Note: Descriptions are shown in the official language in which they were submitted.
106~099 NAR-1581
This invention relates to monitoring of low frequency
magnetic fields within the range of 10 MHz to 300 MHz. More
particularly, it relates to a portable probe system for measur-
in~ the "H" field.
Microwave energy is finding employment in ever increas-
ing areas, both consumer and industrial. This expanding use has
brought into question the effect upon the environment of such
energy, and it is important to accurately and reliably detect
the extent to which such energy is dissipated. The monitoring
of interest, relative to this invention, is the int~nsity of
magnetic field radiation within a particular space or area, with-
out limitation to the polarization of the field in question.
The National Bureau of Standards has been concerned
with the monitoring of magnetic fieIds and a number of magnetic-
field probes have been developed for this purpose. Unfortunate-
ly near-field measurements are often distorted by the measuring
instruments and special precautions and corrections are required
to compensate for the ensuing errors. It has been recognized
that it is desirable to detect fieIds within the frequency range
20 of 10 MHz to 300 MHz.- It has also been recognized that such
detection ~should ideally be isotropic and should be of flat
response over the~entire measurement range.
It is commonly understood that a loop antenna is
responsi~e to the magnetic field. Instruments have been describ-
ed ~oth by the National Bureau of Standards and in the patent
literature, for example, United States Patent 3,721,900 issued
March 20, 1973 to Charles L. Andrews for utilizing such loop
antennas. The cited patent also illustrates the utilization of
orthogonally disposed coils in order to obtain isotropic measurements.
The fact that non-polarization dependent performance
can be obtained by orthogonal disposition of the antenna eIements
has also been disclosed in United States Patent 1,794,914,
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1~60099 NAR-1581
- granted to the assignee of this invention on February 26, 1974.
This patent describes a probe responsive to the electric field
component of an electro-magnetic field and illustrates the
utilization of thin film thermocouple elements for converting
radio frequency induced currents into a direct current voltage
for driving metering equipment. In contrast, the present inven-
tion is designed for response to the magnetic component of
electro-magnetic fieIds. -
The present invention relates to a magnetic field
monitoring probe system using orthogonally disposed loop antennas i : -
terminated in thermocouples.
An object of the invention is to provide an improved
low frequency magnetic field detector.
Another object of the inventi:on is to provide an im-
proved low frequency radiation detector that is principallyresponsive to the magnetic component of an electro-magnetic
field.
Another object of the invention is to provide a low -:
frequency magnetic radiation detector that has a substantially
flat response within the range of lOMHz: to.3aO MHz.
In accordance with:the invention, there is provided a
magnetic fieId radiation detector for measurîng the intens.ity . .
of a magnetic fieId within a predetermined frequency range,
comprising a plurality o~ loop antennas each connected to a
thermocouple positioned substantially within the periphery thereof,
means for tuning said loop antennas to a resonant frequency
below the lower end of said range, and high resistance leads
interconnecting said .thermocouples in series.
A more thorough:understanding of the invention, along
30 ~ with a better appreciation of the ob.jects and novel features
thereof, will be available from the following description and .
the attached drawings.
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~AR-1581
` 10600~9 Canada
FIGURE 1 is a pictorial illustration of a magnetic
field detection system incorporating the present invention;
FIGURE 2 is a cross-sectional view along the lines 2-2
shown in FIGURE 1, revealing the structural arrangement of the
coils and thermocouples used in a particular embodiment of the
invention; and
FIGURE 3 is a lumped equivalent circuit of a coil
used in the detection probe in a particular embodiment of the
invention.
In FIGURE 1, the basic portable probe 10 is designed
for convenient, hand-held use. A particular unit is approximate-
ly one foot in length and the spherical detection portion 13 has
a diameter of less than four inches. As schematically shown in
the break-away portion of the drawing, detection loops 14, 15,
16 are orthogonally mounted within sphere 13. The loops are
covered by a thin film very high resistivity coating as a
shieId against electro-static charges. In a particular embodi-
ment, the resisti~ity of this film was in the order of 0.5
megohn per square.
The voltage deveIoped by the thermocouples within the - -
detection loops is applied to a pre-amplifier (not shown) that
is mounted directly within the handle of the probe. The
specific components of this pre-amplifier are not germane to
the invention; however, it is important that it be an extremely
low noise operational amplifier with a minimum of zero effect
characteristic. The gain of the amplifier establishes the
calibration of the probe.
A multi-conductor, shielded cable 12 connects the
probe to an indicating meter 11. Here too, the particular
meter components are not important. It is simply necessary
that this meter provide a visual or audible output indicative
of the field being measured. This output takes into account
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NAR-1581
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099
the fact that the radio frequency induced currents in the loops
of the detector, will heat the terminating resistive thermo-
couple hot junctions and thereby provide a direct current out-
put voltage which is proportional to the square of the radio
frequency induced current. The shielded cable 12 carries the
pre-amplifier output to the meter and also carries the power
supply voltages and the correction voltage to the probe. The
correction voltage is applied to the input of the amplifier, as
required, to compensate for zero offset.
FIGURE 2 reveals the structure and arrangement of
components within the detection sphere. The entire sphere is -
enclosed within the thin resistive film 20 mentioned above.
Each loop is made up of two coils, e.g. 21, 22, having a thin
film thermocouple element serially connected at one end there-
of. The remote ends of each coil of the pair, are bridged by
a capacitor for tuning the coil to a resonant frequency below
the low frequency of the band to be detected. In a particular
embodiment, operative with a substantially flat response from
10 MHz to 200 MHz, the coils 21, 22 had a diameter of approx-
imately 9 cm. They were terminated with thin film thermocoupleelements formed of antimony and bismuth, deposited between
silver terminals, having a resistance of 30 to 50 ohms. Each
loop had an inductance of 0.73 ~ h and series capacitors of
820~ fd resonated them at about 6.5 MHz. It is a principal
aspect of the invention that the impedance of the loop at the -
low frequency end of the band, be equal to the inductive
reactance of the loop at the low frequency end of the band.
In order to achieve this condition, the series capacitor is
chosen in order to add capacitative reactance that will lower
the total reactance relative to the resistive component. In
other words, the capacitor is added in order to tune the loop
, to a resonant frequency below the lower end of the band.
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NAR-1581
Canada
1060099
-- The loops are orthogonally disposed relative to one
another and they are serially interconnected by means of high
resistive film leads 24, 25. Similar high xesistive film leads
26, 27 interconnect the loops to the pre-amp.
The resistive leads should be monolithic and sub-
stantially rigid. This assures that the leads will not cause
modulation of the signal due to flexing or other movement. In-
sulating tape 28, or the like, may be used to isolate the various
connections from one another.
FIGURE 3 shows the lumped circuit diagram of an
individual loop. In this diagram C is thè lumped resonating
capacitor, L/2 is the inductance of each turn and CL i9 the
loop capacitance of each turn. Rr is the radiation resistance.
R is the resistance of the thin film thermocouple element. Both
the loop capacitance and radiation resistance are extremely
small and are found to be negligible in the operating band. The
input voltage is the induced equivalent open circuit voltage.
By using the teachings disclosed above, low frequency
magnetic field detection monitors have been developed covering
the range of 10 MHz to 300 MHz. These monitors have a 1 db
band width over the measurement range. Clearly, those skilled
in the art may vary parameters and certain structural details
in accordance with desired design criteria. Nevertheless, all
variations and modifications coming within the scope of this
invention are intended to be covered by the following claims.
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