Patent Grant
6944005
1. Field of the Invention
The present invention relates generally to coaxial terminations used to terminate ports that are adapted to receive coaxial cable connectors, and more particularly, to an improved coaxial termination that offers protection against high-voltage surges.
2. Description of the Related Art
RF coaxial cable systems are well known to those in the cable television industry for distributing radio frequency signals to subscribers of cable television service, and more recently, voice and data telecommunications services. The coaxial cables used to route such signals include a center conductor for transmitting a radio frequency signal, and a surrounding, grounded outer conductive braid or sheath. Typically, the coaxial cable includes a dielectric material surrounding the center conductor and spacing it from the grounded outer sheath. The diameter of the center conductor, and the diameter of the outer conductor, and type of dielectric are selected to produce a characteristic impedance, such as 75 ohms, in the coaxial line. This same coaxial cable is sometimes used to provide AC power (typically 60-90 Vrms) to the equipment boxes that require external power to function. Approximately 80% of the cable in a system will carry this AC power.
Within such coaxial cable systems, such coaxial lines are typically coupled at their ends to equipment boxes, such as signal splitters, amplifiers, etc. These equipment boxes often have several internally-threaded coaxial ports adapted to receive end connectors of coaxial cables. If one or more of such coaxial ports is to be left “open”, i.e., a coaxial cable is not going to be secured to such port, then it is necessary to “terminate” such port with a coaxial termination that matches the characteristic impedance of the coaxial line (e.g., a 75 ohm termination). If such a coaxial termination is omitted, then undesired reflected signals interfere with the proper transmission of the desired radio frequency signal.
Coaxial terminations of the type described above are known and available. Typically, such known coaxial termination devices include a metallic outer body which, at a first end thereof, is provided with external threads for mating with the internal threads of a coaxial port on the equipment, box. A center conductor passes through a dielectric secured within the metallic outer body from the first end of the coaxial termination device to an opposing second end thereof. At the second end of the coaxial termination device, a resistor corresponding to the characteristic impedance of the coaxial line is secured, and is coupled between the center conductor and the grounded metallic outer body. If the coaxial line carries AC or DC power, then a low frequency blocking capacitor is typically used to couple the aforementioned resistor to ground. The resistor and capacitor of such known coaxial termination devices are often located outside the controlled characteristic impedance environment, creating an impedance mismatch that reflects some of the forward-transmitted signal back toward its source. These reflections can result in loss of power transfer and interference with, or corruption of, the signal. Accordingly, some signal degradation results from the use of such coaxial termination devices. The degree of such signal degradation at a given frequency, resulting from such impedance mismatch, is sometimes expressed as the RF return loss performance of the coaxial system.
Moreover, when deployed in the field, as in cable TV systems, for example, these known coaxial termination devices can be subjected to power surges caused by lightening strikes and other events. These power surges can damage or destroy the resistive and/or capacitive elements in such a termination, rendering it non-functional. A commonly used surge test, ANSI C62.41 Category B3, specifies that a 6000 Volt open circuit/3000 Amp short circuit surge pulse be injected into the coaxial termination device. At least some of the known coaxial termination devices have difficulty complying with such surge test. Indeed, efforts to make the resistive and capacitive components larger, in order to withstand such power surges, can have the negative impacts of increased costs and/or creating a larger impedance mismatch, and hence, causing poorer levels of RF Return Loss performance. One approach to designing a termination that can withstand the previously mentioned 6,000 Volt surges would be to use a 6,000 Volt capacitor and a high power resistor. Unfortunately, such components are relatively expensive and have a much larger physical size, which tends to increase the size and cost of the housing necessary to contain such components, thereby resulting in a much bulkier and more costly design.
Accordingly, it is an object of the present invention to provide a coaxial termination device capable of maintaining high levels of RF Return Loss performance.
It is a further object of the present invention to provide such a coaxial termination device capable of withstanding power surges without damage to the resistive and/or capacitive elements thereof.
A further object of the present invention is to provide such a coaxial termination device that can simultaneously withstand such power surges without damage, while still maintaining high levels of RF Return Loss performance.
A still further object of the present invention is to provide such a termination device that is relatively compact and inexpensive to manufacture.
Another object of the present invention is to provide such a coaxial termination device that reduces reflection by disposing the resistive component thereof in a controlled characteristic impedance environment.
Still another object of the present invention is to minimize the length of the path between the resistive component of the coaxial termination device and ground (i.e., through the capacitive component) to further minimize inductance and signal reflection.
Yet another object of the present invention is to provide such a coaxial termination device which allows the resistive and capacitive components thereof to be relatively small in size to maintain high levels of RF Return Loss performance while still being able to withstand power surges without damage.
These and other objects of the present invention will become more apparent to those skilled in the art as the description of the present invention proceeds.
Briefly described, and in accordance with the preferred embodiments thereof, the present invention relates to a surge-protected coaxial termination that includes a metallic outer body having a central bore extending therethrough, a center conductor extending into the central bore of the metallic outer body, and a spark gap created within such coaxial termination for allowing a high-voltage power surge to discharge across the spark gap without damaging other components (e.g., resistive and/or capacitive components) that might also be included in such coaxial termination. The central bore of the outer body is bounded by an inner wall, and the center conductor has an outer diameter facing the inner wall of the outer body. Normally, there is a solid dielectric material separating the outer diameter of the center conductor from the inner wall of the outer body; however, in the vicinity of the aforementioned spark gap, the dielectric material is simply air or another ionizable gas.
In a first embodiment of the present invention, the spark gap is created by including an inwardly-directed step upon the inner wall of the outer body. This inwardly-directed step portion of the inner wall is of relatively short axial length and has an inner diameter that is significantly smaller than the inner diameter of the remainder of such inner wall of the outer body. The center conductor extends through the inwardly directed step of the inner wall; at the point where the center conductor passes through the inwardly-directed step, its outer diameter is slightly less than the inner diameter of the inwardly-directed step. This positions the inwardly-directed step of the inner wall in close proximity to the center conductor to form the spark gap therebetween. If desired, the outer diameter of the center conductor can be enlarged somewhat to form an outwardly-directed step at the point where it passes through the inwardly-directed step to facilitate the passage of a spark between the outwardly-directed step of the center conductor and the inwardly-directed step of the outer body.
In a second embodiment of the present invention, the surge-protected coaxial termination again includes a metallic outer body having a central bore extending therethrough, and a center conductor extending into the central bore thereof, but the spark gap is created by forming an outwardly-directed step of relatively short axial length on the center conductor extending toward the inner wall of the outer body. The outer diameter of the outwardly-directed step is slightly less than the inner diameter of the inner wall for positioning the outwardly-directed step of the center conductor in close proximity to the inner wall of the outer body to form a spark gap therebetween.
In a third embodiment of the present invention, the surge-protected coaxial termination again includes a metallic outer body having a central bore extending therethrough, and a center conductor extending into the central bore thereof, but the spark gap is created by a lateral conductor, such as a post or the like. This lateral conductor can be secured to the outer body and extend laterally toward the center conductor, or the lateral conductor can be secured to the center conductor and extend laterally toward the inner wall of the outer metallic body. In either case, the lateral conductor creates a spark gap that can discharge to ground any high voltage surges that appear between the center conductor and the outer conductor.
The creation of the spark gap in the manner described above tends to present a highly-capacitive discontinuity to any RF fields traveling along the transmission line; such a capacitive discontinuity would ordinarily cause reflections of the type that a coaxial termination device is designed to prevent. Accordingly, in the preferred form of the present invention, at least one relatively high characteristic impedance inductive zone is formed adjacent the capacitive spark gap; preferably, such high characteristic impedance inductive zones are formed on both sides of the spark gap. The combination of the capacitive spark gap and the high impedance inductive zones form the equivalent of an electrical T-network low pass filter, wherein the additional inductance of the high impedance zones effectively nullifies the additional capacitance of the spark gap, over the bandwidth of interest.
As mentioned above, coaxial termination devices typically include a resistive component to absorb the RF signal, and prevent the reflection of the RF signal. Accordingly, the preferred embodiments of the present invention include a resistive terminating element electrically coupled between the center conductor and the metallic outer body. This resistor is electrically in parallel with the spark gap, whereby surge currents that jump the spark gap flow around the resistor, avoiding damage thereto. Accordingly, the resistor can be relatively compact and inexpensive.
As also mentioned above, coaxial termination devices typically include an AC/DC power blocking capacitor coupled in series with the resistor between the center conductor and the metallic outer body. Once again, the capacitor can be relatively small and inexpensive because the spark gap protects the capacitor from damaging high voltage power surges.
Another novel feature of the preferred form of the present invention relates to the manner by which such resistive and capacitive components of the coaxial termination device are incorporated therein. Preferably, the resistive component is disposed inside the metallic outer body, and extends co-axially with the center conductor. Ideally, this resistive component is formed inexpensively as a carbon composition resistor. The resistive component may be surrounded by, and supported by, dielectric material disposed inside the central bore of the metallic outer body, thereby maintaining the resistor in a controlled characteristic impedance environment. One end (electrode) of the resistive component is electrically coupled with an end of the center conductor. The opposing second end (electrode) of the resistive component may protrude from the metallic outer body and related dielectric material; the DC blocking capacitor preferably extends radially between the second end of the resistive component and the metallic outer body, or to a grounding post secured thereto. Since the DC blocking capacitor is surge-protected, it may be of a compact and inexpensive design, such as a chip capacitor.
Another aspect of the present invention relates to such a device that is used to couple together two coaxial transmission devices, rather than to terminate a transmission path, while retaining the advantages of providing surge protection. This coupling device uses a similar outer body, center conductor, and spark gap as the aforementioned termination device; in the preferred form of the surge-protected coupler, relatively high characteristic impedance inductive zones are formed adjacent the capacitive spark gap on opposing sides thereof.
A surge-protected coaxial termination constructed in accordance with a preferred embodiment of the present invention is shown in
As shown in
A center conductor 36 extends through central bore 24 of outer body 22, including the reduced-diameter step portion 34 of inner wall 32. Center conductor 36 is supported at one end thereof within central bore 24 by a first supporting insulator 37 of dielectric material; supporting insulator 37 is, in turn, supported by an enlarged annular bore 39 formed in first end 28 of outer body 22. The portion of center conductor 36 that protrudes outwardly from first end 28 of outer body 22 can be cut to any desired length by a user. A typical coaxial port of an equipment box includes a clamping mechanism (not shown) for clamping center conductor 36 and establishing an electrical connection therewith.
Center conductor 36 is also supported at its opposite end by a second supporting insulator 41 of dielectric material, which fits into central bore 24 from second end 30 thereof. The outer diameter of center conductor 36 is preferably selected so that, at any point along its length, given the surrounding dielectric characteristics, and given the diameter of the surrounding inner wall, the characteristic impedance of center conductor 36 will be matched with the desired characteristic impedance of the coaxial cable system (e.g., 75 ohms in a 75-ohm characteristic impedance system). The major exception to the foregoing statement is at the location where center conductor 36 passes through the radial step portion 34 of inner wall 32. Within radial step portion 34 of inner wall 32, the outer diameter of center conductor 36 is preferably equal to, or slightly greater than, the outer diameter of center conductor 36 on either side axially of radial step portion 34. In any event, the outer diameter of center conductor 36 within radial step portion 34 of inner wall 32 is slightly less than the inner diameter of radial step portion 34 for positioning radial step portion 34 of inner wall 32 in close proximity to center conductor 36 to form a narrow spark gap 38 therebetween.
Spark gap 38 is shown in greater detail in the enlarged drawing of FIG. 4. As indicated in
Radial step 34, and spark gap 38, being in close proximity to center conductor 36, represent a highly-capacitive discontinuity in the characteristic impedance of the transmission line relative to RF fields traveling therealong, and would normally cause the RF energy to be reflected, contrary to the purpose of the coaxial termination device. Accordingly, high characteristic impedance inductive zones 40 and 42 are preferably formed on both sides of reduced-diameter radial step 34 to create the equivalent of an electrical T-network low pass filter. High impedance zones 40 and 42 lie on opposite sides of radial step portion 34. The amount of additional inductance introduced by high impedance inductive zones 40 and 42 is designed to precisely offset the additional capacitance caused by reduced-diameter step portion 34. The combined effect of such high impedance inductive zones 40 and 42, together with the highly-capacitive radial step portion 34, effectively nullifies the RF signal reflection that would otherwise occur due to radial step 34 alone. The low pass filter formed by radial step 34 and inductive zones 40 and 42 allows termination device 20 to offer state of the art Return Loss performance over the bandwidth of interest (e.g., 5-1000 MHZ).
As mentioned above, a coaxial termination device typically includes a resistive terminating element coupled between center conductor 36 and grounded outer body 22. Referring to
A blocking capacitor 50, in the form of a so-called “chip capacitor”, extends radially between solder electrode 48 and a second solder electrode 52, or grounding post, that extends from a recess formed in outer body 22. The opposing ends (electrodes) of blocking capacitor 50 are soldered to electrodes 48 and 52 in order to electrically couple center conductor 36 in series with resistor 44 and capacitor 50 to ground (outer body 22), in parallel with spark gap 38. Capacitor 50 is provided to block DC or AC power from flowing through resistor 44 and is not required if AC or DC power is not present on the line; in that case, resistor 44 is connected directly to ground. Chip capacitor 50 is strategically placed to terminate resistor 44 with the shortest possible path to ground, thereby minimizing any parasitic inductance in the connection between resistor 44 and ground.
Since the spark gap 38 is effectively in parallel with resistor 44 and capacitor 50, any power surges are coupled to ground across spark gap 38 to avoid damage to resistor 44 or capacitor 50. In addition, as described above, spark gap 38 and high impedance inductive zones 40 and 42 form a low pass filter that has the additional benefit of reflecting any high-frequency surge energy occurring above, for example, 1000 MHZ, thereby offering additional protection to resistor 44 and capacitor 50.
As shown in
Those skilled in the art will now appreciate that an improved surge-protected coaxial termination device has been described which offers many advantages over known coaxial terminators. As mentioned above, tests conducted by applicant indicate a demonstrated performance of 45 dB Return Loss to 1 GHz, which is about 15 dB better than the current industry state of the art. The disclosed surge protection spark gap allows the present termination device to withstand the 6000 Volt open circuit/3000 Amp short circuit surge test of ANSI C62.41 Category B3, without damage, while still maintaining high levels of RF Return Loss performance. The present invention allows the use of smaller, less expensive, lower voltage, and lower power components that result in a smaller and more economical design. By axially disposing the termination resistor inside the outer metallic body of the termination device, and within the solid dielectric material, the termination device is more compact, and the resistor is maintained within a controlled 75 ohm characteristic impedance environment, for improved return loss performance. There is also a shorter path between the blocking capacitor and ground, thereby resulting in less inductance. Since the spark gap protects the resistor and blocking capacitor from damage due to surges, the resistor and capacitor can be smaller and less expensive.
In addition, while the foregoing description refers to the disclosed device as a coaxial termination, the benefits of the present invention can also be applied to a coupling device used to couple together two coaxial transmission devices. For example, such a coupling device could be used to couple the end of a coaxial cable to a coaxial port of an equipment box; alternatively, such a coupling device could be used to couple together the ends of two coaxial cables. Such a coupling device omits the above-described resistor and blocking capacitor, but retains the spark gap between the center conductor and the outer metallic body. An example of such a coupling device is shown (conceptually) in FIG. 6.
Within
First end 128 of coupler 120 in
The disclosed termination device described in conjunction with
While the present invention has been described with respect to preferred embodiments thereof, such description is for illustrative purposes only, and is not to be construed as limiting the scope of the invention. Various modifications and changes may be made to the described embodiments by those skilled in the art without departing from the true spirit and scope of the invention as defined by the appended claims.
This application is a continuation application of U.S. patent application Ser. No. 09/712,433, filed Nov. 14, 2000 now U.S. Pat. No. 6,751,081, and the benefit of such earlier filing date is hereby claimed pursuant to 35 U.S.C. §120.
| Number | Name | Date | Kind |
|---|---|---|---|
| 5508873 | Knapp et al. | Apr 1996 | A |
| 5724220 | Chaudhry | Mar 1998 | A |
| 5768084 | Chaudhry et al. | Jun 1998 | A |
| 5790361 | Minich | Aug 1998 | A |
| 6751081 | Kooiman | Jun 2004 | B1 |
| Number | Date | Country | |
|---|---|---|---|
| 20040219838 A1 | Nov 2004 | US |
| Number | Date | Country | |
|---|---|---|---|
| Parent | 09712433 | Nov 2000 | US |
| Child | 10856617 | US |
