A lightweight search-coil antenna or sensor assembly for detecting magnetic fields and including a multi-turn electromagnetic induction coil wound on a spool type coil form through which is inserted an elongated coil loading member comprised of metallic glass material wrapped around a dielectric rod. The dielectric rod consists of a plastic or a wooden dowel having a length which is relatively larger than its thickness so as to provide a large length-to-diameter ratio. A tri-axial configuration includes a housing in which is located three substantially identical mutually orthogonal electromagnetic induction coil assemblies of the type described above wherein each of the assemblies include an electromagnetic coil wound on a dielectric spool with an elongated metallic glass coil loading member projecting therethrough

Desch, Michael D.

Farrell, William M.

Houser, Jeffrey G.

NASA Technical Reports Server

United States Patent [191 
Desch et al. 
[54] MAGNETIC ANTENNA USING METALLIC 
GLASS 
[75] Inventors: Michael D. Desch, Greenbelt; William 
M. Farrell, Kensington; Jeffrey G. 
Houser, Silver Spring, all of Md. 
[73] Assignee: The United States of America as 
represented by the Administrator of 
the National Aeronautics and Space 
Administration, Washington, D.C. 
[21] Appl. No.: 372,538 
[22] Filed: Jan. 13, 1995 
[51] 
[52] 
[58] 
Int. C1.6 ....................................................... HOlQ 7/08 
U.S. C1. ............................................. 343/788; 343/787 
Field of Search ..................................... 343/785, 787, 
343/788, 895 
[561 References Cited 
U.S. PATENT DOCUMENTS 
4,148,036 4/1979 Miller ...................................... 343/788 
4,458,248 7/1984 Lyasko .................................... 343/719 
4,658,263 4/1987 Urbanski ................................. 343/788 
US005521609A 
[ i l l  Patent Number: 5,521,609 
[45] Date of Patent: May 28, 1996 
4,746,926 511988 Stem et al. ............................. 3431788 
5,250,944 10/1993 Urbas et al. ............................ 343/788 
5,396,698 3/1995 Orthmann et al. ........................ 29/600 
Primary Examiner-Donald T. Hajec 
Assistant Examiner-Tan Ho 
Attorney, Agent, or Firm-Keith L. Dixon 
[571 ABSTRACT 
A lightweight search-coil antenna or sensor assembly for 
detecting magnetic fields and including a multi-turn elec- 
tromagnetic induction coil wound on a spool type coil form 
through which is inserted an elongated coil loading member 
comprised of metallic glass material wrapped around a 
dielectric rod. The dielectric rod consists of a plastic or a 
wooden dowel having a length which is relatively larger than 
its thickness so as to provide a large length-to-diameter ratio. 
A tri-axial configuration includes a housing in which is 
located three substantially identical mutually orthogonal 
electromagnetic induction coil assemblies of the type 
described above wherein each of the assemblies include an 
electromagnetic coil wound on a dielectric spool with an 
elongated metallic glass coil loading member projecting 
therethrough. 
17 Claims, 4 Drawing Sheets 
10 
22 
https://ntrs.nasa.gov/search.jsp?R=20080005964 2019-08-30T03:02:29+00:00Z
U.S. Patent May 28,1996 Sheet 1 of 4 5,521,609 
22 
FIG. 
U.S. Patent May 28,1996 Sheet 2 of 4 5,521,609 
U.S. Patent May 28,1996 Sheet 3 of 4 
* 
LL, 
5,521,609 
U.S. Patent 
.I 
.05 
EFFECTIVE .02 
HEIGHT 4cm) 
.01 
.005 
.002 
May 28,1996 Sheet 4 of 4 
FIG, 5 
t- 
5,521,609 
I 
I I 
I I I I l l l l  I I 
10 30 I00 
FREQUENCY-(~Hz)  
300 
5,521,609 
2 1 
MAGNETIC ANTENNA USING METALLIC 
GLASS 
ORIGIN OF THE INVENTION 
This invention was made by employees of the United 
States Government, and may be manufactured and used by 
or for the Government for governmental purposes without 
the payment of any royalties thereon or therefor. 
BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates generally to apparatus for sensing 
electromagnetic radiation and more particularly to a search- 
coil antenna for detecting magnetic fields. 
2. Description of the Prior Art 
A search-coil is a device that measures fluctuating mag- 
netic fields and comprises a magnetic antenna and associated 
electronics therefor. When used on sounding rockets or on 
spacecraft, search-coils are typically mounted on a boom in 
order to escape the electronic noise generated by other 
scientific instruments andor subsystems in the immediate 
vicinity. 
As a general rule, scientific instrumentation should impact 
mass constraints as little as possible; however, boom- 
mounted instruments in particular must be lightweight to 
survive deployment under high stress conditions when used 
in connection with spacecraft. This weight constraint can 
limit both the size of the sensor and its placement on the 
boom. Both factors translate into severe constraints on the 
capability of the instrument to measure weak electromag- 
netic fields encountered, for example, in space. 
For this type of task, the known prior art typically 
comprises relatively heavy ferrite-loaded loop type antennas 
shown and described, for example, in the section entitled 
“Loop Antennas” by G. S. Smith, which appears in Antenna 
Engineering Handbook, edited by R. C. Johnson et al, 
McGraw-Hill, Inc., New York, 1961. 
SUMMARY 
Accordingly, it is an object of the present invention to 
provide an improvement in search-coil type receiving anten- 
nas. 
It is a further object of the present invention to provide a 
lightweight metallic glass antenna for detecting AC mag- 
netic fields generated by natural and man made sources. 
It is another object of the invention to provide a light- 
weight search-coil antenna for detecting AC magnetic fields 
in outer space. 
It is yet another object of the invention to provide a 
metallic glass antenna having an improved effective height 
characteristic in the frequency range below and above 100 
kHZ. 
The foregoing and other objects are realized by a light- 
weight search-coil antenna for detecting magnetic fields and 
including a multi-turn electromagnetic induction coil wound 
on a spool type coil form through which is inserted an 
elongated coil loading device comprised of metallic glass 
material wrapped around a dielectric rod. The dielectric rod 
is comprised of a plastic or a wooden dowel having a length 
which is relatively larger than its diameter so as to provide 
a large length-to-diameter ratio. Furthermore, a tri-axial 
search-coil antenna configuration exists and includes a hous- 
ing in which is located three substantially identical mutually 
5 
10 
15 
20 
25 
30 
35 
40 
45 
50 
55 
60 
65 
orthogonal electromagnetic induction coil assemblies of the 
type described above wherein each of the assemblies include 
an electromagnetic coil wound on a dielectric spool with an 
elongated metallic glass coil loading sub-assembly project- 
ing therethrough. 
BRTEF DESCRIPTION OF THE DRAWINGS 
The following detailed description of the invention will be 
more readily understood when considered together with the 
accompanying drawings, wherein: 
FIG. 1 is a perspective view generally illustrative of one 
preferred embodiment of the invention; 
FIG. 2 is a perspective view illustrative of a coil loading 
member comprised of a metallic glass sheet being wrapped 
around a dielectric rod; 
FIG. 3 is a central longitudinal cross sectional view 
illustrative of a second preferred embodiment of the inven- 
tion; 
FIG. 4 is an electrical schematic diagram illustrative of a 
buffer amplification circuit for use in connection with a 
search-coil shown, for example, in FIG. 1; and 
FIG. 5 is a graph illustrative of the effective height vs. 
frequency characteristic of a search-coil constructed in 
accordance with the teachings of the subject invention. 
DETAILED DESCRIPTION OF THE 
PREFERRED EMBODIMENTS 
This invention is directed to a lightweight magnetic 
antenna structure or sensor for detecting AC magnetic fields 
in the extra low frequency to low frequency ranges of the 
electromagnetic spectrum (10 Hz-500 kHz). The sensor 
includes a loaded induction coil and is similar to known 
ferrite-loaded receiving coils; however, rather than utilizing 
a femte rod as the coil loading member, the magnetic flux 
is now concentrated using a new coil loading material 
comprising metallic glass, a material manufactured by Met- 
glas Products, Parsippany, N.J. and which is currently iden- 
tified by the product line called “Metglas Ribbon” and sold 
under Part No. 2705111. 
Refemng now to FIG. 1, shown thereat is a search-coil 
antenna configuration according to a first embodiment of the 
invention which is comprised of an induction coil 12 con- 
sisting of many turns of electrically conductive wire wound 
on a dielectric coil form 14 which is in the form of a spool 
of circular cross section and having a central circular aper- 
ture or opening 16 through which is inserted an elongated 
sensor coil loading device 18. The coil loading device 18 is 
in the form of a relatively long, thin circular cylindrical 
metallic glass rod comprised of one or more sheets of 
metallic glass 20, as shown in FIG. 2, wrapped around a 
lightweight dielectric dowel 22 fabricated from plastic, 
wood or the like. The loading device 18 has a length which 
is greater than the width of the spool 14 so that it projects 
through the circular opening 16 and extends outward from 
each side thereof a predetermined distance. Typically the 
width of the induction coil 12 is in the order of 0.4 in., and 
comprising two thousand turns of wire providing a diameter 
of 0.875 in. The loading device 18, for example, is at least 
6.0 in. long and has a diameter of 0.5 in. A relatively high 
length to diameter ratio structure is provided thereby. 
The use of metallic glass sheets 20 provides a distinct 
advantage over the known prior art in that the weight of a 
magnetic antenna using magnetic glass as the coil loading 
material is substantially reduced. The intrinsic density of 
5,521,609 
3 
“Metglass” is nearly a factor of 2 greater than that of ferrite, 
for example. However, for a configuration of equal size, only 
7% as much material is needed when metallic glass sheets 
are used. Further, ferrite substances tend to be brittle, 
shearing under torques and shattering under extreme pres- 
sure. With sheets of metallic glass 20 wrapped on a light- 
weight plastic or wooden dowel 22, this inherent limitation 
is overcome with the structural integrity limited only by 
adhesive fastening of the wrap to the dowel. Such a sensor 
is desirable in many applications, particularly in space flight 
where space science and communication studies are cur- 
rently being carried out. 
Referring now to FIG. 3, shown thereat is a second 
embodiment of the invention comprised of a tri-axial metal- 
lic glass search coil assembly for measuring field strength 
along three mutually perpendicular X, Y and Z axes (Car- 
tesian coordinates). As shown, the assembly includes three 
substantially identical magnetic antenna structures lo,, 10, 
and lo,, one of which is shown in FIG. 1, oriented in an 
orthogonal configuration and mounted within a tubular 
dielectric housing comprising an outer shell 30 including a 
tapered nose portion 32. The nose portion 32 also includes 
a central axial bore 34 for the passage of the loading member 
18, therethrough. The outer shell 30 also includes a trans- 
verse set of holes 36 and 38 for the passage of the coil 
loading member 18, therethrough. An additional opening or 
hole 40 is also provided in the outer shell 30 for the passage 
of a wiring harness, not shown, therethrough which is 
adapted to be connected to the various electrical components 
26 and 28 located on circuit wiring boards 24,, 24,, and 24,, 
respectively attached to coil spools 14,, 14,, and 14,. 
It should also be noted that in the tri-axial search-coil 
assembly shown in FIG. 3, the three search coils 12,, 12,, 
12,, their respective mounting spools 14,, 14, and 14, and 
circuit boards 24,, 24, and 24, are further located in and 
fastened to generally circular type dielectric spool enclosure 
members 42,, 42, and 42, which fit inside the outer shell 30. 
Each of the enclosure members 4 5 ,  42, and 42,, moreover, 
are covered with electrical shielding 44,, 44, and 44, and 
which may comprise, for example, metallic foil or other type 
of electrical shielding material. A grounding wire, not 
shown, extends from each circuit board 24,, 24, and 24, to 
the electrical shielding 4 5 ,  42,, 42,, respectively. 
Each loaded sensor coil 12,, 12, and 12, has an intrinsic 
inductance L, distributed capacitance C and an inherent 
resistance R. As is well known, the combination of the 
inductance and capacitance controls the frequency of reso- 
nance where the inductive reactance X, and the capacitive 
reactance X, cancel each other, leaving only a resistive 
impedance X,. At the resonant frequency, the output signal 
voltage is amplified by a factor Q, often referred to as a 
resonance quality factor, which is functionally dependent 
upon the inherent resistive losses. As the resistance losses 
decrease, Q increases. 
Since the characteristics of antenna structures such as 
shown in FIGS. 1 and 3 are easily altered by any load, a 
buffer or a preamplifier is normally connected to the ends of 
the coils 12 in FIG. 1 and the coils 12,, 12, and 12, of FIG. 
3 to electrically isolate the antenna(s) from subsequent 
electronic circuitry. FIG. 4 is illustrative of one such ampli- 
fier. 
Referring now to FIG. 4, shown thereat is an amplifier 46 
having its + and - inputs coupled across a search-coil 12. 
The amplifier 46 typically comprises a Precision Monolithic 
Inc. AMP-01 instrument amplifier which has high common 
mode rejection characteristics for enhanced signal to noise 
4 
performance. A four diode limiting network 48 is also shown 
coupled across the + and - inputs to the amplifier 46. The 
amplifier 46, moreover, includes a low impedance output 
terminal 50 which is shown connected to the inner conductor 
5 52 of a coaxial cable 54 whose outer conductor 56 is shown 
connected to ground. The output of the amplifier 46 is thus 
connectable via terminal 57 to subsequent amplifier stages, 
for example, not shown. Power supply potentials V,, and 
V,, are applied to the amplifier 46 through respective RC 
Referring now to FIG. 5, shown thereat is a pair of 
characteristic curves 66 and 68 illustrating the “effective” 
height as a function of frequency of a magnetic antenna as 
shown in FIGS. 1 and 3. Curve 66 is illustrative of a femte 
15 loaded antenna while curve 68 is illustrative of a metallic 
glass loaded antenna in accordance with the subject inven- 
tion. 
The effective height is defined as the equivalent length of 
a short electric dipole antenna required to obtain the same 
20 output voltage as the magnetic antenna. The effective height 
h can be defined mathematically as: 
1o filter networks 58 and 60 via terminals 62 and 64. 
h=iop,,QNA/c 
25 where i implies the imaginary portion of a complex number 
x+iy, -2Xf where f is the frequency, pmd is effective 
permeability of the material, Q is the RLC quality factor, N 
is the number of turns of the inductor, A is the cross sectional 
area of the coil and c is the speed of light. Accordingly, the 
3o magnitude of h is a measure of the sensitivity of the 
magnetic antenna. Thus the sensitivity of a ferrite type 
sensor as shown by the characteristic curve 66 for a ferrite 
type antenna is more sensitive in the 10-20 lcHz range, while 
a metallic glass loaded antenna characteristic as shown by 
35 reference numeral 68 is more sensitive in the range of 
frequencies above 30k and which is of particular interest for 
studies involving space science such as the high latitude 
polar region of the earth’s ionosphere. 
Thus what has been shown and described is a metallic 
glass antenna having low ohmic loss and high magnetic 
permeability particularly adapted for use in detecting AC 
magnetic fields associated with VLF wave activity above the 
earth’s surface. 
Having thus shown and described what is at present 
considered to be the preferred embodiments of the inven- 
bon, it should be known that the same has been made by way 
of illustration and not limitation. Accordingly, all modifica- 
tions, alterations and changes coming within the spirit and 
scope of the invention as set forth in the appended claims are 
40 
45 . 
5o herein meant to be included. 
netic fields, comprising: 
We claim: 
1. A lightweight search-coil antenna for detecting mag- 
a multi-turn electromagnetic induction coil wound on a 
cylindrically shaped coil form wherein said coil form is 
of a predetermined width dimension measured along a 
central axis and includes a spool located in a dielectric 
enclosure with an elongated loading device extending 
out of said dielectric enclosure; and 
said elongated loading device for said coil inserted 
through the center of said coil, said elongated loading 
device including a metallic glass material wrapped 
around a dielectric support member with a thickness 
dimension and a length dimension wherein said length 
65 dimension is relatively larger than said thickness 
dimension so as to provide a relatively large length to 
thickness ratio. 
55 
60 
5,521,609 
5 
2. An antenna according to claim 1 wherein said dielectric 
support member comprises a dielectric rod and said metallic 
glass material comprises at least one sheet of metallic glass 
wrapped around said rod. 
3. An antenna according to claim 2 wherein said at least 
one sheet of metallic glass is wrapped around said rod so as 
to provide a laminated coil loading structure. 
4. An antenna according to claim 1 wherein said loading 
device has a transverse cross sectional dimension and 
wherein the ratio of said length dimension to said transverse 
cross sectional dimension is relatively large. 
5. An antenna according to claim 1 and additionally 
including means located on the spool for mounting electrical 
circuit components thereon. 
6. An antenna according to claim 5 wherein said means 
for mounting comprises a circuit board. 
7. An antenna according to claim 1 wherein said dielectric 
enclosure includes electrical shielding located on the outside 
thereof. 
8. An antenna according to claim 1 and additionally 
including a dielectric outer shell for supporting said enclo- 
sure and said spool with said induction coil wound thereon. 
9. A tri-axial search-coil antenna structure, comprising: 
a housing; 
three mutually orthogonal electromagnetic induction coil 
sensor assemblies located in said housing; 
each of said sensor assemblies including a coil wound on 
a coil form, and 
a coil loading device projecting through said coil and 
comprising an elongated dielectric cylindrical support 
member having at least one layer of metallic glass 
wound thereon. 
6 
10. An antenna structure according to claim 9 wherein 
said layer of metallic glass comprises at least one sheet of 
metallic glass wrapped around said support member in a 
plurality of turns so as to provide a plurality of layers of 
metallic glass. 
11. An antenna structure according to claim 9 wherein 
each said dielectric support member is cylindrical in cross- 
section. 
12. An antenna structure according to claim 9 wherein 
said dielectric support member has a thickness dimension 
and a length dimension for providing a relatively large 
length to thickness ratio. 
13. An antenna structure according to claim 9 wherein 
said coil form comprises a dielectric spool type member. 
14. An antenna structure according to claim 13 wherein 
said coil and spool type member are located in a dielectric 
15. An antenna structure according to claim 14 wherein 
said housing comprises a dielectric outer shell for supporting 
said coil sensor assemblies. 
16. An antenna structure according to claim 14 wherein 
said outer shell comprises a circular cylindrical housing 
member. 
17. An antenna structure according to claim 16 wherein 
said housing member includes a tapered nose portion at one 
end thereof. 
lo 
l5 
20 enclosure. 
25 
30 
$ * * * $  


Magnetic antenna using metallic glass

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