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1. WO1996034428 - COAXIAL CABLE TAP

Note: Text based on automatic Optical Character Recognition processes. Please use the PDF version for legal matters

[ EN ]


This invention relates to coaxial cable taps.

Coaxial cable is widely used in the communication industry to distribute television (TV) and other signals. This cable typically comprises: a center conductor along which the signals are transmitted; a dielectric surrounding the center conductor; a rigid outer conductor cylinder which shields the signals from leakage and interference; and, optionally, a protective insulating outer jacket.

As used herein, a "tap" is a means by which a signal can be extracted from and/or inserted into a coaxial distribution cable so that signals can be passed between the distribution cable and a subscriber's premises. As used herein, a "tap assembly" is a device which is connected to a coaxial cable to form a tap in the cable.

The known procedure for inserting a tap into a coaxial cable is time consuming and labor intensive, and, when a tap is being added to an operating cable TV system, interrupts service. Thus, the coaxial cable is severed, both free ends of the severed coaxial cable are prepared with a coring tool, a connector is installed on each of the two prepared ends, and a tap assembly is inserted between the two connectors. In order to accommodate environmental stresses, a length of the coaxial cable surrounding the tap must be formed into an expansion loop. The tap assembly typically provides connections for subscriber drops, such as RG59 and/or RG6 drop cables.

It is common practice to interrupt cable television service downstream of the point where a tap is being added to an operating distribution cable. However, so that cable companies may expand to provide, for example, telephone or other services requiring uninterrupted operation, it is necessary that taps be capable of being made without interrupting the signal passing on the cable.

We have discovered a method of installing a tap into a coaxial cable without having to sever the coaxial cable, and a coaxial cable tap assembly suitable for use in the installation method. The invention permits the installation of a tap into an operating coaxial distribution cable without interrupting the signal carried on the cable. The method and tap assembly of the invention may be used for installation of taps in new cable installations as well as for adding taps to operating systems.

In a first aspect, the invention provides a method of forming a tap into an intermediate point on a continuous coaxial cable comprising an inner conductor, a dielectric surrounding the inner conductor, and an outer conductor surrounding the dielectric, the method comprising the steps of:
a. forming an opening in the outer conductor of the coaxial cable;
b. forming a cavity in the dielectric under the opening; and
c. mounting a tap insert to the coaxial cable, the tap insert comprising a signal chip, so that the signal chip is located in the cavity adjacent to the inner conductor of the coaxial cable, and can (i) extract signals from the inner conductor, or (ii) insert signals into the inner conductor, or (iii) do both (i) and (ii).

In a second aspect, the invention provides a coaxial cable tap housing, suitable for use in the method of the first aspect of the invention, the tap housing comprising:
a. a first end portion,
b. a second end portion;
c. an intermediate portion having a recess;
d. means for fixing the housing around an intermediate point on a continuous coaxial cable;
e. means for electromagnetically and environmentally sealing the tap housing to the coaxial cable;
f. an alignment element having a known spatial relationship with the recess;
g. means for mounting a tap insert to the recess of the tap housing.

In a third aspect, the invention provides a tap insert, suitable for use in the method of the first aspect of the invention, the tap insert comprising:
a. a tap faceplate comprising
i. an inner face, ii an outer face,
iii. means for mounting the tap faceplate to a cable tap housing, and
iv. means for environmentally and electromagnetically sealing the tap
faceplate to the cable tap housing;
b. a signal chip mounted on the inner face of the tap faceplate; and
c. means for providing RF input and/or output, said means mounted on the outer face of the tap faceplate.

In a fourth aspect, the invention provides a kit of parts suitable for forming a tap into an intermediate point on a continuous coaxial cable, the kit comprising:
a. a coaxial cable tap housing capable of attaching around an intermediate point in a continuous coaxial cable; and
b. a tap insert comprising a signal chip, the tap insert capable of attaching to the tap housing after an opening has been made in the outer conductor, and a cavity has been made in the dielectric, of the coaxial cable.

In the method of installation, an opening is cut in the outer conductor of the coaxial cable and a cavity is cut in the dielectric beneath the opening in the outer conductor. In preferred embodiments, the opening and the cavity are elongate and longitudinal; however, other shapes may be used. Typically the opening and the cavity in the dielectric beneath it are cut at the same time using a cutting tool, e.g. a rotary cutting tool, preferable a rotary saw blade sized to correspond to the desired dimensions. Preferably a guide is used to position the tool and to help to control the dimensions of the opening and the cavity. Preferably the tap housing is first attached to the cable and is then used as a jig to guide the cutting tool. The tap housing may have a recessed area through which the blade of the cutting tool is inserted, and an alignment element having a known spatial relationship with the recess. The alignment element serves to position the cutting tool with respect to the recess, and to define the dimensions of the cut by limiting the range of motion of the cutting tool. If desired, the cavity in the dielectric may be enlarged after the cutting tool has been used, preferably using a non-conductive tool so that signals carried by the inner conductor are not disturbed. After the opening and the cavity have been cut, a tap insert may be attached to the tap housing, the tap insert comprising a signal chip which enters the cavity in the dielectric of the coaxial cable through the recess in the tap housing and the opening in the outer conductor of the cable.

The signal chip, mounted to an inner face of the baseplate of the tap insert, is electrically connected to means for outputting and inputting RF signals. Typically connectors such as F-type connectors are mounted on an outer face of the baseplate of the tap insert, and would connect to drop cables to subscriber premises.

In preferred embodiments, the tap housing is comprised of a metal such as aluminum. However, other suitable materials such as a plastic with a conductive coating on its inner surface could be used.

In preferred embodiments, the tap housing is used as a jig for the cutting tool as described above. However, it is also possible to cut the opening and/or the cavity, in the same or separate operations, before the tap housing is applied to the cable, or to do so after the tap housing has been applied but without using the tap housing to guide the cutting tool. Any jig suitable for positioning the cutting tool and controlling the dimensions of the opening and/or the cavity may be used. For example, a separate jig can be used to cut the opening and cavity in the cable, and then the tap housing installed on the cable, with the recess in the tap housing aligned over the opening and cavity previously cut in the cable.

It is desirable to use a tap housing with a recess and a means to attach a tap insert to the tap housing. This facilitates removal of the tap insert for adjustment, maintenance and the like. However, in some embodiments it is also possible to use a tap housing to which the signal chip is attached, and which does not, therefore, need a recess through which the signal chip is inserted. Such a tap housing is applied to the cable after the opening and cavity have been cut in the cable.

It is preferred that the tap assembly provide an electromagnetic seal around the opening cut in the outer conductor in order to shield against signal leakage out of, and interference into, the opening. An inner surface of the tap housing makes electrical contact with the outer surface of the conductor of the cable in an area surrounding the opening in the outer conductor. The two halves of the clamshell-style tap housing are sealed with an EMI gasket (not illustrated in any of the figures), and the interface between the tap insert and the tap housing is also sealed with an EMI gasket (not illustrated in any of the figures).

The preferred cutting tool is a rotary saw. However, other suitable tools, e.g. a router, may be used in conjunction with a suitable corresponding jig and/or alignment element on the tap housing. The edge of the recess may provide the alignment means, e.g. for a router fitted with a bit having a top bearing.

It is desirable to provide a flat signal strength over the transmitted frequency spectrum along the length of the coaxial cable. A traditional tap assembly has a flat coupling efficiency over the frequency range. The slope of the signal strength of a tapped off signal corresponds to the slope of the signal strength in the coaxial cable. In a traditional cable TV transmission, the signal slope is positive when the signal leaves the amplifier. At the end of the cable, the signal slope is negative. In addition, signal transmission in the cable generally has a higher attenuation at the high frequency end.

In the tap assembly of the invention, the signal strength of the tapped-off signal is preferably field adjustable without signal interruption. The invention permits the tap assembly to be designed to compensate for non-uniform cable attenuation, to provide a flat signal response over the length of the cable, and flat signal extraction over the transmitted high frequency and low frequency spectrum over the length of cable. The extracted signal strength is tunable by the signal chip design and/or by adjusting the gap between the signal chip and the center conductor. The signal chip functions at least in part as a receiving antenna. The signal chip is designed with multiple coupling paths to ensure the required signal strength. Inductive and capacitive coupling are more efficient at higher frequency. The multiple coupling paths can be optimized to take into account the coupling efficiency differences at the high and low frequencies. The tapped-off signal from a traditional tap assembly is not balanced in the signal strength of the frequency spectrum. The preferred tap assembly provides the advantages due to the capability to balance the signal strength.

The tap of the invention is preferably designed to have a high selectivity for coupling the desired signal traveling in a first direction on the distribution cable, and to reject reflected signals which travel in an opposite direction on the distribution cable. In preferred tap embodiments, the inductive coupling cancels one direction of the capacitive coupling, thereby enabling extraction of the desired signal without reflected ghost signals.

A preferred embodiment of the invention will be described with reference to Figures 1-8. Figure 1 illustrates a hinged clamshell-style tap housing 100 which fits over a cable 1000 and receives a tap insert 20 (Figure 5). Although a hinge 11 is illustrated, the tap housing 100 can have two separate parts which are bolted together. The housing is preferably fabricated from a conductive and electromagnetically shielding material. The housing 100 includes body halves 10a and 10b. The internal configuration of the body halves 10a 10b preferably has a diameter substantially equal to the outer diameter of the cable 1000 for an environmentally and electromagnetically tight fit. A body half includes a recess 14 through which an inserted signal chip 22 (Figure 5) passes. In the preferred embodiment, the recess 14 is shown as an elongate longitudinal recess. However, other suitable shapes may be used. The housing 100 is robust enough to round an out of round cable upon tightening of the housing 100 to the cable 1000. A way to achieve this effect is through the use of a harder metal alloy than the shielding layer on the coaxial cable 1000. The housing 100 additionally includes an aligning hole 12 into which an installation tool 500 (Figure 2) is inserted. The aligning hole 12 helps align and control the depth of a cut made into the cable 1000 through the recess 14 by the cutting tool 500. However, any suitable structure to permit forming an opening in the outer conductor may be used.

The outside diameter of a hard line distribution cable ranges from 1.016 cm (.400 in) to 3.810 cm (1.5 in). Standard cable sizes are, for example, 1.046 cm (.412 in), 1.372 cm (.540 in), 1.588 cm (.625 in) and 1.905 cm (.750 in). A typical cable used for illustration of this invention is 1.372 cm (.540 in) (e.g., Comscope 540 cable). The body halves include passageways 16 to permit the tap housing 100 to be securely bolted around the cable 1000. The bolting operation forces the inner face 102 of the tap housing 100 against the outer conductor of the cable 1000 to make electrical contact between the two surfaces. The inner face 102 of the housing 100 may include contact elements such as piercing spikes 18a 18b 18c 18d which are driven into the outer conductor of the cable 1000. Although four spikes are illustrated, one spike is suitable if a good contact is formed between the tap housing 100 and the cable 1000.

Distribution cables come in many different configurations. Generally the cables contain a center conductor surrounded by a dielectric region and a rigid outer conductor and, optionally, an outer electrically insulating cable protective jacket. In the event the distribution cable contains this outer insulating protective jacket, a portion of the protective jacket is preferably removed prior to attaching the housing 100. In preferred embodiments, the insulating jacket is removed from an area about the cable 1000 slightly longer than the length of the opening to be cut in the cable 1000. In preferred embodiments the insulating jacket is left intact on the cable 1000 in the areas corresponding to the two end portions of the housing 100 to help form an environmental seal with the inner face 102 of the housing 100. The housing 100 is sealed to the outer surface of the cable 1000 by any suitable means such as a mastic, adhesive or sealant, e.g. an epoxy resin, or (to provide re-enterability) suitable gel or rubber materials. Examples of suitable gels include those described in U.S. Patent No. 4,600,261 and US Patent No. 5,350,057, the disclosures of which are incorporated herein by reference for all purposes, and the materials available from Dow Corning under the trade name Sylgard®, in particular Sylgard® 527. Optionally, thermoplastic elastomer sealing gels such as Septon® based styrene-ethylene-butyelene-styrene and/or styrene-ethylene-propylene-styrene oil extended gel materials can be used. These thermoplastic gels are generally 1% to 20% by weight polymer and the rest to 100% an extruder fluid. Optionally, the crosslinked or thermoplastic gels can include up to 2.5% additives such as antioxidants, corrosion inhibitors, fungicides, and the like.

Figure 2 illustrates the tap housing 100 installed around the cable 1000 exposing the recess 14. The tap housing 100 firmly grasps and seals to the cable 1000 to provide a fixture for the installation tool 500 to make the cable cut. Any suitable installation tool can be used. A preferred installation tool 500 is illustrated. For example a standard tool 510 is available from the Makita Corporation as Model 9500D with the modification of the alignment fixture 512. The fixture 512 is configured to cut an opening about .445 cm (.175 in) wide by about 3.810 cm (1.5 in) long. Larger diameter cables provide the option to have a longer and/or wider opening. The longitudinal recess 14 is preferred to semicircular cuts because it is easier to seal and limits stray signal loss. This type of cut also helps locate and align the signal chip 22 (Figure 5) upon insertion. The cutting blade 514 is adapted for precise depth of cut into the outer conductor of the cable 1000. The depth will be about .508 cm (.20 in) for a 540 cable. Larger cables require a deeper cut because the distance from the center conductor is important to achieve proper signal strength reception and/or injection.

Figure 3 illustrates an alternative perspective of a tap housing 100 attached to the distribution cable 1000. The installation tool 500 is illustrated with the alignment pin 512 aligned for insertion into the alignment hole 12 in the tap housing 100. The depth of the cable cut is controlled by the depth control face 516 on the installation tool 500 such that the cutting blade 514 cuts to a predetermined depth of about .508 cm (.200 in) for 1.372 cm (.540 in) hard line coaxial cable. Figure 4 illustrates the tool 500 inserted in the recess 14 cutting into the distribution cable 1000 and through the dielectric material 1020 toward the center conductor 1010. The depth control face 516 bottoms on the tap housing 100 to ensure that the cutting blade 514 does not contact the center conductor 1010. With the installation tool 500 attached, the installation tool 500 is rotated toward the cable 1000 as illustrated by the arrow at the base of the tool 510 until the depth control face 516 bottoms onto the tap housing 100.

After an opening about .445 cm (.175 in) wide by about 3.810 cm (1.5 in) long by about .508 cm (.200 in) deep is made in the cable, a tap insert 20, illustrated in Figure 5, is attached to the tap housing 100. The tap insert 20 includes a tap face plate 21, a signal chip 22 and RF ports 24a 24b 24c 24d. The tap face plate 21 attaches into the housing 100 at fixture points 17a and 17b on the housing 100. The tap face plate 21 is sealed to the housing by any suitable means 28 such as mastics, epoxies, or for re-enterability, gel/rubber sealing materials as previously described. Preferably the tap face plate 21 is bolted or screwed through holes 26a 26b to tapped holes 17a 17b in the tap housing 100 to compress and shielding seal thereto as illustrated in Figure 6. Figure 6 illustrates the tap housing 100 with the tap insert 20 installed on the cable 1000.

Figures 7 and 8 illustrates the signal chip 22 inserted into the cable 1000 and represents an inductive coupling as well as a capacitive coupling through which a resistor R balances the capacitively coupled signals and inductively coupled signals, thereby coupling the direct signal and rejecting reflected signals. The main components for the signal chip are a conductive path and a balancing resistor. The conductive path functions as an antenna to extract a portion of the signal from the center conductor 1010. The proximity of the antenna to the center conductor 1010 means that it has both an inductive and a capacitive function. The presence of a resistor 32 and ground 30 on the signal chip 22 allows the antenna to have both inductive and capacitive functions operating at the same time. By selecting the proper value of the resistor 32, a balanced signal (inductive and capacitive) will be extracted and/or inserted. Thus, the antenna becomes selective by accepting the source signal and rejecting reflected signals.

The signal chip 22 in the tap insert 20 includes an end attached to the RF ports 24a 24b 24c 24d shown in Figure 5 appropriately sized. The antenna can have a size from about .635 cm (0.25 in) to tens of centimeters (inches) long. For convenience, a 1.524 cm (0.6 in) long antenna was used. The antenna is illustrated in Figures 7 and 8. The resistor R is adjusted to ensure rejection of reflected signals. In a preferred embodiment illustrated, the antenna has about .762 cm (.300 in) straight region 22a adjacent to the center conductor 1010 with adjacent .762 cm (.300 in) radius 1/4 section curves 22b on either side of the straight region 22a. These curves 22b are connected to .381 cm (.150 in) straight regions 22c and a final 1.524 cm (.600 in) semicircular connection region 22d for the preferred shape of the multi-turn antenna.

The .762 cm (0.3 in) flat section 22a adjacent the center conductor 1010 is chosen to have a long section of the coil that could have a close proximity to the center conductor 1010 of the coaxial cable 1000. The longer the interaction section ensures the stronger the signal pick-up. The two 1/4 circles 22b at both ends of the .762 cm (0.3 in) straight line 22a are for smooth transitions of the straight section 22a, so that a compact multiple turn of coil is formed. The curvatures of the transition sectors control the reactance of the coil. Smooth transitions allow more turns on the coil to allow more signal pick-up. In this preferred example, a six-turn-coil with the shape as illustrated creates a flat frequency response for the pick-up signal. The coil designed may have alternative shapes, as long as proper reactance values are built in to balance the signal response.

The antenna is screen printed on, for example, a printed circuit board to include the resistor for reflection cancellation. The resistor R will have a value generally between 10 and 500 ohms but any suitable resistance or variable capacitance is to be used to couple the signal out. In the described embodiment, a 75 ohm resistor was found to have best directivity for signal strength withdrawal. The signal chip 22 can be adjustable such that the distance between the center conductor 1010 and the antenna can be adjusted to obtain a signal strength of about lOdBmV over the desired frequency range.

To improve the signal strength of the extracted signal and to balance the signal strength at both high frequency (UHF) and low frequency (VHF), the antenna can be made in the form of a coil described above. The coil is designed to extract the signals and to act as a reactive load to balance the signal pull off at high frequency and low frequency. The coil could be a .043 cm (.017 in) thick (26 gauge) magnetic wire wrapped around a suitable mandrel with 1.52 cm (.6 in) diameter and flat region as shown in Figure 7 which faces the center conductor. The coil diameter can be .254 cm (.1 in) to several centimeters (inches). The choice of about 1.52 cm (.6 in) is preferably chosen for compactness and signal reception. In order to increase the coupling strength, multiple turns of the 0.6 inch pick-up coil have been chosen. In order to balance the pull off signal frequency response and the coupling efficiency, a 6 turn pick-up coil was constructed. For a 6 turn pick-up coil the extracted signal strength at high frequency is the same as the signal strength at low frequency, i.e. with 6 turns there is the preferred flat response over the desired frequency range. More turns would provide a downward slope of extracted signal while fewer turns would create an upward slope. To obtain the preferred flat response, the number of turns of the pick-up coil is adjusted to create a mirror image of the existing signal strength along the cable.

Figure 8 is a schematic of the inductive coupling 35 of the antenna with the resistor adjustment 32 and ground 30. The RF signal out is illustrated by 34 and capacitive drain portion is illustrated as 37.

The invention was used to insert a new tap into a TV distribution cable between two operating taps, each connected to a television set, without interrupting any of the signals to the television sets. The new tap was connected to a television set and also received the TV signal.

The invention is useful in the system described in copending, commonly assigned US Serial No. 08/353,541 filed December 9, 1994 and entitled Distributed Digital Loop Carrier System Using Coaxial Cable, the entire disclosure of which is incorporated herein by reference.