line geometry with a single conductor trace on one side of a dielectric substrate and a single ground plane on the opposite side. Since it is an open structure, microstrip line has a major fabrication advantage over stripline. It also features ease of interconnections and adjustments.
In a microstrip line, the electromagnetic fields exist partly in the air above the dielectric substrate and partly within the substrate itself The effective dielectric constant of the line is greater than the dielectric constant of air and less than that of the dielectric substrate. It is apparent that a basic (unshielded) microstrip line is not really a practical structure. It is open to the air and, in reality, it is desirable to have circuits that are covered to protect them from the environment as well as to prevent radiation and electromagnetic interference (EMI). Covering the basic microstrip configuration with a metal housing and a metal plate on the top leads to a shielded microstrip line. In the case of high power applications (which is a current trend), the main purpose of the housing is to provide electromagnetic shielding or, in other words, to prevent RF leakage. The other purposes are to provide mechanical strength, germetization, heat sinking, and to protect the circuitry from moisture, humidity, dust, salt spray, and other environmental contaminants. In order to protect the circuit, certain methods of sealing can be used: conductive epoxy, solder, welding, gasket materials, and metallization tape.
One of the most often used unit incorporated microstrip line printed circuit board (PCB) in microwave applications is a combiner/divider. This unit usually includes a housing having a bottom, two connectorized ends (input-output), two side walls and a cover. In combination with the covered housing the microstrip line is converted into shielded microstrip line. There is a relationship between the effective dielectric constant and physical dimensions of the shielded microstrip line. The side walls should be sufficiently spaced so that only weak fringing fields being seen and, therefore, a negligible effect on the effective dielectric constant being produced. The cover tends to lower the effective dielectric constant and reduce the impedance. The cover enables electric fields in the air above the strip conductor thereby giving the air more influence in determining the propagation characteristics. Only when the ratio of the distance from the cover to the dielectric substrate and the substrate thickness is greater than 10, the cover effects can be considered negligible. In practice, where the overall dimensions of the unit are usually very limited, this ratio is difficult to hold at 10. Designers increase the distance to the cover for the expense of cover's thickness.
There are three types of losses that occur in microstrip lines: conductor losses, dielectric losses, and radiation losses. The latest two contribute to field emissions. Those losses can be minimized, but they can not be totally eliminated. The only protection from RF leakage into environment is covered housing. Usually, the housing is a structure having a bottom, two side walls and two opposite ends with openings to receive connectors. Top of the structure constitutes an aperture. This aperture can be closed by a cover. In order to make the structure more compact (which often is a requirement), the press-in type connectors are used, and the cover is situated as close to the connectors as possible. With minimum size structure, where the cover is thin to a strength limit and there is no room for a gasket in the housing, the most vulnerable place in terms of RF leakage is a cover-housing contact.
Covers for passive microwave structures that are used without any gaskets and provide good contact with the housing are known (see, for example, U.S. Pat. No. 6,044,538). In the known structure the cover periphery having notch with a beveled edge is engaged with a housing aperture periphery having slanted face. The cover dimensions across the cover exceeds the aperture dimensions. The cover can be engaged with the housing by having a temperature differential that is sufficient to permit the cover to be positioned across the aperture. This temperature differential can be created by heating the housing.
Heating the housing of the combiner/divider when PCB and electronic components are already in place is very problematic. Another problem is the notch in the cover. The notch requires additional thickness of the cover material and associated space. Additional machining, equipment, labor and associated expenses are also required.
Accordingly, what is needed is a structure that can provide for the cover maximum proximity to connectors along with reliable shielding against RF field emittance. The cover and the housing should have minimal thickness and provide shielding without use of gaskets. A simple flat structure for the cover is also needed. Compactness of the structure in combination with mechanical strength is also needed.
BRIEF SUMMARY OF THE INVENTION
The present invention relates to a compact passive RF structure, mainly to a combiner/divider comprising a housing having a bottom portion, two side walls and two ends that in combination form a pocket inside the housing. In the housing a microstrip line PCB is disposed. A cover closes the pocket in the housing. There are holes in the ends with push-in type connectors pressed in them. The pocket comprises a bed all around its periphery where the cover is seating. The bed comprises blind threaded holes at side wall areas where it is wider than at end areas. There is no room for threaded holes as well as for a gasket at end areas. The cover overhangs push-in portion of the connectors leaving minimum metal thickness for the housing material that is just enough to withstand stress in the connectors' pressing process. The cover is secured to the housing by screws.
According to the present invention the bed of the housing is slanted toward the pocket with PCB and comprises threaded holes that are perpendicular to the bed surface. The cover that is thin and originally flat is bent along its periphery by force of tightened screws. The cover and the bed have rounded corners. Because of this, the force is spread all around the cover's edge. Stress caused by this bend does not exceed a proportional limit on the stress-strain curve for a given material of the cover that is metal. It means that, when all screws are tightened, the cover still tends to spring back to its original flat shape. As a result, specific force applied to the bed by an edge of the cover is immense because of very small area of the contact between the cover's edge and the bed. Therefore, this spring back action of the cover provides good sealing against RF leakage without use of any gasket.
By definition, a combiner/divider combines several channel into one, or divides one channel into several ones. Therefore, there are several connectors in one end of the structure and one connector in the other end. According to the present invention the pocket's aperture in the housing and, accordingly, the cover has shape of a funnel with “a spout” facing the end having one connector. For better conveying the spring back action of the cover from screwed sides to the non-screwed ends the cover corners are rounded.
Thus, the structure according to the present invention is a passive RF device such as a combiner/divider having a housing with a pocket and a cover seated on a slanted bed formed all around the pocket. The pocket's aperture and the cover have funnel-like shape with rounded corners. The housing contains a microstrip line PCB disposed in the pocket and push-in type connectors disposed in the ends.
It is an object of the present invention to have a structure with maxim
Gandhi Jayprakash N.
Renaissance Electronics Corporation
Tran Thanh Y.
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