U.S. patent number RE36,506 [Application Number 09/168,826] was granted by the patent office on 2000-01-18 for antenna design using a high index, low loss material.
This patent grant is currently assigned to California Microwave. Invention is credited to Richard C. Dempsey, Daniel W. Drago, Carl O. Jelinek.
United States Patent |
RE36,506 |
Dempsey , et al. |
January 18, 2000 |
Antenna design using a high index, low loss material
Abstract
Antenna elements and systems and other radio and microwave
frequency devices are constructed with a high index of refraction
medium having high matched values of relative permeability and
relative permittivity, and a low loss tangent. By making the
permeability of the transmission medium substantially equal to its
relative permittivity, the impedance of the material is matched to
that of the surrounding free space or air. By immersing a radiating
element in such a material, and/or by using such a material between
adjacent radiating elements or between a radiating element and a
reflective ground plane, the physical size and/or the spacing of
the elements may be substantially reduced without appreciable
performance loss, thereby resulting in a more compact device that
is particularly desirable for mobile applications. At least one
exemplary such material is formed in layers and has electrical
properties which are anisotropic and homogeneous and which vary as
a function of frequency; the layers of such a material are
preferably oriented such that the particular frequencies of
radiation propagating through each layer are presented with high
matched values of relative permittivity and relative permeability,
and low values of dielectric and magnetic loss tangents.
Inventors: |
Dempsey; Richard C.
(Chatsworth, CA), Drago; Daniel W. (Beaver Creek, OH),
Jelinek; Carl O. (Camarillo, CA) |
Assignee: |
California Microwave (Woodlane
Hills, CA)
|
Family
ID: |
22784418 |
Appl.
No.: |
09/168,826 |
Filed: |
October 8, 1998 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
|
Reissue of: |
210829 |
Mar 18, 1994 |
05563616 |
Oct 8, 1996 |
|
|
Current U.S.
Class: |
343/753;
343/700MS; 343/756; 343/909 |
Current CPC
Class: |
H01Q
1/38 (20130101); H01Q 9/285 (20130101); H01Q
15/244 (20130101) |
Current International
Class: |
H01Q
1/38 (20060101); H01Q 9/28 (20060101); H01Q
9/04 (20060101); H01Q 15/24 (20060101); H01Q
15/00 (20060101); H01Q 019/00 () |
Field of
Search: |
;343/753,756,755,7MS,795,787,909,911R |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Le; Hoanganh
Attorney, Agent or Firm: Burns, Doane, Swecker & Mathis,
L.L.P.
Claims
What is claimed is:
1. A device for use with radio or microwave frequency radiation
including a predetermined frequency of interest, said device
comprising:
at least one radiating element operatively coupled to radiation at
said predetermined frequency of interest and
one or more layers of a transmission medium having dielectric and
magnetic loss tangents each .[.substantially.]. less than 0.3 and
matched values of relative permittivity and relative permeability
.[.substantially.]. greater than 10 for said predetermined
frequency of interest,
wherein the radiating element is oriented with respect to the
transmission medium such that at least some of the radiation
coupled to the radiating element is propagated through the
transmission medium at a velocity less .[.that.]. .Iadd.than
.Iaddend.the velocity of said radiation in free space by a factor
substantially equal to said relative permittivity.
2. The device of claim 1 wherein said loss tangents are an order of
magnitude less than 0.3.
3. The device of claim 1 wherein said relative permittivity and
relative permeability are an order of magnitude greater than
10.
4. The device of claim 1 wherein said radiating element comprises
two arms of a dipole antenna element, said device further comprises
a reflective ground plane, and said transmission medium is disposed
between said arms and said ground plane.
5. The device of claim 1, further comprising a reflective ground
plane, wherein:
said transmission medium is an anisotropic transmission medium;
and
a substantial portion of the radiation propagating through the
transmission medium in the vicinity of the ground plane has its
electrical and magnetic components aligned with an axis of the
transmission medium having said high relative permittivity and with
an axis having said high relative permeability, respectively.
6. The device of claim 5 wherein said radiating element is a
capacitively loaded monopole antenna.
7. The device of claim 1, wherein:
said transmission medium is an anisotropic transmission medium;
and
a substantial portion of the radiation propagating through the
transmission medium in the vicinity of the radiating element has
its electrical and magnetic components aligned with an axis of the
transmission medium having said high relative permittivity and with
an axis having said high relative permeability, respectively.
8. The device of claim 7 wherein said radiating element is a
capacitively loaded monopole antenna.
9. The device of claim 1, wherein a substantial portion of the
radiation at an interface between the transmission medium and the
surrounding air or free space has associated electrical and
magnetic fields aligned with respective axes of the transmission
medium having said substantially equal respective values of
relative permittivity and relative permeability.
10. The device of claim 9 wherein said radiating element is a
capacitively loaded monopole antenna.
11. A direction finder device for use with radio or microwave
frequency radiation including a predetermined frequency of
interest, said device comprising:
first and second receiving elements each operatively coupled to
radiation at said predetermined frequency of interest; and
one or more layers of a transmission medium having dielectric and
magnetic loss tangents each substantially less than 0.3 and matched
values of relative permittivity and relative permeability
substantially greater than 10 for said predetermined frequency of
interest, said transmission medium surrounding said first receiving
element and oriented with respect to the first receiving element
such that substantially all of the radiation coupled to the
receiving element from a remote source is propagated through the
transmission medium in a propagation direction which is
perpendicular to an interface surface between the transmission
medium and the surrounding air or free space, said interface
surface being separated from said first receiving element along
said propagation direction by a propagation distance which is a
function of the angular orientation of said source relative to said
first receiving element.
12. A device for use with radio or microwave frequency radiation
including a predetermined frequency of interest, said device
comprising:
an antenna aperture; and
a paraboloid lens formed from one or more layers of a transmission
medium having loss tangents dielectric and magnetic loss tangents
each .[.substantially.]. less than 0.3 and matched values of
relative permittivity and permeability .[.substantially.]. greater
than 10 for said predetermined frequency of interest,
wherein the aperture is coupled to the lens such that substantially
all of the radiation passing through the aperture is propagated
through the transmission medium at a velocity less .[.that.].
.Iadd.than .Iaddend.the velocity of said radiation in free space by
a factor substantially equal to said relative permittivity.
13. A polarizer usable at a predetermined frequency of interest,
said polarizer comprising a stacked array of multi-layer
meander-line polarizer plates, each plate consisting of a
conductive surface defining a plurality of meander-lines and a
layer of a transmission medium, wherein said transmission medium
has dielectric and magnetic loss tangents each .[.substantially.].
less than 0.3 and matched values of relative permittivity and
permeability .[.substantially.]. greater than 10 for said
predetermined frequency of interest.
Description
FIELD OF THE INVENTION
The present invention relates generally to the use of a high-index
of refraction, low loss transmission medium in an electromagnetic
device, and more particularly to relatively compact antenna
elements and systems containing a high index of refraction medium
having matched values of relative permittivity and relative
permeability and low values of dielectric and magnetic loss
tangents.
BACKGROUND ART
Antenna elements and systems, and other radio and microwave
devices, are conventionally constructed from conductive radiating
elements, transmission lines and ground planes, and non-conductive
spacers, mechanical supports, and other components. Their design
typically requires the selection of appropriate materials. Size,
weight, electrical properties and environmental resistance are
primary parameters of interest.
For example, the current trend in mobile antenna designs, such as
those required by aircraft, ships and other vehicles, result in a
need for low profile, directional antenna configurations which can
conveniently be made to conform to the shape of a mobile unit, such
as an airplane wing, while providing excellent beam steering and
electromagnetic properties. Moreover, safety and fuel economy have
become important factors in vehicle mounted antenna design.
Projections from mobile antennas mounted on such vehicles are not
only hazardous, but also cause drag and instability to the vehicle
and vibration while the vehicle is in motion.
However, radiating elements must typically be positioned at least
one quarter wavelength away and parallel to a ground plane (such as
the metallic skin of an aircraft) to prevent unwanted cancellation
between the radiated signal from the radiating elements and the
reflected signals from the ground plane. When the plane of the
elements are brought closer to the ground plane, the reflected
waves from the surface interferes with the directed waves,
producing a loss in signal strength and in radiation efficiency.
Placing a dielectric substrate having a high dielectric constant
between the ground plane and the plane of the elements has been
used to minimize such losses. When a high dielectric material is
placed between the ground plane and the radiating element, the
incident radiation is slowed down by the index of refraction (H) of
the material; however, increasing the dielectric constant (relative
permittivity--.epsilon..sub.r) to 10 or more without a similar
increase in relative permeability (.mu..sub.r) results in a severe
impedance mismatch and thus is not technically desirable for many
broadband applications.
To achieve sufficient bandwidth, conventional meander-line
polarizers require multiple layers of material spaced at least one
quarter wavelength apart, and thus tend to be a wavelength in
length or longer. When such devices are applied to apertures which
are less than approximately one wavelength in size or when they are
forced into the flares of small horns, a serious deterioration in
performance results. Conventional radio and microwave frequency
polarizers are also subject to losses caused by high loss tangents
and severe impedance mismatching at the entrance and exit ports.
Prior art radio frequency and microwave lenses and other
electromagnetic devices operating in the radio and microwave
frequency ranges suffer from similar drawbacks.
It is known to reduce the size of a conventional loop or whip
antenna by embedding it in a ferrite loading material. Although it
utilizes materials which are quite lossy, such a loaded design more
than compensates for the mismatch losses that would otherwise
result between the maximum practical antenna size for a portable AM
radio (tens of centimeters) or other handheld device designed to
receive a signal in the kilohertz range, and the optimal antenna
size that would be required as those frequencies (tens of meters)
in the absence of any loading material.
It has also been proposed to use a commercially available surface
wave absorber material having a relatively high refractive index to
microwave radiation as a low propagation velocity material between
various planar radiating elements of a broadband antenna and their
respective ground planes; however, the heretofore known such
materials had a relatively high loss tangent (on the order of 0.3)
and the resultant efficiency is an order of magnitude less than
acceptable for most commercial applications.
U.S. Pat. No. 3,540,047 (Walser) discloses radiation absorbing
layers forming a three dimensional array of thin ferromagnetic
elements with all the elements having a common uniaxial anisotropy
axis, which is usable at microwave frequencies (200 mHz to 2 gHz).
Although the patent hints at the possibility of other uses
requiring a "reduced" magnetic loss tangent, the disclosed examples
are intended only to absorb incident radiation and do not appear to
have either matched values of relative permittivity and relative
permeability, or low magnetic loss tangents, at the microwave
frequencies of interest. U.S. Pat. No. 5,047,296 (Miltenberger)
discloses another anisotropic radiation absorption material formed
from layers of individual blocks of amorphous magnetic films, with
the different layers having crossed magnetic axes. That material
also does not appear to have either matched values of relative
permittivity and relative permeability, or low magnetic loss
tangents. However, at least from the latter patent, it is apparent
that the real and imaginary permeability components of the array
are primarily dictated by the corresponding properties of the bulk
material from which the individual elements are formed, and thus it
should be possible to manufacture similar materials with other
electrical properties.
DISCLOSURE OF INVENTION
The preceding and other shortcomings of prior art electromagnetic
devices are addressed and overcome by the present invention which,
in its broadest aspect provides a radio or microwave frequency
device having at least one radiating element and one or more layers
of a transmission medium having low loss tangents and high, matched
values of relative permittivity and relative permeability, with the
radiating element being oriented with respect to the transmission
medium that at least some of the radiation associated with the
radiating element is propagated through the transmission medium. In
connection with the foregoing, it should be noted that as used
herein, "radiating element" may refer to either a receiving element
which is illuminated by radiation from an external source or to a
transmitting element which radiates radiation in the direction of
an external receiver, or to a reflector (such as a conductive
ground plane) for such radiation, or to a radiation transforming
device such as a lens, prisms, or polarizer which is illuminated by
radiation from an external source and which transmits that
radiation in modified form to an external receiver.
In accordance with a first specific aspect, the invention increases
the effective spacing between the antenna element and a reflective
ground plane by orienting at least one layer of an anisotropic
transmission medium such that a substantial portion of the
radiation propagating through the transmission medium in the
vicinity of the ground plane has its electrical and magnetic
components aligned with an axis of the transmission medium having
high relative permittivity and with an axis having high relative
permeability, respectively.
In accordance with a second specific aspect, the present invention
increases the effective size of the radiating element by orienting
at least one layer of an anisotropic transmission medium such that
a substantial portion of the radiation propagating through the
transmission medium in the vicinity of the radiating element has
its electrical and magnetic components aligned with an axis of the
transmission medium having high relative permittivity and with an
axis having high relative permeability, respectively (i.e., the
ratio is essentially equal to one).
In accordance with a third specific aspect, the present invention
provides an improved impedance match between the transmission
medium and the surrounding free space by ensuring that a
substantial portion of the radiation at an interface between the
transmission medium and the surrounding air or free space has its
electrical and magnetic components aligned with respective axes of
the transmission medium having substantially equal respective
values of relative permittivity and relative permeability.
BRIEF DESCRIPTION OF THE DRAWINGS
The foregoing and additional features and advantages of this
invention will become further apparent from the detailed
description and accompanying drawing figures, in which numerals
indicate the various structural elements of the invention, like
numerals referring to like elements.
In the drawings:
FIG. 1 is a view of a dipole antenna element over a ground plane
with a high index of refraction medium between the plane of the
element and the ground plane, in accordance with one aspect of the
present invention;
FIG. 2 is a side view of the dipole antenna element shown in FIG.
1;
FIG. 3 is another side view of the dipole antenna element shown in
FIG. 1, showing the relationship between the incident radiation and
refracted radiation;
FIG. 4 is a side view of a capacitively loaded monopole antenna
element over a ground plane modified in accordance with another
aspect of the present invention, showing the orientation of the
magnetic and electrical components of the radiation;
FIGS. 4A and 4B are respective plan views of the antenna of FIG. 4,
showing two possible orientations of a layered anisotropic
transmission medium relative to the radiating element;
FIG. 5 is a view of a multi-layer meander-line polarizer modified
in accordance with the present invention;
FIG. 6 comprising FIG. 6A and FIG. 6B shows a direction finder
including a spiral of a high index of refraction transmission
medium in the transmission paths to one of its two receiving
elements employing phase difference to measure angle of arrival;
and
FIG. 7 shows a lens of a high index of refraction transmission
medium inside a radome above an antenna aperture.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENT(S)
In its broadest aspect, the present invention matches a high index
of refraction, low loss transmission medium to electromagnetic
radiation propagating from, to, or within a radio or microwave
frequency device, to thereby decrease the effective size of and/or
distance between one or more conductive elements within the device,
and is adaptable to various antenna systems, as well as to other
radio and microwave frequency devices, such as waveguides,
polarizers, diffraction gratings, prisms and lenses.
Referring now to FIG. 1, dipole element 10 includes two bow-tie
shaped arms 12 and 14 positioned on high index of refraction
substrate 18, the opposite surface of which is covered by ground
plane 16. Signal power is applied to (or received from) arms 12 and
14 by balanced feed lines 20 and 22, respectively. The construction
of dipole element 10 is similar to that of a conventional dipole
element in that it is formed by depositing, plating or etching the
metal arms 12 and 14 on the substrate 18. The surface of dipole
element 10 may be covered by the same high index of refraction
transmission material as used for substrate 18, by a conventional
radio frequency transparent material (not shown) or the exposed
surface of dipole element 10 may be positioned above the surface of
a vehicle skin so that it radiates outward directly into the
surrounding air or free space.
In accordance with one important aspect of the present invention,
the high index of refraction transmission medium forming substrate
18 has matched values of relative permittivity and relative
permeability at the frequency of interest, so that there is a good
impedance match between the transmission medium and the surrounding
air or free space.
The characteristic impedance Z of a medium is given by the
equation: ##EQU1## where Z.sub.1 =impedance of free space or dry
air=377 ohms;
.mu..sub.r =relative permeability; and
.epsilon..sub.r =relative permittivity.
From equation (1) it will be seen that, when the relative
permittivity is equal to the relative permeability, the
characteristic impedance of the medium will be the same as that of
free space or air, and the losses due to impedance mismatch will be
negligible.
FIG. 2 is a side view of dipole element 10 shown in FIG. 1 over
ground plane 16 with high index transmission medium substrate 18
filling the entire space between the plane of dipole element 10 and
ground plane 16. The outer ground conductors 20a, 22a of coaxial
feed lines 20 and 22 are electrically connected to ground plane 16,
while the inner signal conductors 20b, 22b of feed lines 20 and 22
pass through respective holes 24 and 26 in the high index
transmission medium substrate 18 and are electrically connected to
the respective inner ends of arms 12 and 14 of dipole element 10.
In a conventional dipole antenna over a ground plane, optimum
performance is obtained when the distance 38 between ground plane
16 and the plane of dipole element 10, is equal to one quarter
wavelength and the back radiation from dipole element 10 reflecting
off of (and thereby subjected to a phase delay of 180.degree.) the
ground plane 16 is in phase with, and thus reinforces, the forward
radiation from dipole element 10.
The index of refraction is given by the equation:
where
v=velocity of electromagnetic waves in the medium; and
c=speed of light in free space.
The velocity of propagation in a nonconductive material is given by
the equation: ##EQU2## where .epsilon.=permittivity; and
.mu.=permeability.
Thus, the index of refraction n is equal to: ##EQU3##
In the exemplary embodiment of FIG. 1, .epsilon..sub.r and
.mu..sub.r are both greater than 10, and preferably are
substantially greater than 10. Accordingly, the transmission medium
forming substrate 18 will have an index of refraction substantially
higher than 10 and radiation will propagate through the substrate
18 at a reduced velocity relative to its velocity in air or free
space, substantially less by a factor equal to its index of
refraction. Moreover, the physical distance through the substrate
18 corresponding to a quarter wavelength will also be substantially
less than a quarter wavelength of the same frequency in free space,
by the same factor.
FIG. 3 generally corresponds to FIG. 2, but is a ray diagram
showing incident radiation 28 propagating through free space 40,
and impinging upon the surface of high index of refraction
substrate 18. As in the example of FIGS. 1 and 2, incident
radiation 28 typically is an electromagnetic wave propagating
through free space at velocity c, with a frequency within the radio
to microwave frequency range.
Substrate 18 permits incident radiation 28 to penetrate into, and
interact with, the substrate 18. As noted previously, substrate 18
preferably has the properties of low loss tangent and high matched
values of relative permittivity and relative permeability.
Accordingly, it provides a lower velocity of propagation to
electromagnetic waves, such as incident radiation 28, and a matched
impedance to free space or air. Incident radiation 28 propagates
through free space 40 at velocity c. At the boundary 29 between
free space 40 and high index transmission medium substrate 18,
incident radiation 28 refracts due to the discontinuity between the
velocity of propagation through free space and the velocity of
propagation through substrate 18, with the refracted radiation 32
propagating at a velocity v inversely proportional to the
refractive index n of the medium.
Still referring to FIG. 3, at the boundary 36 between high index
transmission medium substrate 18 and ground plane 16, refracted
radiation 32 is reflected off of ground plane 16 at an angle
.theta.; the reflected radiation 34 intercepts arm 12 of dipole
element 10. Thus refracted radiation 32 in substrate 18 recombines
with the radiation induced in the antenna elements at a shorter
position due to the change in the index of refraction n between
free space 40 and substrate 18, resulting in a longer effective
element length for a given distance between the arms 12,14 and the
ground plane 16.
The above discussion assumes an homogeneous transmission medium 18.
Although the known high index, low loss transmission mediums at the
wavelengths of interest (radio frequency to microwave) are
fabricated in layers and have anisotropic electromagnetic
properties, in principal it should be possible to fabricate a
transmission medium from variously oriented smaller units of an
anisotropic material, such that at larger scales the material would
appear isotropic. In any event, the loss tangent, an additional
loss term due to the complex quantities of the physical constants,
should preferably be made significantly lower than found in
conventional radiation absorption materials. The loss tangent is
determined by the ratio of the imaginary component of the
permeability (or permittivity) to the real component of the
permeability (or permittivity), and can be adjusted by selecting
the mixture of materials used and by appropriate binding and curing
of the materials and designed for specific frequency bands.
In accordance with yet another aspect of the present invention, as
shown in FIG. 4, rather than using an isotropic material as the
high index transmission medium 18, the transmission medium 42 may
be oriented with respect to the E and H vectors associated with the
radiation propagating through the material such that the desired
high matched values of the relative permittivity and relative
permeability (which will in general be different for different axes
of the material) are associated with axes aligned with the E and H
vectors, thus providing the desired lower velocity of propagation
to electromagnetic waves and a matched impedance to free space or
air. In particular, FIG. 4 shows a capacitor loaded monopole
antenna including a vertical radiating element 44 extending through
a hole 24 in the ground plane 16 and connected to the inner
conductor 20b of a single coaxial conductor 20 whose grounded outer
conductor 20a is connected to ground plane 16. The other end of the
vertical radiating element 44 is terminated by a circular cap 46,
which capacitively loads the element 44. In accordance with the
present invention, it is desirable to immerse vertical element 44
in one or more layers of high index transmission medium 42 with an
axis of the material 42 having a high relative permittivity aligned
the E vectors (which as shown by the solid arrows, are vertical in
the vicinity of the vertical conductor 44 and in the vicinity of
the ground plane, and have a vertical component throughout the
transmission medium 42) and having a high value of relative
permeability aligned with the H vectors (which form concentric
circles about vertical radiating element 44 perpendicular to the E
vectors, and which in the cross section shown in the figure, are
also perpendicular to the plane of the figure). Assuming that the
anisotropic transmission medium 42 is formed in layers with the
desired high value of permeability being associated with only one
axis H and that axis is the plane of each layer and that the
desired high value of permittivity is associated with at least one
other axis E also in the plane of each layer and perpendicular to
the axis of high relative permeability, then (as illustrated in the
top view of FIG. 4A) the required orientation can be accomplished
by orienting each of the layers forming the anisotropic
transmission medium 42 as one or more separate concentric
cylindrical segments 42a about the vertical radiating element 44,
with the high relative permeability axis H' perpendicular to the E
vectors and generally parallel to the ground plane. On the other
hand, if the desired high value of permittivity is associated with
an axis perpendicular to the individual layers, and the desired
high permeability is associated with at least one axis H in the
plane of each layer, then (as illustrated in FIG. 4B) the layers
can arranged as stacked layers 42 parallel to ground plane 16, each
layer 42 comprising a plurality of circular segments 42b
surrounding vertical element 44 with its high permeability axis H"
perpendicular to the radius. In a similar manner, any high index of
refraction transmission medium having at least two perpendicular
axes associated respectively with a desired high permeability and a
desired high permittivity can be decomposed into layers of
individual cylindrical segments 42a, circular segments 42b, or
other similar layer-like geometrical elements such that the
mutually perpendicular E and H vectors in the transmission medium
42 may be substantially aligned with mutually perpendicular axes of
the transmission medium having the desired high permittivity and
permeability values.
As a practical matter, because the electrical and magnetic fields
in the near field of a radiating element depend on the charge
distribution and current density at different portions of the
radiating element and are therefore difficult to calculate a
priori, it is preferable to measure the relevant electric and
magnetic field intensity vectors E and H experimentally for a
particular configuration of antenna elements at the frequencies of
interest. Once the near field electric and magnetic vectors have
thus been determined experimentally, the individual layers forming
the high index of refraction substrate can each be oriented with
the relevant axes aligned with those vectors.
The present invention is not limited to the application of high
index of refraction transmission medium to the antenna
configurations described above. Using the same concepts and
principles described above, high index transmission medium may be
used in the construction of radio and microwave frequency devices
which are smaller and lower loss. For example, such a high index of
refraction, low loss transmission medium can be used in polarizers,
radio frequency lenses, prisms, diffraction gratings, loaded wave
guides, and other radio and microwave devices which are smaller and
lower loss.
In particular, the present invention can be used to design smaller
and lower loss polarizers. Referring to FIG. 5, multi-layer
meander-line polarizer plate 100 includes meander-lines 102 etched
on a conductive upper surface 103 of high index of refraction
transmission medium layer 104. The physical configuration of
polarizer plate 100 is similar to that of conventional multi-layer
meander-line structures, and may be formed as a bonded sandwich of
a plurality of such etched sheets of high index of refraction
transmission media 104. In general, a greater number of such layers
yields greater bandwidth.
It will be appreciated that, in accordance with the present
invention, polarizer plate 100, fabricated from a high index of
refraction, low loss transmission medium, is substantially smaller
than conventional multi-layer meander-line structures. In
particular, polarizer plate 100 can be made electrically shorter by
an amount equal to the index of refraction n of the transmission
medium 104.
FIG. 6A is an isometric view of a prior art direction finder
modified in accordance with the present invention. In particular,
it includes a stacked pair of vertical receiving elements 50, 52,
with the lower element 50 immersed in a spiral 54 of transmission
medium 56 having an index of refraction greater than that of the
surrounding free space. Accordingly, as shown in the plan view of
FIG. 6B, radiation from a remote source will have to travel through
a thickness of the transmission medium 56 which is a linear
function of the angle of arrival .dwnarw., and therefore will be
delayed in time or phase by a linear function of theta, relative to
the time or phase the same signal is received by upper element 52.
Therefore, by measuring the phase or time difference of arrival
between the loaded and unloaded antenna elements, the angle of
arrival may be calculated. However, unlike the known direction
finder, the transmission medium has loss tangents substantially
less than 0.3 and matched values of relative permittivity and
relative permeability substantially greater than (and preferably an
order of magnitude greater than) 10 for a predetermined frequency
of interest. Since the delay is a function of the index of
refraction times the distance, the present invention permits a more
compact and efficient unit than would otherwise be possible. FIG. 7
shows yet another application of some of the principles underlying
the present invention, this time to a lens 60 inside a radome 62.
Sidelobe absorbers 64 of a convention high loss material are
provided at either side of an antenna aperture 66. The lens 60 is
circularly symmetric about the antenna boresight 68 and may be
formed from a stack of paraboloid-shaped layers 60a of a low loss,
high index of refraction transmission medium having equal high
values of permittivity and permeability in the plane of the
material and low dielectric and magnetic loss tangents.
Persons skilled in the art should realize that the scope of the
present invention is not limited to what has been shown and
described hereinabove, but only by the claims which follow.
* * * * *