U.S. patent number 3,742,509 [Application Number 05/197,989] was granted by the patent office on 1973-06-26 for subsurface traveling wave antenna.
This patent grant is currently assigned to Raytheon Company. Invention is credited to Joseph T. DeBettencourt, Carson K. H. Tsao.
United States Patent |
3,742,509 |
DeBettencourt , et
al. |
June 26, 1973 |
SUBSURFACE TRAVELING WAVE ANTENNA
Abstract
A subsurface traveling wave antenna for generating and receiving
primarily surface waves in either the vertical or horizontal
position below the surface. The antenna comprises an insulated
linear radiating element terminated with a matched load for
coupling a portion of the surface wave component. Impedance
elements interconnecting portions of the radiating element at
periodic intervals along its extent provide speed matching between
the phase velocity of the wave propagating down the radiating
element and the surface wave component.
Inventors: |
DeBettencourt; Joseph T. (West
Newton, MA), Tsao; Carson K. H. (Braintree, MA) |
Assignee: |
Raytheon Company (Lexington,
MA)
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Family
ID: |
22731549 |
Appl.
No.: |
05/197,989 |
Filed: |
November 11, 1971 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
|
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48889 |
Jun 15, 1970 |
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742074 |
Jul 2, 1968 |
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Current U.S.
Class: |
343/719; 343/739;
343/749 |
Current CPC
Class: |
H04B
7/00 (20130101); H01Q 1/04 (20130101) |
Current International
Class: |
H04B
7/00 (20060101); H01Q 1/00 (20060101); H01Q
1/04 (20060101); H01q 001/04 () |
Field of
Search: |
;343/719,739,749 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Lieberman; Eli
Parent Case Text
This is a continuation of application Ser. No. 48,889, filed June
15, 1970, which is a continuation of application Ser. No. 742,074,
filed July 2, 1968, both now abandoned.
Claims
We claim:
1. An end fed linear antenna submerged beneath the earth's surface
comprising:
a series of elongated electrically conducting elements;
a series of reactive impedance elements, each of said reactive
impedance elements being periodically placed between adjacent ones
of said conducting elements of said series of conducting elements
each of said reactive impedance elements coupling adjacent
conducting elements of said series of conducting elements;
a series of cylindrical dielectric insulating elements, each of
said insulating elements being positioned about and enclosing
respective ones of said conducting elements, a conducting element
at one end of said series of conducting elements being positioned
for coupling to a source of electromagnetic energy, said series of
conducting elements and said series of insulating elements
cooperating with material of said earth's surface for propagating a
wave of electromagnetic energy along said series of conducting
elements and said series of insulating elements, the values of said
reactive impedance elements being selected to equalize the phase
velocity of said electromagnetic wave with that of a surface wave
propagating along the earth's surface; and
a matched load coupled to a conducting element at the other end of
said series of conducting elements for absorbing said
electromagnetic wave.
2. An antenna system comprising:
a first and a second end fed linear antenna each of which comprises
a series of electrically conducting elements, a series of reactive
impedance elements which are periodically positioned between the
termini of respective ones of said series of conducting elements
for coupling electromagnetic energy between successive ones of said
series of conducting elements, a cylindrically shaped jacket of
insulating material which encloses each conducting element of said
series of conducting elements, and a matched load coupled to a
conducting element at one end of said series of conducting elements
for absorbing electromagnetic energy propagating along said series
of conducting elements and said insulating jacket; said system
further comprising
a source of electromagnetic energy coupled to an end of said series
of conducting elements opposite said matched load in said first
antenna, said first antenna being submerged beneath the earth's
surface to permit an electric field to be established from a
conductive region of the earth through said jacket to said
conducting elements, the values of said reactive impedance elements
being selected to equalize the phase velocity of an electromagnetic
wave propagating along said first antenna with that of a surface
wave propagating along the earth's surface; and
a receiver of electromagnetic energy coupled to a conducting
element at an end of said second antenna opposite said matched load
for receiving energy propagating along said second antenna, said
first and said second antennas being spaced apart and disposed
substantially parallel to a common axis for coupling energy
therebetween.
Description
BACKGROUND OF THE INVENTION
This invention relates to subsurface antennas, and more
particularly to highly efficient buried antenna systems which may
be rendered immune to military bombardment.
When an antenna is buried in the ground, it suffers energy losses
in several ways. First, the signal energy suffers attenuation
through rock. This appears as an exponential loss. Second, there is
a "refraction" loss at the interface between the earth and air.
Third, the received power density is reduced by the "spread"
loss.
In the ground, the reduction in power density and the efficiency of
an antenna have lead to the use of the concept of a "modified power
gain." The power gain of an antenna in air may be defined as the
ratio of the power density at a receiver at a distance R from the
transmitting antenna to the power density in the antenna, this
ratio being multiplied by 4.pi.R.sup.2. For the gain measurement in
the ground, the gain in air is multiplied by e.sup.2aR where "a" is
the attenuation constant. Now, the power density at the receiver is
Ke.sup.-.sup.2aR. Thus, the "modified power gain" G.sub.m may be
considered independent of distance in the the rock and expressed as
G.sub.m - (Ke.sup.-.sup.2aR (4.pi.R.sup.2) e.sup.2aR)/power in
antenna.
Generally, the modified power gain as defined above will vary with
the type of transmission medium. If the transmission medium is
unbounded, homogenous, and isotropic and the receiver is spaced at
a large distance from the transmitter, then the linear antennas
should preferably be placed parallel to each other for maximum
power transfer. This is termed broadside radiation.
It is well known that buried antennas near the surface may transfer
and receive energy by three mechanisms. The first mechanism is the
generation of a surface wave, which is a substantially vertically
polarized wave propagating, along the earth's surface. In the
second mechanism, one buried antenna generates a space wave which
upon reflection may communicate with another buried antenna in the
so-called "up-over-and-down" propagation scheme. The third
transmission mechanism is directly "through the rock" in which
waves are polarized in directions parallel to the buried antenna.
Reference is made in this regard to the Proceedings of the
Conference of the British Institute of Electrical Engineers, 8-10
Nov. 1967, at pages 313-317, in an article entitled, "Subsurface
Radio Communications," by Tsao and deBettencourt and "Progress in
Radio Science" by J. T. deBettencourt in the International Radio
Scientific Union (URSI) Berkeley, California, 1967, part I, pages
697-767.
In survivable communications, such as at missile sites, it is
anticipated that the requirements to survive in the event of
nuclear attack would indicate the use of subsurface antennas for
radio communications. Furthermore, it is contemplated that any one
or all of the three propagation mechanisms would be used. Needless
to say, these three mechanisms are subject to one or more of the
aforementioned power or energy losses.
In the prior art, such as U. S. Pat. No. 3,346,864, issued to G. J.
Harmon on Oct. 10, 1967, it is taught that an underground antenna
may be positioned at an inclination greater than 10.degree. to the
horizontal and at a depth greater than 1/10 the wavelength. If the
electrical conducting antenna surface is placed in close contact
with the subsurface rock, then a space wave is propagated. However,
this antenna system produces a space wave component in the
direction of interest. Such a system requires surfaces inclined at
a specific angle of inclination.
It is accordingly an object of this invention to devise a
subsurface antenna useful in transmitting in the three principal
propagation mechanisms and capable of being placed in both plane
and irregular topography.
In addition to the Harmon reference, another patent relating to
underground antennas is U. S. Pat. No. 3,183,510. This patent does
not show systems which optimize the surface wave, space wave, and
broadside radiation so as to increase power gain of the antenna.
The prior art furthermore deals only with antennas 1/2 wavelength
long and of the standing wave type.
It is accordingly another object of this invention to devise a
geometrically simplified subsurface antenna structure which
optimizes antenna gain and is capable of directionality.
SUMMARY OF THE INVENTION
The foregoing objects of this invention are satisfied in an
embodiment in which an insulated linear radiating element
terminated in its characteristic impedance, that is matched
termination, is positioned below the surface for coupling a portion
of the surface wave component of an incident electromagnetic wave.
A plurality of reactive impedance elements are serially distributed
and interconnect portions of the radiating element at periodic
intervals along its extent. This permits matching the phase
velocity of the induced wave to the phase velocity in the
propagation medium, be it air or ground. This matching permits the
most effective antenna gain and control of directionality.
In one embodiment, the means for speed matching comprise a
plurality of capacitors periodically interposed along a
horizontally placed radiating element. In another embodiment, a
plurality of inductances are periodically interposed along the
extent of a vertically placed radiating element. In both instances,
the radiating element has a matched termination. This results in a
traveling wave because of the freedom from reflections at the
termination. In this regard, it is helpful to consider the
radiating element as a transmission line terminated in its
characteristic impedance. By using capacitive elements in the
horizontal case, the induced wave is speeded up to match the
surface wave. In the vertical case, the induced wave is slowed down
to match the propagation through the rock.
Either a space wave for the up-over-and-down mode of propagation
may be generated or a surface wave generated by a subsurface
antenna. However, in the vertical case, because of the depth of
attenuation effect in the ground, the lower portion of the vertical
antenna is not as effective as the upper portion in contributing to
the field in air. Thus, there is a maximum useful length in the
vertical extent for these transmission mechanisms.
If the radiating element current propagation constant k.sub.2 is
equal to B.sub.2 - ja.sub.2 where B.sub.2 is equal to the phase
constant and a.sub.2 is the attenuation constant, then the useful
radiation element length L.sub.eff is governed largely by the
exponential factor e.sup.-.sup.a2L, where L is the physical length
of the element. Thus, when the a.sub.2 L has the value of several
nepers, further increase in length L will no longer be useful.
However, by proper design of the antenna element, B.sub.2 can be
made to greatly exceed a.sub.2. Then, the useful physical length of
the radiating element can be many wavelengths long.
The embodiments of this invention transmit in the three principal
propagation modes and, because of directionality, are capable of
being placed in both plane and irregular topography. The structure
by using serially interposed reactive impedance elements epitomizes
geometrical simplicity. Since the phase and distance are related
along the antenna propagation axis, the vectoral addition may be
controlled to effect directionality.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1a shows a vertically disposed subsurface traveling wave
antenna;
FIG. 1b shows a horizontally disposed subsurface traveling wave
antenna;
FIG. 1c shows a subsurface communication system.
FIG. 2 shows the interaction between a surface wave and horizontal
subsurface antenna;
FIG. 3 shows experimental curves between a bare linear radiating
element and a capacitively loaded linear radiating element in a
dissipative medium;
FIG. 4 shows the amplitude of current distributions on unloaded and
the capacitor loaded bare linear raditating elements at cut-off
frequency;
FIG. 5 shows the phase shifts of the current waves on the bare
linear radiating elements;
FIGS. 6a and 6b show the amplitude and phase angle of current with
and without capacitive loading on an insulated linear radiating
element.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
In FIG. 1a, an insulated linear radiating element, such as a type
RG58/U cable (outer metallic braid and protective neoprene jacket
removed) is vertically disposed at a distance D under the ground. A
plurality of inductances 2a interconnect portions of the inner
conductor 3 along its extent. The inner conductor 3 is surrounded
by an insulating dielectric 4. The surface end of the inner
conductor 3 is shown terminated in a matched load 1. The other end
of inner conductor 3 is terminated in either a receiver or
transmitter 5 and hence to ground 6.
In FIG. 1b, inner conductor 3 has periodically interposed along its
extent a plurality of capacitors 2b. One end of the inner conductor
is terminated in load 1 with the other end of the inner conductor
being terminated in ground 6 through either a receiver or
transmitter 5.
If a bare antenna wire, such as inner conductor 3 was placed in
intimate contact with the ground, the wavelength of energy
.lambda..sub.r propagating down the antenna would approximate the
wavelength of energy .lambda..sub.g propagating in the ground. When
inner conductor 3 is insulated by dielectric 4, then the wavelength
on the wire .lambda..sub.r approximates the wavelength in the
dielectric at best if it unloaded.
In FIG. 2, receiver 5 is shown terminating one end of an antenna 3.
Load r terminates the other end of inner conductor 3 to ground. The
antenna is assumed sufficiently near the surface such that
substantial attenuation effects due to depth may be ignored. Taking
the propagation of a wave in air and earth as two of the media of
interest, the following remarks will illustrate the relevent
principles.
A surface wave with a substantial vertically polarized component 10
is assumed to be propagating along the earth-air interface from
right to left. Arbitrary points 1e, 2e, 3e, and 4e are arbitrarily
selected along the extent of antenna 3. Corresponding points 1, 2,
3, and 4 are taken at the interface. Now, the phase delay B in time
over a horizontal distance d is represented, for example, as
d.sub.43 /.lambda..sub.a (2.pi.). .lambda..sub.a is the wavelength
of the surface wave propagating in air. A portion of the surface
wave is refracted into the earth and induces a wave upon the
antenna. The phase delays are identical for distance 1 to 1e, 2 to
2e, etc. On the antenna, the wave propagates again from right to
left but with a velocity v.sub.e. The corresponding phase delay is
(2.pi.d.sub.5,3e)/.lambda..sub.e, where .lambda..sub.e is the
wavelength on the antenna. If the total delay for the wavelength
from point 4 to point 5 (the receiver) is the same regardless of
whether the path involves 4, 4e, 3, 3e, etc., then the waves will
combine in phase in the receiver. To achieve this, it is necessary
that the wavelength in the air .lambda..sub.a be the same as the
wavelength on the antenna .lambda..sub.e. That is, the wavelengths
must be equal. Since the velocity v.sub.a in air is higher than the
velocity on an antenna in the earth (v.sub.a > v.sub.e), it is
desirable to adjust the phase velocity on the antenna to match that
of the surface wave. This is accomplished in the invention by
capacitive loading.
This demonstration may be repeated for points 3, 2, and 1. If the
phase velocities are matched, then it is possible to have a
predetermined vectoral reinforcement at the receiver 5. From this
analysis it is clear that, given the use of an insulated wire and
further given that the wave velocity on the insulated wire is
normally intermediate between the free space velocity in air and
the lossy surrounding medium, then the velocity on the insulated
wire may be increased if capacitive elements are used and decreased
if inductive elements are used.
For surface wave radiation using a traveling wave antenna, the
phase constant of the antenna should be nearly equal to the phase
constant in the propagation medium. One embodiment of this
invention was tested in a salt water model tank to verify the
effects of capacitor loading on the characteristics of linear
antennas in a dissipative medium. The experiments were conducted in
a tank 18 feet in diameter and 3 feet deep. The test frequencies
were in the 2-30 megahertz range. The conductivity of the water was
maintained at 4 mhos per meter at room temperature. Under these
conditions, a 15 inch bare linear radiating element will be
electrically long, and the depth of the tank will be much greater
than the skip depth.
If the bare antenna is appropriately loaded, the input impedance
will be capacitive at frequencies below cut-off and inductive above
cut-off. The cut-off frequency is the frequency at which the phase
velocity is infinite. That is, the wavelength on the antenna is
infinite. The resistance has a minimum value at cut-off.
Measurements were made with a bare radiating element suspended
below a floating ground plane and driven against it. The capacitor
loaded monopole is essentially a series connection of 0.15
microfarad condensers spaced along the monopole length at every 1.7
inches.
In FIG. 3, the impedance versus frequency characteristics of the
loaded and unloaded radiating elements are compared. It is observed
that resonance occurred at 2 megahertz. Resonance in this context
occurs at the cut-off frequency. In order to make true measurements
of the attenuation and phase constants, the radiating elements were
lengthened to 30 inches. The long radiating elements were placed
horizontally in the tank a few inches below the water surface. The
ground plane was taken perpendicular to the antenna axis and water
surface.
In FIG. 4, for either antenna the current decreases exponentially
with distance from the feed point. The corresponding attenuation
constant was 5.7 nepers per meter for the unloaded antenna. This
was reduced to 3.8 nepers per meter for the capacitor loaded
antenna.
In FIG. 5, the phase retardation on the bare unloaded antenna
corresponded to a phase constant of 5.5 radians per meter. The
phase shift on the capacitor loaded antenna was not measurable.
This implies that the wavelength on the antenna due to loading has
become infinite.
In FIGS. 6a and 6b the data on the curves shown are measurements
taken on an antenna without matched termination. Thus, the current
in the antenna exhibits a standing wave pattern having maximum and
minimum current amplitudes separated by a distance of a quarter
wavelength. These figures clearly demonstrate the loading effects
of the propagation constant in the manner previously discussed.
In FIG. 6a on the left hand side a normalized current amplitude
I/I.sub.reference, plotted against the antenna length in meters.
Phase angle in degrees lag is plotted against antenna length in
meters on the right hand side. The applied signal frequency was 21
megahertz.
FIG. 6b shows experimental measurements of the same antenna of
three meters length. The antenna is insulated and capacitively
loaded with 400 uuf capacitors spaced at 20 centimeters apart. The
applied signal frequency was taken at 21 megacycles. The normalized
current gain is scaled on the left while phase lag is scaled on the
right. Comparison of the figures shows that the current amplitude
on the capacitively loaded antenna shows an almost linear relation
with length. In contrast, the unloaded antenna is
characteristically non-linear. Similar results are apparent when
comparing phase. Extrapolating to the case of terminating the
antennas in their characteristic impedance, it is clear that the
current phase components will add in the capacitor loaded case.
In summary, a subsurface antenna useful in transmitting or
receiving a surface wave, space wave, or ground wave energy
components has been shown in which the velocity of the wave either
radiating from or induced upon the antenna is matched to that of
the propagating medium. This matching permits a more efficient
energy transfer and further increases directionality.
* * * * *