Note: Descriptions are shown in the official language in which they were submitted.
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METHOD OF AND ELECTRICAL TERMINATION FOR COAXIAL CABLE
This invention relates to an electrical termination for a
coaxial cable having a square-cut end portion and more
particularly to a terminating clamp member which
interconnects with the cable to improve axial securement
and reduce undesirable impedance mismatch.
BA~KGROUND OF THE INVENTION
Many approaches have been suggested for electrically
terminating and or securing an electrical connector to a
coaxial-type electrical cable.
Some approaches have required that the cable be
prepared by removing forward portions of the outer
jacket, the braid, the dielectric core insulator layer,
and the central conductor in progressively longer lengths
(i.e., describing a "stepped" arrangement) prior to
insertion thereof into an electrical connector. Such a
"stepped'l arrangement of the conductor is undesirable in
that it requires additional time and effort by the user
and provides a chance that the preparation would not be
properly done. Such an arrangement is shown in U.S.
Patents 3,054,981; 3,107,135; 3,209,287; 3,264,602 and
3,634,815.
Another approach provides a clamp member with a
frusto-conical shape that is pushed axially inward over
the central conductor and within the jacket and braid.
When the cable jacket is thick or made of a heavy, non-
yielding material, inward movement of the clamp member
can be resisted and the clamp pushed axially outward.
Such an approach suggested in U.S. Patent No. 3,373,243.
~hen the clamp member is not fully enqaged with the
braid and/or the central conductor, a poor electrical
connection is made. Undesirable resistance can develop
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between the braid and the clamp. High frequency impedance
mismatch can develop if an air ~ap is formed between the
clamp member and the central-conductor. Provision of ribbed
grooves have not been entirely satisfactory in that a crimping
process usually is required to form a tight fitment between
the clamp and the cable.
Other solutions to poor electrical termination
problems have been suggested respectively in Canadian Patent
Application serial number 343,890, filed on January 17, 1980
and in U.S. Patent 3,110,756. While variously providing
mechanical securement and electrical terminations, each
separate connection approach did not coact to provide both a
reliable electrical termination having a secure positional
fitment between a connector member and the cable, as well as
a termination having high frequency impedance matching.
The present invention is a connector assembly having
a clamp member for terminating a coaxial cable which is easy
and quick to assemble to the cable, which overcomes the
limitations and undesirable features of the prior art, which
resists rearward axial movement of the clamp member from the ~;
cable prior to complete assembly and which provides improved
high frequency impedance match between the cable and the
connector.
According to the present invention there is provided
an electrical termination between an electrical connector
and an end of a coaxial cable characterized by a dielectric
core, a braided outer conductor and an outer external jacket
square-cut, and by a central conductor extending forward of
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the square-cut. The electrical connector includes a
rotatable clamp member for engaging the cable end, the clamp
member having a forward face and a rear face with a generally
frusto-conical body portion having tapered sides and a central
passage extending between the faces for receiving the dielectric
core and central conductor. The central passage includes a
first helical thread having raised edge portions for
progressively engaging the dielectric core and the tapering
portion including a second helical thread having a raised edge
portion for progressively engaging the braid. The first and
second threads are of like sense such that after the dielectric
core is received in the central passage and the clamp member
is rotated about the cable, the raised edge portions coact
to progressively engage the dielectric core and the braid to
secure and to electrically connect the cable to the clamp
member. The first threads substantially eliminate any separation
between dielectric core and clamp passage to improve high
frequency impedance mismatch.
In a specific embodiment of the invention the passage
wall includes an axial groove portion that is disposed within
the helical thread to receive insulative material scraps
as a result of biting engagement by the first threads.
According to another aspect of the present invention
there is provided a method of terminating a coaxial cable of
the type having a square-cut end and including a conductive
braid circumposing a dielectric core surrounding a central
conductor having an exposed end portion extending from the
square-cut. The method includes the step of assembling
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a frusto-conical member having a central passage over the
dielectric core such that the frusto-conical portion faces
the conductive braid and the central passage faces the
dielectric core, the central passa~e conical portion being
provided with progressively spiralling thread of the same
sense and having raised edge portions. The frusto-c~nical
member is rotated to progressively engage the raised edge
portions of a helical threads with the dielectric core and
the braid to prevent removal of the member from the cable
and to reduce high frequency impedance mismatch caused by
separation between the dielectric core and the clamp member.
A rear nut and a forward contact element are assembled over
the frusto-conical member to complete the contact for the
coaxial termination and captivate the member therebetween.
The assembly of the present invention is advantageous
in that it uses a "square cut" cable termination in which the
jacket, braid conductor and insulation layer are cut to a
uniform length (i.e., in a single cut for ca~le preparation).
The central conductor has an exposed portion that extends
forwardly of the single cut. Such a square cut termination is
an easy and quick one to accomplish in preparation of the cable
and economizes on labor costs.
The connector and clamp of the present invention is
compatible with any type of jacket and does not require the
slitting of the jacket maintaining the cable's integrity is
advantageous in that it increases the life of the cable and the
termination.
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The connector of the present invention has an
advantage in that the el~ectrical termination between the
insulation and the connector is improved in its reliability
and uniformity.
Yet another advantage is the improved high frequency
impedance matching achieved by the electrical termination
provided by a clamp member of the present invention.
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As a further advantage of the present invention, the
connector of the present invention involves the use of
only three separate assemblies of relatively large size
to be handled conveniently. The use of a minimum number
of pieces reduces the chance of inadvertent loss of a
part durinq packaging or assembly. With only three dis-
similar parts, assembly instructions can be quite simple.
The connector of the present invention has an advan-
tage in that the clamp member is merely rotated to draw
it into progressive engagement with the cable. Such an
engagement minimizes possible damage to the cable.
Other ob;ects and advantages of the present inven-
tion will be apparent to one skilled in the art in view
of the following detailed description and claims in the
accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIGURE 1 is a cross sectional view of an electrical
connector assembly terminated to a square-cut coaxial
cable.
FIGURE 2 is an enlarged side view of a clamp member
used in the connector assembly of FIGURB l showing
external helical thread portion.
FIGURE 3 is a cross sectional view of the clamp
member looking along the lines III-III of FIGURE 2 and
showing a central passage having an internal wall
provided with helical~thread and an axial slot.
FIGURE 4 is an end view of the clamp member looking
along lines IV-IV of FIGURE 3.
FIGURE 5 shows the clamp member of the present
invention partially inserted over the coaxial cable with
and both helical threads coacting and respectively in
engagement with the cable braid and the dielectric core
insulation.
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FIGURE 6 is a cross sectional view of the coaxial
cable and clamp member of FIGURE 4 when the clamp member
is in complete engagement within the coaxial cable and
held in place therewith by the coacting helical threads.
DETAILED DESCRIPTION OF THE INVENTION
FIGURE 1 shows a cross sectional view of an
electrical termination between a coaxial cable 10 and an
electrical connector assembly 100 according to the
present invention.
The coaxial cable 10 includes a core of dielectric
material 14 disposed between a central conductor 12 and
an outer braid conductor 16. A protective outer jacket
18 surrounds the conductors and the dielectric core. The
outer jacket 18 and the dielectric material 14 are both
made of electrically insulating materials to electrically
isolate the conductors 12 and 16 from each other and from
the environment.
The electrical connector assembly 100, in the
~ embodiment shown, includes three components which are
supplied and handled as separate pieces: a forward body
200, an internal clamp member 300 and a rear nut 400.
The forward body 200 includes electrically insulating
material 220 and an internal thread at the rear end
thereof for coupling to the nut 400. An outer electrical
contact 230 is disposed outside the insulating material
220 and is electrically isolated from the central contact
210 upon assembly. When the connector 100 is attached to
the cable 10, the central contact 210 is electrically
coupled to the central conductor 12 of the cable 10 and
the outer contact 230 is electrically coupled to the
braid conductor 16 o the cable 10.
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The nut 400 may be of the type described in U.S.
Patent 3,373,243 and include a rear portion 410 adapted
to be grasped during installation and an external screw
thread portion located forward of the rear portion 410
for engaging the internal thread of the body to secure
the connector assembly 100 together. The nut 400 further
includes a central aperture 430 through which the cable
10 extends and steps 440 whic:h provide the central
aperture 430 in the forward region with increasingly
larger diameters. When used in conjunction with the
frusto-conical shaped clamp of the present design and as
described later the steps 440 enable the assembled
connector 100, to captivate the jacket 18 and the braid
16 and thereby to retain the cable 10 therein to provide
increased resistance to axial decoupling forces applied
on the cable. Captivation of the cable jacket 18 and the
braid 18 occurs between the steps 440 and a frusto-
conical surface of the clamp member 300.
The clamp member 300 includes an electrically con-
ductive body 310 and an insulator body 320 and central
mating contact 210. Preferably and in accord with the
present invention the insulator body 320 includes a
central bore 321 which is large enough to receive only
the central conductor 12 therein. The insulator body 320
fits within an undercut portion 312 extending rearwardly
from the front end of the conductive body 310. The
insulator body 320 electrically isolates the exposed
portion of the central conductor 12 of the cable and the
contact 210 from the body 310. The insulator body 320 is -
retained within the undercut by one of many known
expedients (e.g., adhesively bonded or interference fit).
- ~ portion of solder 700 would be provided between
the central conductor 12 and the rearward end of the
conductive body 210 to complete an electrical intercon-
nection between cable and connector.
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FIGURE 2 is an enlarged view of the clamp member 300
of the present invention showing the external surface of
the clamp body 310. The clamp body 310 has a constant
diameter forward portion 313 and a tapered frusto-conical
portion rear portion 314, rear portion 314 extending from
the forward portion 313 rearwardly and having forward end
314b of greater diameter than a rear end 314a.
A helical groove 500 or threaded portion having
raised edge portions and undercuts spirals progressively
rearward around the frusto-conical portion 314 making
several revolutions between the ends 314a and 314b.
FIGURE 3 is a cross sectional view of the clamp body
310 without the insulator body 320 and shows the forward
undercut portion 312 for receiving the insulator body.
Preferably and in accord with the present invention,
the bod~ 310 of the clamp includes a central passage 311
sized large enough to receive the layer of insulation 16
of the cable 10. The internal wall 315 of the central
passage 311 is provided with a helical groove 600 or
threaded portion having raised edge portions and adjacent
undercuts that spiral progressively rearward around the
passage, making several revolutions. The sense of each
respective thread or helical groove 500 and 600 would be
identical. That is, both grooves would be left-handed or
both would be right-handed. It has been found that for
the external thread 50û a continuous spir al of
right-handed buttre~es-type thread of 40 pitch having a
depth of 0.005 inches works to advantage. Although
left-handed threads could be substituted and used to
advantage in the present design, these threads would be
unconventional and more difficult to install manually
inasmuch as an operator is accustomed to install a screw
member with right handed threads. The threads 600 on the
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passage wall 311 could be of the same type (i.e.,
buttress) as on the clamp or, more advantageously,
V-shaped. Each V-shaped groove would act to bite sharply
into the core of dielectric material.
Further, a pair of axially extended grooves 318
disposed at substantially diametrically opposite portions
of the central passage wall could be provided if desired
to receive any scraps of insulative material as a result
of the sharp teeth 600 biting into the cable insulation.
OPERATION
In FIGURE 5 The cable 10 has been prepared in a
manner which is referred to in the industry as a "square-
cut" end wherein the central conductor 12 is exposed to
- extend forwardly of the rest of the cable (outer layer
18, braid conductor 16 and dielectric core 14), these
other portions being cut-off square and at a position
rearwardly of the exposed end of conductor 12.
The tapered outer frusto-conical surface of the
clamp 300 is pressed between the dielectric core 14 and
the conductive braid layer 16, urging the braid 16 and
the outer jacket 18 slightly radially outward.
At the stage shown in FIGURE 5, the helical grooves
500 and 600 begin to coact to threadably engage with the
braid and the dielectric core. Because the braid 16 is
also being urged radially inward by the surrounding
jacket 18, the braid 16 is partially urged into the
groove 50n.
As the clamp 300 is rotated in the direction of
helical grooves 500, 600, the raised edge portions of
grooves 500 coact with the braid 16 to draw the body 310
inwardly in t:he nature of a screw being drawn into wood
as it rotates. Continued rotation of the body 310 causes
additional grooves to engage the braid 16 and secures the
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clamp and cable 10 together by the grooves against
unwanted axial movement out of the cable 10 because the
braid 16 is engaged by the grooves 500.
Further, and in coaction with the external grooves
500, rotation of the clamp member 300 causes the raised
edge portions of the interior V-shaped teeth 600 to bite
into the dielectric core 14, thereby eliminating the
slight air gap circumposing the dielectric core 14 that
otherwise would exist if the dielectric core 14 were to
be slidably slipped within passage 311 in a clearance
fit. At extremely high frequencies (e.g. 18 gigahertz)
an air gap defined by the passage 315 can develop an
impedance mismatch from the typically desired 50 ohms.
Since 10 gigahertz corresponds to approximately 1 wave
length with respect to passage 315 and inasmuch as any
impedance mismatch of length 1/20 wavelength (i.e. at 0.5
gigahertz) begins to adversely effect performance of the
transmission line as measured by VSWR, this mismatch must
be reduced to tolerable limits. Accordingly, the length
of V-shaped teeth 600 in the passage 311 serves to
eliminate both this air gap as well as to provide
increased resistance to unwanted axial withdrawal.
FIGURE 6 shows the clamp 300 when the body 310 is
fully engaged with cable 10. The braid 16 and the outer
jacket 18 extend forwardly on the clamp body 310 over the
enlarged portion 313 thereof, fully cove`ring the tapering
frusto-conical portion 314, with the helical groove 500
being embedded along its length within the braid 16. The
dielectric core 14 is well forward within the clamp body
and in the central passage 315 thereof with the helical
thread 600 bitingly engaged along its length within the
core to eliminate any air layer thereabout.
While other types of grooves and threads could be
used - either exposed external threads, discontinuous
grooves, or varying shapes of grooves, the threads of the
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type shown in FIGURE 6 are believed the preferred embodi-
ment as they can be formed quite easily on automatic
screw machines. Of course, the threads could be formed
using other techniques.
While a preferred embodiment of the invention has
been disclosed, it will be apparent to those skilled in
the art that changes may be made to the invention as set
forth in the appended claims and, in some instances~ cer-
tain features of the invention may be used to advantage
without corresponding use of other features. For exam-
ple, additional members could be used in the present
assembly. The clamp of the present invention could also
be used for tri-axial cables with appropriate modifica-
tions. Accordingly, it is intended that the illustrative
and descriptive materials herein be used to illustrate
the principles of the invention and not to limit the
scope thereof.
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