Note: Descriptions are shown in the official language in which they were submitted.
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LENS/POLARIZER/RADOME
Background of the Invention
This invention pertains generally to directive
antennas for radio frequency energy, and particularly
to a Lens/Polarizer/Radome used in conjunc~ion with
other types of antennas.
It is sometimes necessary to modify the shape of
the antenna pattern of an array of antennas. In such
case it would be standard practice to redesign the array
to attain the desired modified antenna patternO However,
such an approach could be relatively difficult and
expensive to iinplement, especially if implementation were
to require retrofitting an appreciable number of systems
in the field.
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Summar~_s~____e_~n _ ntion
With the ore~oiny background in mind, it i~ a pr~mary
object of ~his invention to provide a Lens~Polarizer/Radome ~hat
may be easily attached to an existing array antenna to modify khe
antenna pattern in a desired way without significantl~ affe~ting
the other operating characteristies of such array antenna.
The foregoing and other objects of this lnverltlon are
attained generally by providing a Lens/Polarizer/Radome
incorporating an appropriately shaped dielectric lens alony with
lo impedance matching and filtering structures, such
Lens~Polarize.r/Radome being adapted for mounting on the existing
array antenna to form a unitary structure.
More particularly, the presen~ invention pr~vides in an
antenna system whereln the phase distribution of radio frequency
energy across ~he aperture of an antenna array is ~o be changed
from a first to a second phase distribution, khe improvemen~
comprising, (a) a dielectric lens formed o~ a material havin~ a
dielectric constant greater ~han 2.0, such lens hav:Lny a ~lrst
side with a ~urvature substantial:ly corresponcling to the curvature
~0 of the aperture of the antenna array and a second sld~ shaped to
change the phase distrlbutian of radio frequency eneryy from the
first to the second phase distrlbution; (b) impedance matching
means overlying the first side o~ ~he dielectric lens and
overlying the second side of the dielectric lens, such means being
fabricated from a sheet of dielectric material having a dielec~ric
constant substanti~lly equal to the square roo~ of the dielectric
constant of the ma~erial o~ the dielectric lens and a thickness
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substantially equal to one-quarter waveleng~h of the radlo
frequency ener~iy; (c) absorbing means disposed around the
periphery of the dielec~ric lens to control sidelobes and pattern
nulls; and (d) supporting means ~or holdiny the lens, the
lmpedance matching means and the absorbing means in the path of
radio frequency energy passing to and from the aperture, ~he
supporting means further being adaptecd to cause the first slde o~
the lens to be til~ed with reæpec~ to the aper~ure o~ the antenna
array.
2a
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Brief Descr1ption of the _ wings
For a more complete understanding of this invention,
reference is now made to the following description of the
accompanying drawings wherein:
FIG. 1 is an isometric drawing, partially cross-
sectional, showing a Lens/Polarizer/Radome according to
a preferred embodiment of this invention in place over
an array antenna; and
FIGS. 2 and 2A show a polarizer here contemplated.
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Description of the Preferred Embo_iment
Referring now to FIG. l, it may be seen that the
elements of the contemplated Lens/Polarizer/Radome are
mounted within a flan~ed rame 10 that is dimensioned to
permit mountin~ in any convenient ~anner on the ace of
an array antenna 12, here a linear array of sectoral horns
~not numbered). The elements of the contemplated
Lens/Polari~er/Radome are a dielectric lens 13, a
quarter-wave matching element 15, a polarization filter 17
and a polarizer l9. In addition, absorbers 21, 23, 24 are
- ~ provided as shown.
The dielectric lens 13, here fabricated from polyethylene
having a dielectric constant of approximately 2.3, is shaped
to have a first surface 13a complementary in shape to the
ends of the sectoral horns (not numbered). To put it
another way, irst surface 13a is shaped to present nearly an
equiphase surace to ields produced by the s0ctoral horns
(not numberedj. A second surace 13b o the dielectric
lens 13 is shaped to adjust the phase delay of rays passinq
through the dlelectric lens 13 as required to attain a
desired distribution across the aperture (not numbered) oE
the Lens/Polarizer/Radome. As is known, the phase
delay at any point through the dielectric lens 13 is
directly related to the thickness of the dielectric
lens and to the square root of the dielectric constant
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,
and inversely related to the wavelength of the electroma~netic
energy being transmitted or received. In the illustrated
example, where it is desired to increase the elevation
angle of the up~er 3 dB point of the antenna pattern,
i.e., increase the coverage in elevation, the cross-section
of the dielectric lens 13 is shaped as shown. It is
noted here that the first surface 13a of the dielectric
lens 13 need not be concentric with the end of the
sectoral horns (not numbered). As a matter of fact, in
order to optimize elevation sidelobes it is here preferred
that the dielectric lens 13 be rotated so that the
upper end of the first surface 13a is sli~htly closer
to the sectoral horn than the lower end of the first
surface 13b.
The quarter-wave matching element 15 here is a sheet of
foam rubber having a thickness of one-~uarter wavelength of
electromagnetic energy passing throu~h the dielectric
lens 13 in either direction. The dielectric constant of
the foam rubber is equal approximately to the square root
of the dielectric constant of the polyethylene of the
dielectric lens 13. The quarter-wave matching element 15
is aEfixed with an electrically thin layer of R. F . transparent
adhesive to the first and second surfaces 13a, 13b of the
dielectric lens 13.
The polarization filter 17 and polarizer 19 here are
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used to convert circularly polarized energy to linearly
polarized enerqy and vice ~ersa and to compensate for
changes in the cross-polarization component of the
electromagnetic energy out of each sectoral horn (not
numbered). As is known, such a cross-polarized component
increases with non-principal plane angles. The polarization
filter 17 is conventional, here being made up of parallel
metal plates spaced at about 0.4 wavelengths at the upper
end of the frequency band of interest and about 3/4 inches
deep. The polarization filter 17, as shown, conforms with
the polarizer 19. On transmission, then, only horizontally
polarized energy is passed through the polarization filter 17
to the polarizer 19.
Referring now to FI~S. 2 and 2A, it will be seen that
the polarizer 19 here consists of four sheets o dielectric
material essentially transparent to the radio frequency
energy passing throu~h the Lens/Polarizer/Radome, Before
assembly a metallic meanderline l9a, l9b, l9c, l9d, l9e is
formed on each one o the sheets in accordance with the table
shown in FI(,. 2A. The meanderlines are oriented so that each
is inclined at an angle of 45 to the horizontal. As a result,
then, linearly polarized energy passing through the polarizer 19
is converted to circularly polarized energy. Because the
polarizer 19 is a reciprocal device, circularly polarized
enerqy passing through the ~olarizer 19 is converted to
linearly polarized energy.
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To complete the contemplated Lens/Polarizer/Radome,
absorbers 21, 23, 24 fabricated from any known absorbin~
material are affixed (as by cementing with an electrically
thin layer of R~Fo transparent adhesive) to the perimeter
of the dielectric lens 13 and adjacent areas. The absorbers
21, 23, 24 then are effective to prevent unwanted nulls in the
antenna pattern and radiation from the ends of the dielectric
lens 13. In addition, spaces between the elements of the
just-described Lens/Polarizer/Radome preferably are filled
with dielectrlc material (not shown) having a dielectric
constant approximating 1Ø Such a filler then has no
appreciable electrical effect, but rather serves only to
make the Lens/Polarizer/Radome a unitary structure.
Having described apparatus that may be used to implement
the contemplated invention, it will now be apparent to one
of skill in the art that modifications may be made without
departin~ from the inventive concept. It is felt, therefore,
that this invention should not be restricted to its disclosed
embodiment, but rather should be limited only by the spirit
and scope of the appended claims.
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