Note: Descriptions are shown in the official language in which they were submitted.
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CURRENI DETECTION DEVICE HAVING AN
EXTENDED FREQUENCY RANGE OF RESPONSE
FIELD OF THE INVENTION
The present invention relates generally to a
device for detecting and measuring the magnitude of an
electric current passing along a coaxial conductor,
and, more particularly, to such a device capable of
detecting and measuring electric current over an
extended frequency range and in a non-invasive manner.
BACXGROUND OF THE INVENTION
ThC detection and measurement of electric
currents through application of the general principle
on induction is well known, as exemplified by clamp-on
meters used for relatively low frequency current
measurement. Briefly, these known devices incorporate
a magnetic material loop arranged to encircle a
current carrying conductor, which loop has an electric
potential induced therein that can be detected by a
galvanometer, the latter being optionally calibrated
for direct readout. At low frequencies, these devices
have found wide use and are relatively insensitive to
secondary inductions and do not require precise
location of the current carrying conductor with
respect to any of the detection equipment parts.
However, as the frequency of the current
being measured increases, various factors which can be
substantially ignored at lower frequencies must now be
taken into account in order to obtain accurate
determinations. First of all, although conductor
resistance loss is responsible for most of tne
attenuation at low frequencies, at higher frequencies
dielectric loss is the primary cause of attentuation.
Moreover, the series resistance of a radio frequency
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(or higher) line is controlled by a physical
- phenomenon referred to as "skin effect" and it can be
shown to be proportional to the square root of the
frequency. Still further, the coaxial characteristic
impedance is substantially independent of frequency,
ranging between 20-300 ohms for coaxial conductors or
transmission lines. The combined effect of all of
these factors makes current detection and measurement
more difficult as the frequency increases.
10In addition to desiring to unobtrusively
determine the magnitude of coaxial transmission line
current, there are situations in which it is
advantageous to be able to test the effectiveness of
such apparatus as cable termination means. Exemplary
of what is referred to here, there are many
en~ironments (e.g., aboard a ship) where shielded
cables are exposed to relatively large interference
electromagnetic fields which induce correspondingly
large interference currents in the cable shield. If
these currents are not terminated satisfactorily, they
can impair or even destroy the equipment to which the
cables are connected. Termination means of
considerable variety have been devised to achieve
termination for cables as vell as other equipment and
devices, and it is a desideratum to have non-invasive
test equipment which can determine the effectiveness
of a particular termination or grounding device prior
to its installation and actual on-site utilization.
An essential part of apparatus for measuring
termination means effectiveness is a wide range
electrical current detection and measurement device as
described herein.
OBJECTS AND SUMMARY OF THE INVENTION
It is, therefore, a primary o~ject and aim of
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the present invention to provide a coaxially current
detection device having an exceptional wide range of
frequency response, namely, D.C. to 1 gigahertz.
Another object is the provision of a current
detection and measurement device which is incorporated
into a test arrangement for termination apparatus.
In accordance with the practice of the
described invention, a current detection device is
provided including an annular dielectric substrate
with a thin (e.g., less than one skin depth at maximum
frequency) resistive foil or film laid down on the
substrate outer surface and extPnding completely
thereabout. First and second annular metal rims are
affixed to the substrate edges respectively contacting
the metal foil or film. More particularly, the foil
edges are turned up into a plurality of tabs which
press against the metal rims. A dielectric member is
located over the thin foil or film for protection
against physical contact.
A portion of the dielectric member is removed
to form a recess within which a resistor is located
with one terminal interconnected with one side of the
foil and the other terminal of the resistor
interconnected with the center lead of a length
coaxial cable. The outer conductor of the coaxial
cable is connected to the opposite side of the foil.
The coaxial cable extends outwardly of the dielectric
member for interconnection with a suitable voltage
measuring and display means for measuring the voltage
drop developed across the foil.
A conductive strip of appropriate dimensions
is received onto the outer edges of the metal rims and
extending about the complete circumference of the
rims. A resistive material is used to secure the
conductive strip to the edges of the rims so that the
entire resistance of the strip and resistance material
exceeds that of the sensing metal foil to prevent
shorting out of the foil.
In use for determining effectivity of
termination means, the annular current detection and
measurement device is mounted onto a grounded
conductive plate which, in turn, has a termination
means conductively secured to the opposite plate
surface. Specifically if the termination means is to
terminate a cable shield, the termination means will
interconnect the cable shield to the plate and the
cable will be allowed to extend through an opening in
the plate as well as the central opening in the
current detecting device. A test voltage is applied
to the cable shield on the termination side of the
conductive plate. If the termination is lossless, no
current will be detected by the described device. If
termination is incomplete, some currents in the cable
shield will be detected by the device, the value of
the currents being a direct measure of the termination
means effectiveness. The return current through the
current detection device produces a voltage drop
across the metal foil related to the current I on the
cable shield by Ohm's law (i.e., E=IR) where E is the
D.C. voltage output of the device as installed with a
known coaxial input of direct current. Accordingly,
an unknown test sample can be accurately measured
since the device output does not change significantly
with frequency.
DESCRIPTION OF THE DRAWINGS
Figure l is a perspective view of the current
detection and measurement device of the present
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invention shown schematically connected to means for
displaying current detected.
Figure 2 is a side elevational, sectional
view taken along the line 2--2 of Figure 1.
Figure 3 is a further elevational, sectional
view taken through the coaxial connector to the
current detecting and measuring device along the line
3--3 of Figure 2.
~igure 4 is a perspective view of the metal
foil used in the current detecting and measuring
device.
DESCRIPTION OF THE PREFERRED EMBODIMENT
~ urning now to the drawings and particularly
Figure 1, the current detection device to be described
is enumerated generally as 10 and seen to be generally
annular in shape and contemplated for detecting and
measuring the current of a conductor that extends
through the device central opening. A coaxial cable
or lead 12 interconnects the device with a voltage
measuring and display means 13 which is depicted
schematically by a dial instrument. As is well known
in the electrical arts, the means 13 may be calibrated
for direct readout of electric current.
The current detecting and measuring device 10
has an insulative substrate 14 in the form of a solid
flat ring enclosed at each side by metal rims 15 and
16 which extend radially outwardly from the
substrate. The substrate can be made in one piece by
molding or machining, or in a suitable cementitious
material (e.g., epoxy). Also, the substrate may be
made of any of a number synthetic plastics or ceramics
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having good strength and rigidity as well as being a
good electrical insulator.
A thin metal foil or film 17 preferably of a
metal such as Inconel (80% Nickel, 14% Chromium and 6%
Iron) is laid down on the substrate 14 outer surface
and is in intimate contact with the entire surface.
In addition, the film or foil edges are turned
upwardly in a plurality of slotted tabs which contact
the respectively adjacent rims 15 and 16 (Figure 2).
The film or foil thickness should preferably not
exceed 0.0005 inches in order to avoid difficulties
from skin effect at higher frequencies and to provide
sufficient resistance to produce a significant voltage
drop from currents induced in the film by leakage
currents. A shielding conductive strip 18 extends
across the two rims 15 and 16 and completely about the
rims' circumference. The strip 18 is secured to the
rim edges by a relatively resistive material 19 such
that the total resistance measured across the strip
width is sufficient so as not to form a short circuit
across the rims. The strip is important, however, in
preventing stray field inductions in the film 17 which
can result from such things as back loops.
Still referring to Figure 2, the coaxial
cable or lead 12 is seen to have its central conductor
connected to one edge of the film ox foil 17
through a 50-ohm resistor 21. The opposite film edge
either connects directly with coaxial cable outer
conductor 22 or through the immediately adjacent rim.
A quantity of a dielectric material 23 fills the space
between the rims and above the film 17. A plurality
of aligned openings 24 in the rims 15 and 16 are used
in practice receive bolts 25 therethrough which are
electrically isolated from the rims by a glass fiber
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tube 26 and washers 27, for mounting the entire device
onto a fixture or the like.
The described current detection and measuring
device can be effectively employed for de~ection of
return electric currents passing along a coaxial cable
28 extending through the device opening 11. In a
manufacturing or laboratory test mode, it is
contemplated that the device will be most extensively
employed detecting electric currents in coaxial
transmission lines providing a direct measure of the
transmission line current by utilizing a diode with a
sensitive galvanometer (not shown) mounted directly to
the device output coaxial connector 12. In this case,
the entire device is preferably packaged with
lS appropriate interconnection means for the applicable
coaxial transmission line being monitored.
Exemplary of but one practical employment of
the described current detecting and measuring device~
a conductive plate 29 of dimensions substantially
equal to the outer diameter of the device 10 is
secured to one side of the device by a plurality of
bolts 25 passed through the device openings 24 and
electrically isolated from the rims 15 and 16. The
cable 28 having, say, a shield conductively connected
(terminated) to the plate 29 by a suitable termination
means 30 extends through an opening 31 in the plate as
well as through the device opening 11.
Then, the incoming cable 28 as well as a
return path (not shown) are coaxially connected to an
~0 electromagnetic signal, typically a swept frequency
source which induces interference electric currents in
the cable 28, or more particularly the cable outer
shield. Those interference currents not terminated by
termination means 30 at the plate 29 appear on that
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part of the cable shield extending within the device
opening 11, which is further identified as 28'. The
plate may be conductively related to other metal parts
to prevent creation of loops from the plate 29 through
the film 17 to the conductor 28 causing erroneously
high current readings. This latter feature is
indicated by the ground symbol 32.
Currents present on the cable shield 28' will
largely return through the film or foil 17
establishing a voltage drop across the film width that
is measured by the meter 13. Ideally, if termination
were total, then there would be zero current measured
on 28'.
