U.S. patent number 3,818,490 [Application Number 05/277,932] was granted by the patent office on 1974-06-18 for dual frequency array.
This patent grant is currently assigned to Westinghouse Electric Corporation. Invention is credited to Henry C. Leahy.
United States Patent |
3,818,490 |
Leahy |
June 18, 1974 |
DUAL FREQUENCY ARRAY
Abstract
An antenna array having two repetitive radiator systems in a
single aperture, operating in two distinct frequency ranges. Each
radiator system includes an open-ended, circular waveguide and a
parallel plate waveguide. A monopole or a dipole is situated
between the plates of the parallel plate waveguide and normal to
them. The dimensions of the waveguides and the spacings between
them are chosen to provide isolation between the frequency
ranges.
Inventors: |
Leahy; Henry C. (Glen Burnie,
MD) |
Assignee: |
Westinghouse Electric
Corporation (Pittsburgh, PA)
|
Family
ID: |
23062997 |
Appl.
No.: |
05/277,932 |
Filed: |
August 4, 1972 |
Current U.S.
Class: |
343/727; 343/779;
343/846; 343/786 |
Current CPC
Class: |
H01Q
21/06 (20130101); H01Q 21/24 (20130101); H01Q
5/42 (20150115) |
Current International
Class: |
H01Q
21/24 (20060101); H01Q 5/00 (20060101); H01Q
21/06 (20060101); H01q 021/00 () |
Field of
Search: |
;343/725-730,854,786,846 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
Other References
Hsiao, Multiple Frequency Phased Array Of Dielectric Loaded
Waveguides; 1970 G-AP International Symposium September 1970,
Columbus, Ohio.
|
Primary Examiner: Lieberman; Eli
Attorney, Agent or Firm: Hinson; J. B.
Claims
I claim:
1. An antenna adapted to operate in first and second frequency
ranges comprising:
a. a plurality of radiating horns for radiating energy within said
first frequency range;
b. a plurality of dipole radiating elements transversely mounted in
a plurality of sections of open waveguide, each of said dipoles
being designed to radiate energy in said second frequency
range;
e. means for mounting said horns and said sections of open
waveguide to form a unitary antenna aperture comprising rows of
radiating elements, the type of radiating element comprising the
rows alternating between horns and dipoles in a direction
substantially ninety degrees with respect to said waveguides, with
said first and second frequency ranges, the dimension of said
horns, the dimension of said dipole elements and the dimension of
said open waveguide being selected to reduce the coupling between
said horns and dipoles.
2. An antenna adapted to operate in first and second frequency
ranges, said antenna comprising a plurality of array elements
assembled to form a unitary aperture structure with each element
being substantially identical and including a first horn radiating
element comprising an opening in a conductive strip for radiating
energy within a first selected frequency band and a dipole element
for radiating energy within a second selected frequency band, said
dipole element being mounted substantially parallel to said strip
and transverse across and near the open side of a section of open
waveguide, the dimensions of said openings in said conductive
strip, said dipole and said open waveguide being selected such that
coupling between said horns and dipole is minimized, said elements
being assembled such that said dipoles and horns form rows with the
type of radiating element alternating between rows.
3. An antenna array in accordance with claim 1 in which the spacing
between adjacent radiating horns is less than one half of the
wavelength of the highest frequency within said first frequency
range.
4. An antenna in accordance with claim 1 in which the mechanical
spacing of said dipoles is the same as the spacing of said
radiating horns.
5. An antenna adapted to operate in first and second frequency
ranges, said antenna comprising a plurality of array elements
assembled to form a unitary aperture structure with each element of
said array being substantially identical and including, a first
radiating element comprising an opening in a conductive strip to
form a horn for radiating energy within a first selected frequency
band and a second dipole radiating element mounted transversely
across and near the open edge of a section of open waveguide for
radiating energy within a second selected frequency band, the
dimension of said opening in said conductive strip, said second
radiating element and said waveguide being selected such that
coupling between said first and second radiating elements is
minimized said elements being assembled to form said aperture such
that said first and second radiating elements form rows with the
type of radiating element alternating between adjacent rows.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
In general, the present invention pertains to a new antenna
structure. More specifically, it pertains to an antenna array which
couples to two distinct frequency ranges while using the same
aperture. That is, it pertains to a situation where it would be
necessary to have two systems operating on two different
frequencies but where there is a shortage of space. In such a
situation, it is highly desirable to minimize the antenna aperture
-- that is, the area of the antenna.
The invention described hereinafter is particularly useful in a
phased array -- that is, an antenna which has a large number of
radiators which are individually controllable to some extent. In a
phased array, the excitation of space is controlled by a large
number of independent, point variables. Each of the independent
point variables is an individual element of the phased array, the
individual excitation of each element being adjustable. If desired,
a phased array can be used for scanning purposes. That is, it is
possible to steer the beam and change its shape by changing the
excitation functions of each element. When the excitation functions
of each element are changed very fast, scanning occurs very
fast.
Because a phase array can operate very quickly, not retarded by
mechanical inertia they are very desirable pieces of equipment.
However, they are also very expensive. Therefore, it is very
desirable to make the most efficient use of the antenna aperture
and the electronics equipment required to operate the antenna. For
example, it is desirable to have the antenna perform two distinct
functions substantially simultaneously.
For example, it might be desirable for an airplane to carry an
antenna system both for the purpose of mapping the terrain below it
and, simultaneously, keeping track of all other aircraft in the
immediate vicinity in order to avoid a mid-air collision. While
performing a mapping function, angular resolution is extremely
important in order to determine exactly where the antenna is
pointing within predetermined tolerances. A narrower beam width is
necessarily a requirement of increased angular resolution. Because
higher frequencies provide narrower beam widths, the greater the
angular resolution required for the particular mapping function,
the higher the frequency required.
In providing a search function, one of the main objectives is
detection of objects of interest at larger and larger ranges. In
order to increase the range of detection, the antenna must be
operated at a lower frequency as range increases. When an antenna
is operated at lower frequencies, its energy is more easily able to
penetrate clouds, water, and water vapor. The difficulty of
penetrating such atmospheric disturbances increases as frequency
increases. Therefore, a search radar uses frequencies in a
relatively lower frequency range in order to increase its range
capabilities.
As discussed above, the present invention pertains to a system
which permits a single antenna aperture to perform two functions.
For example, the antenna to be described hereinafter could provide
both the above described mapping function and search function
because each requires a different, distinct frequency range of
operation. However, the invention to be described could be used for
any two functions which can be performed by using two distinct
frequency ranges and which are separated by a sufficiently large
frequency range.
2. Description of the Prior Art
A commonly considered method of designing the transmitter
electronics of an active antenna array is the use of a transmitter
amplifier cascaded with a varactor multiplier. Because an antenna
array utilizes large numbers of radiators, construction of such an
array has posed a problem of combining large numbers of low powered
sources with minimal loss. An excellent solution to this problem
has been to associate a system having individual array elemental
radiators with individual transmitters. In a system thus
configured, the varactor multipliers constitute the major source of
inefficiency in converting raw power into usable radiated energy.
Nevertheless, their use is mandatory in most of such systems where
the radiated frequencies lie above the capabilities of existing
transistors.
As explained above, there are situations in multi-mode radars where
high frequencies are required for some functions but not for
others. In such cases, one approach is to use the amplifier output
as the directly radiated signal thereby eliminating the necessity
of transmission through the varactor multipliers. The varactor
multipliers often reduce radiated energy by 50 percent to 80
percent. However, this new approach has led to problems in other
areas of design, one of which, radiation structure, is a subject of
this invention.
BRIEF SUMMARY OF INVENTION
The antenna to be described hereinafter in detail is made up of a
plurality of juxtaposed basic radiator structures. Each of the
radiator structures is, in turn, made up of a plurality of first
radiator elements and a plurality of second radiator elements. The
first and second radiator elements are each capable of coupling
only to their respective frequency ranges. When the basic radiator
structures are arranged in a predetermined manner, the result is a
first repetitive radiator system and a second repetitive radiator
system which, together, form an antenna array.
The first radiator system is made up of a plurality of rows of a
certain type of radiator elements. Interspersed between the rows of
these elements are rows of a second kind of radiator element. For
each element in the first system there is a corresponding element
in the second system. Each row of the first system has a conductive
strip which helps to form the first radiator elements. The second
system is made up of a plurality of parallel plate waveguides with
either a monopole or a dipole situated between the plates of the
waveguides and normal to the plates.
BRIEF DESCRIPTION OF THE DRAWINGS
For a better understanding of the invention, reference may be had
to the preferred embodiment, exemplary of the invention, shown in
the accompanying drawings, in which:
FIG. 1 is a normal view of the basic radiator structure of the
antenna array;
FIG. 2 is an end view of the basic radiator structure;
FIG. 3 is a perspective view of the parallel plate waveguide
section of the basic radiator structure;
FIG. 4 is a perspective view of an alternative embodiment of the
parallel plate waveguide portion of the basic radiator
structure;
FIG. 5 is a normal view of a portion of an antenna array comprising
a plurality of basic radiator structures; and
FIG. 6 is an end view of the basic radiator showing the R.F. energy
coupling lines.
DETAILED DESCRIPTION OF THE INVENTION
The antenna described herein is composed of a plurality of basic
radiator structures. When these basic radiator structures are
juxtaposed in such a manner that each basic structure is in
physical contact with a least one other basic radiator structure,
the totality of these structures form an antenna array. Referring
briefly to FIG. 5, an antenna 100 is shown as being composed of a
plurality of radiator elements 10 and 12. For analysis purposes, in
order to break down the array 100 into a basic radiator structure,
an elemental, repetitive component has been chosen. Examples of
such a component are indicated in FIG. 5 as dashed blocks 80, 85,
and 90. Each of these dashed blocks represent a predetermined
portion of the antenna aperture -- that is, a predetermined portion
of the antenna area. Each of the blocks covers an equal area. For
purposes of explanation herein, block 90 is the easiest to
explain.
Referring to FIG. 1, block 90 is shown as the basic radiator
structure. It will be understood, however, that any component of
like size could be chosen to explain the structure and operation of
the present invention. As explained above, block 80 or block 85
could have been chosen.
Referring to FIG. 1, the basic radiator structure 90 is enclosed by
and includes three parallel plates 15, 16, and 17. Plates 15 and 16
are part of and enclose a first radiator element 18 which is
operable to couple to a first frequency range -- a high frequency
range such as 9-10 GHz. Plates 16 and 17 are part of, and enclose,
a second radiator element 19 which is operable to couple to a
second frequency range -- a low frequency range such as 1.8-2.0
GHz.
A fundamental problem which is solved by the present invention is
isolation of the two radiation systems. Isolation is quite
essential if two plane scan is to be accomplished with maintenance
of element drive point impedance and/or more than trivial bandwidth
is to be realized. Isolation is accomplished by the present
invention which uses crossed linear polarizations and cut off
phenomena.
The first radiator element 18, hereinafter referred to as the high
frequency radiator, includes a conductive strip 20 composed of
metals, copper, brass, aluminum or silver, for example. An aperture
22 is made in the conductive strip 20 in order to couple to the
high frequency energy. Because most antennas are reciprocal -- that
is, they can be used either to transmit or receive energy, the
function of the present invention is not to be construed as being
limited to either of these functions. Accordingly, the term
"couple" will be used throughout the present application to connote
both transmitting and receiving functions.
The aperture 22 in the conductive strip 20 can be referred to as a
small horn or as an open ended waveguide. While it is recognized
that some people skilled in the antenna arts might make definite
distinctions between what they call a "waveguide" and a "horn", it
is difficult to determine exactly when a radiator becomes so small
that the terminology used to describe it can be changed from horn
to waveguide. Therefore, even though the two terms can be used
interchangeably in the present application, no significance is
intended to be attached to one term over the other.
In the embodiment shown in FIG. 1, the aperture 22 of the high
frequency radiator 18 has a circular cross-section -- that is, it
can be said to be X and Y symmetric. It excites, in space, a far
field TEM wave with a horizontal E field. Because of the technique
of cross-polarization, the horizontal E field coupled to the high
frequency radiator 18 cannot couple to the low frequency radiator
19 as will be described in more detail below.
Although FIG. 1 shows a circular aperture 22, the shape of the
aperture need not be circular. For example, the shape could be
square or rectangular. In the case of the circular aperture 22,
shown in FIG. 1, the diameter of the aperture must be greater than
one-half the wavelength of the highest frequency in the high
frequency range. If it is desired to make the basic radiator
structure 90 as small as possible, the actual physical diameter of
the aperture 22 can be made to be less than one-half of the
wavelength but electrically, the effective width must be greater
than one-half of the wavelength. Such an effective width is
essential to obtain propagation of the high frequency energy
through the high frequency radiator 18 -- that is, through the
aperture 22 and the waveguide connected to it. In order to increase
the effective cross-section or width of the aperture 22, the
aperture must be filled with a suitable, low loss dielectric
material. An example of such material is polystyrene
(.epsilon..sub.r = 2.6) or Teflon (.epsilon..sub.r = 2.07). When
such a dielectric material is used, the effective diameter of the
aperture 22 is determined by the relationship D.sub.eff = D.sub.act
.sqroot..epsilon..sub.r .
If a shape other than circular is used for the aperture 22, one
effective dimension can be arbitrary but the other effective
dimension must be determined by other considerations. One
consideration is the cross polarization effect to prevent coupling
of the low frequency energy into the high frequency radiator.
Accordingly, the dimension of the aperture which is perpendicular
to the direction of the E field of the low frequency radiator (that
is, parallel to the direction of the E field of the high frequency
radiator) must be less than 1/2.lambda..sub.l
/.sqroot..epsilon..sub.r where .lambda..sub.l is the wavelength of
the lowest frequency of the high frequency range. In one
embodiment, the E field of the low frequency radiator will be
vertical and the E field of the high frequency radiator will be
horizontal the E field of the high frequency. As explained
previously, propagation cannot occur unless the effective
electrical width is greater than one-half of the wave-length of the
frequency being propagated. Because wavelength increases as
frequency decreases, the dimension of the aperture perpendicular to
the E field, must be greater 1/2.lambda./.sqroot..epsilon..sub.r
for the greatest usable wavelength. In the present invention, the
largest wavelength in the high frequency range will therefore occur
at the lowest frequency in that range.
In addition to the consideration of cross polarization, the high
frequency range must be chosen so that it cannot couple into the
low frequency radiator. That is, the high frequency being
propagated must be well beyond the cut off of the lower frequency
parallel plate waveguide. Stated another way, the lowest frequency
in the high frequency range must be substantially greater than the
highest frequency of the low frequency range.
Referring briefly to FIG. 2, it can be seen that the aperture 22
extends for an arbitrary distance in depth. It need only be
extended far enough so that the energy can be coupled from the high
frequency generator (not shown) by any convenient means such for
example a mixer or a transmitter.
Referring again to FIG. 1, the second radiator element 19,
hereinafter referred to as the low frequency radiator, is shown
juxtaposed to and touching the high frequency radiator 18. It can
be said to be touching the high frequency radiator 18 because of
the common plate 16 which is common to both the high frequency
radiator and the low frequency radiator. In describing the low
frequency radiator 19, reference will be made to both FIG. 1 and
FIG. 2.
The low frequency radiator 19 includes two parallel plates 16 and
17 which make up a parallel plate waveguide. The spacing between
the plates is determined by the high frequency range. That is, the
cut off frequency of the parallel plate waveguide is determined by
the high frequency range. The cut off frequency is then set by
proper spacing between the plates.
The spacing between the plates 16 and 17 of the parallel plate
waveguide is made small compared to the wavelengths of both
frequency ranges. Specifically, the spacing between the plates 16
and 17 is made less than one-half of the wavelength of the highest
frequency of the high frequency range. Consequently, the parallel
plate waveguide will be able to propagate TE waves with the E field
normal to the plates at all frequencies. TE modes with the E field
parallel to the planes of the plates (or any other modes) can
propagate only at frequencies above the cut-off frequency of the
appropriate mode. The lowest cut-off frequency is defined by the
relationship c/.sqroot..epsilon..sub.r, where c is the free space
velocity of light, L is the spacing between the plates and
.epsilon..sub.r is the free space normalized dielectric constant of
the material between the plates. L and .epsilon. are chosen to
yield a cut-off frequency above the highest frequency of the high
frequency operating band. Therefore, there will not be any
frequencies which can couple to the high frequency radiator which
will also be under the cut-off frequency of the low frequency
radiator. To summarize, to prevent cross coupling between high and
low frequency radiators, the field coupled to the low frequency
radiator has an E field which is normal to the plates of the
parallel plate waveguide. On the other hand, the E field of the
high frequency radiator 18 is parallel to the plates 15 and 16 and,
consequently, parallel to the plates of the parallel plate
waveguide 16 and 17. Partially as a result of this cross
polarization and because of the above described cut-off features of
each radiator segment, the energy of the high frequency radiator
cannot be coupled into the low frequency radiator and vice
versa.
The low frequency radiator 19 also includes a monopole 24 which is
disposed between the plates 16 and 17 essentially in the same plane
as the conductive strip 20 of the high frequency radiator 18. The
specific structure and location of the monopole 24 will be
described in greater detail below.
Referring to FIG. 2, it can be seen that the parallel plates 16 and
17 are short circuited by a conductive strip 26. The short circuit
26 is located at a distance which is measured from the center of
the monopole 24. The specific distance is one quarter of the
wavelength of the midband frequency of the low frequency range. If
the monopole 24 is placed sufficiently close to the plane of the
conductive strip 20, the parallel plate waveguide appears as an
open circuit or of small reactance at the plane of the conductive
strip 20 for TE waves with the E field normal to the plates. As a
result, during transmission, all of the transmitted energy will
radiate out from the front of the waveguide instead of only half of
the energy. That is, the monopole will radiate into half-space. All
other x polarized waves impinging on this structure impinge on a
waveguide beyond cut-off and the surface appears as an inductive
surface with evanescent fields existing in the space between the
plates.
FIG. 3 is a cut-away, perspective view of the parallel plate
waveguide previously discussed in FIGS. 1 and 2. FIG. 3 shows the
monopole placed across the open end of the waveguide and normal to
the waveguide plates 16 and 17. The waveguide is open circuited for
the fields the monopole can excite and the free space on the other
side of the monopole has a real impedance into which energy is
radiated. The radiation resistance of the monopole is the impedance
of free space multiplied by the ratio of the unit cell dimensions.
The unit cell can be defined by the unit vectors describing the
monopole location.
The precise location fore and aft of the monopole is not important.
However, it is desirable to locate it as close as possible to the
edges 16a and 17a of the plates 16 and 17 -- that is, so that it is
essentially in the same plane as the conductive strip 20 which was
described in FIG. 1 and FIG. 2. It is desirable to place the
monopole in such a position because the driving point impedance of
the parallel plate waveguide becomes increasingly frequency
sensitive as the monopole is moved away from the edges 16a and 17a
of the parallel plates.
The monopole 24, in an operative embodiment, is constructed of the
inner cable of a coaxial cable. However, it could just as well be
constructed of any wire conductor. It is not necessary to
physically connect the monopole 24 to the plate 17. However, it is
preferable to have the monopole 24 physically and electrically
connected to said plate. The connection can be made by a simple
soldering operation. If the monopole is not physically connected to
the plate 17, a high capacitance electrical connection between the
lower end of the monopole and the plate 17 is required. The
diameter of the monopole 24 is significant only insofar as its
physical strength is concerned. That is, it should not be so thin
that it is unable to remain in its preset position without falling
over or that it breaks easily.
In order to connect low frequency energy to the low frequency
radiator 19, the low frequency generator (not shown) is connected
to two parts of the low frequency radiator by means of two wires 26
and 28, which may form the inner and outer conductors of a coaxial
cable.
In order to connect the wires 26 and 28 to the appropriate places
of the low frequency radiator 19, a small hole is cut into the
plate 16. In order to more easily illustrate the structure, FIG. 3
shows only a cut-away view of the upper plate. However, it will be
understood that FIG. 3 only shows half of the hole 30 as
illustrated by solid line 31. In fact, the hole is complete as
illustrated by dotted line 32. The dimension of the hole does not
matter as long as it is significantly smaller than the aperture 22
associated with the high frequency radiator 18 described in FIG. 1.
Furthermore, it does not matter if the monopole 24 is thin enough
so that it can extend up into the hole 30. Or, if desired, the
length of the monopole 24 can stop just short of the underside of
the plate 16. A wire 28 is attached to the upper end of the
monopole 24 and is extended through the hole 30 for connection to
one terminal of the low frequency generator. The other wire, 26, is
connected to the other terminal of the low frequency generator and
is connected to the edge 31 of the hole 30.
FIG. 4 shows an alternative embodiment to the use of a monopole --
a dipole. The dipole location is to the monopole location which was
described in FIG. 3. That is, it is located as close as possible to
the edges 16a and 17a of the plates 16 and 17. In an operative
embodiment, the dipole consists of a solid conductor 34 and a
hollow metal sleeve 36 which extends downwardly from the plate 16
toward the plate 17. The sleeve 36 has a circular cross section and
has an aperture in its center. The sleeve 36 forms the upper half
or arm of the dipole and the solid conductor 34 forms the other of
the dipole. Although sleeve 36 is shown in FIG. 4 in a cross
sectional view, it will be understood that it forms a complete
cylinder. Wire 28 is connected to the top of the solid conductor 34
and the other end of conductor 28 is connected to one terminal of
the low frequency generator (not shown). The other terminal of the
low frequency generator is connected to the wire 26 which, in turn,
is connected to the inside face of the sleeve 36.
In order to construct the antenna array 100 shown in FIG. 5, a
plurality of the above-described basic radiator structures are
juxtaposed to one another. It will be noted, however, as discussed
previously, that the basic radiator structure could have been
considered to be block 80 or block 85 of FIG. 5. If either of those
blocks were used, the basic radiator structure would have had
portions of more than one low frequency radiator such as shown by
block 80 or it would have had portions of more than one high
frequency radiator as shown by block 85. Irrespective of which type
of basic radiator structure is used in the analysis, the antenna
array 100 shown in FIG. 5 is composed of a plurality of them.
FIG. 5 shows only a portion 100 of the entire antenna array
aperture. The array includes two repetitive systems. A portion of
the first repetitive system is shown as row 40 which includes and
is enclosed by plates 43 and 45 each of which extend, as shown in
FIG. 5, in a horizontal direction to their respective ends of the
antenna array. The first repetitive radiator system is adapted to
operate in the first or high frequency range. Its structure
consists of a plurality of high frequency radiators each of which
is similar to the high frequency radiator 18 of FIG. 1. That is, it
includes a conductive strip 44 and a plurality of apertures 46
located in the conductive strip. The dimensions of the apertures 46
are chosen in the same manner as explained with respect to aperture
22 of the high frequency radiator 18 in FIG. 1. Each such radiator
is connected to a different high frequency generator.
The antenna array 100 also has a second repetitive radiator system
which is adapted to operate in the second or low frequency range.
One portion of the second repetitive radiator system is indicated
as row 42 which includes and is enclosed by the plates 45 and 47.
These plates also extend in a horizontal direction to their
respective ends of the antenna array. These plates, 45 and 47, form
a parallel plate waveguide the dimensions of which are determined
by the same considerations which were explained above with respect
to low frequency radiator 19 shown in FIGS. 1 and 2. At
predetermined locations throughout the parallel plate waveguide are
located a plutality of monopoles or dipoles for the same reasons
and of the same dimensions and installed into the parallel plate
waveguide in the same manner as was explained in FIGS. 1, 2, and 3.
Each such radiator may be connected to a different low frequency
generator. For every aperture 46 there is a corresponding monopole
or dipole 48. That is, the two are present in their respective
repetitive systems on a one to one basis.
As explained above, the row 40 is part of a first repetitive
radiator system and the row 42 is part of a second repetitive
radiator system. Each row is substantially repeated in alternate
rows. That is, the structure of row 40 occurs both above and below
row 42. The row occurring after row 42 has been designated as 40a.
Likewise, a row substantially identical to row 42 occurs both above
and below row 40a. The row below row 40a has been designated row
42a. Similarly, row 40b is substantially the same as row 40 and
occurs after row 42a; and row 42b is substantially the same as row
42 and occurs after row 40b.
In order to insure that the array operates properly, a number of
constraints are necessary. Although it is not necessary that the
monopoles 48 be located exactly as shown in FIG. 5 with respect to
the apertures 46, once a particular location has been chosen for
each, relative to one another, that relative position must be
maintained throughout the array. For example, if the longitudinal
axis of the monopole 48 is exactly aligned with the vertical
diameter of the aperture 46 in one particular one to one
relationship, the same relative position must be maintained between
all of the monopoles 48 and apertures 46.
In the following discussion of the antenna array, it is easier if
we define the high frequency radiator elements to be the apertures
46 and the low frequency radiating elements to be the monopoles 48.
However, it will be understood that whenever the term "high
frequency radiator" is used that it is also describing all of the
structure and factors considered with respect to FIGS. 1, 2, and 3.
In addition, whenever the term "low frequency radiator" is used it
will incorporate by reference all of the information discussed
above with respect to FIGS. 1, 2, and 3.
The space between each of the high frequency radiators 46 must be
on the order of one-half of the wavelength of the highest frequency
of the high frequency range. That is, the spacing between the
apertures 46 cannot be greater than said distance. The spacing
between the monopoles 48 is the same as the spacing between the
high frequency apertures 46. This is done partially in order to
more easily obtain the basic radiator structures described above.
It is also done to prevent coupling of energy from the high
frequency radiators into the low frequency radiators and vice
versa. Moreover, at the frequency at which it is operating, that
is, at a low frequency, the spacing of the low frequency radiators
cannot be greater than one-eighth of a wavelength. This latter
constraint, then, partly determines the low frequency range. The
maximum frequency of the low frequency range cannot be greater than
one-quarter of the minimum frequency of the high frequency range
because the drive point impedance of the low frequency radiators
becomes highly reactive for wider spacings. The spacing between the
low frequency radiators has been chosen to be not greater than
one-eighth of a wavelength of the lowest frequency of the low
frequency range because at low frequencies, the Q (the ratio of
reactance to resistance) gets better -- that is, the Q gets lower.
When designed in such a manner, the low frequency repetitive system
is equivalent to a current sheet. The current can be controlled as
a function of position in the sheet.
The only coupling possibilities are monopole (dipole) and/or
waveguide to near field vertically polarized waves. Since these
waves do not exist in the high frequency far field, the total
contribution to such waves of each elemental high frequency
radiators near field must be zero.
Typical frequency ranges for the high frequency radiator is 9 to 10
GHz, and 1.8 to 2 GHz for the low frequency radiators.
* * * * *