A low loss dichroic plate is disclosed for passing radiation within a particular frequency band and reflecting radiation outside of that frequency band. The dichroic plate is comprised of a configuration of dipole elements defined by slots formed in a conductive plate. The slots are dimensioned so as to pass radiation of a selected frequency and are shaped so as to minimize the relationship between that frequency and the tilt angle of the plate relative to the direction of radiation. The slots are arranged so as to minimize signal power loss due to cross polarization effects

Ludwig, A. C.

Woo, R. T.

English

NASA Technical Reports Server

NATIONAL AERONAUTICS AND SPACE ADMINIStRATION
WASHINGTON. D.C. 20546
REPLY TO ,
 29 1973ATTN OF. GP *•
TO: KSI/Scientif ic & Technical Information Division
Attention: Miss Winnie M. Morgan
PROM: GP /Off ice of Assistant General Counsel for
Patent Matters
SUBJECT: Announcement of NASA-Owned U.S. patents in STAR
In accordance with the procedures agreed upon by Code GP
and Code KSI, the attached NASA-owned U.S. Patent is being
forwarded for abstracting and announcement in NASA STAR.
The following information is provided:
U.S. patent No. ; J H L°l
Government or
Corporate Employee
Supplementary Corporate ~r O /
Source (if applicable) : 0 ' ^
NASA patent Case No. ; A-'Pt) - I
NOTE - If this patent covers an invention made by a corporate
employee of a NASA Contractor, the following is applicable:
Yes /.X/ No / /
Pursuant to Section 305(a) of the National Aeronautics and
Space Act, the name of the Administrator, of NASA appears on
the first page of the patent; however, the name of the actual
inventor (author) appears at the heading of column No. 1 of
the Specification, following the words "... with respect to
an invention of .
Elizabeth A. Carter
Enclosure
Copy of Patent cited above
NASA-HQ
https://ntrs.nasa.gov/search.jsp?R=19740002887 2020-03-17T14:20:40+00:00Z
United States Patent
Fletcher et al.
[19] mi 3,769,623
[45] Oct. 30, 1973
[54] LOW LOSS DICHROIC PLATE
[76] Inventors: James C. Fletcher, Administrator of
the National Aeronautics and Space
Administration with respect to an
invention of; Richard T. Woo;
Arthur C. Ludwig, both of La
Canada, Calif.
[22] Filed: Sept 21, 1972
[21] Appl. No.: 290,915
[52] U.S. Cl 343/909, 343/781
[51] Int. Cl HOlq 15/02
[58] Field of Search 343/755, 781, 909,
343/911
[56] References Cited
UNITED STATES PATENTS
3,165,749 1/1965 Cushner 343/909
Primary Examiner—Eli Lieberman
Attorney—Monte F. Mott et al.
[57] ABSTRACT
3,148,370 9/1964 Bowman 343/909
^<NASfi-Case-Np6^13l71^TjLOfTLOSiT
PLATE Patent (Jet Propulsion Lab,
A low loss dichroic plate is disclosed for passing radia-
tion within a particular frequency band and reflecting
radiation outside of that frequency band. The dichroic
plate is comprised of a configuration of dipole ele-
ments defined by slots formed in a conductive plate.
The slots are dimensioned so as to pass radiation of a
selected frequency and are shaped so as to minimize
the relationship between that frequency and the tilt
angle of the plate relative to the direction of radiation.
The slots are arranged so as to minimize signal power
loss due to cross polarization effects.
9 Claims, 8 Drawing Figures
DICHBOIC
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PATENTEDOCT30I973 3.769,623
SHEET 1CF 3
FIG. 1
,30
32_JLJt JIJL
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36 AJLAJL
PERPENDICULAR
POLARIZATION
1 \
CROSS
POLARIZATION
/Tl _PARALLEj- ^
VLpOLARlZATTON >
_^ MAIN
• POLARIZATION
FIG. 2 FIG. 3
PATENTED OCT 3 OfflH 3,769.623
SHEET 2 OF 3
40
46
48
50
52
54
42
44 44c ^  44d
FIG. 4
44a 44b
\3SS
JL
5
i 1
40
FIG. 5
FIG. 6
MAIN POLARIZATION
CROSS POLARIZATION
MAIN POLARIZATION
PATENTEOOCT30I973 3,769,623
SHEET 3 OF 3
66
60-
62
64
MAIN
POLARIZATION
CROSS
POLARIZATION
MAIN
POLARIZATION
FIG. 7
80
82
84-
FIG. 8
3,769,623
1 2
• LOW LOSS DICHROIC PLATE The novel features of the invention are set forth with
particularity within the appended claims. The invention
The invention described herein was made in the per- will best be understood from the following description
formance of work under a NASA contract and is sub- when read in conjunction with the accompanying draw-
ject to the provisions of Section 305 of the National 5 ings wherein:
Aerbnauticsf and Space Act of 1958, Public Law FIG. 1 is a schematic diagram of a system in which
85-568 (72 Stat. 435; 42 USC 2457). a dichroic plate in accordance with the present inven-
BACKGROUND OF THE INVENTION tk>n Ca" be advanta6eous|y utilized;BACKGROUND Oh 1 Hfc IN VfcN I ION
 F,G 2 jg a schematic jiiustration of a known linearly
1. Field of the Invention 10 polarized dichroic plate;
This invention relates to a low loss dichroic plate par- FIG. 3 is a diagram depicting the cross polarization
ticularly useful in a dual frequency feed system for Cas- effects occurring in the plate of FIG. 2;
segrainian antennas for passing polarized radiation FIG. 4 is a plan view of a linearly polarized dichroic
within a selected frequency band. plate in accordance with the present invention;
2. Description of the Prior Art 15 FIG. 5 is a sectional view taken substantially along
U.S. Patent Application Ser. No. 290,022, filed on the plane 5—5 of FIG. 4;
Sept. 18, 1972 by Dan A. Bathker, Samuel A. Brun- FIG. 6 is a schematic view depicting the polarization
stein, and Arthur C. Ludwig entitled "Dual Frequency effects occurring within the slots of the dichroic plate
Microwave Reflex Feed" discloses a feed system for a of FIGS. 4 and 5;
Cassegrainian antenna wherein X and S band signals 20 FIG. 7 is a schematic illustration of a first circularly
are simultaneously transmitted or received along a polarized- dichroic plate embodiment in accordance
common boresight of a single antenna. That system em- with the present invention; and
ploys a dichroic plate able to reflect signals of a first FIG. 8 is a schematic illustration of an alternative cir-
frequency (e.g., S band) and pass signals of a second cularly polarized dichroic plate in embodiment accor-
frequency (X band). Dichroic plates intended for use 25 dance with the invention.
in such systems have been discussed in the literature; INSCRIPTION OF THF PRFFFRRFD
e.g., (1) Ohio State University Technical Report DESCR p°Jl°£, t£REFERRED
2148-6, 1967 and (2) IEEE Transactions on Antennas EMBODIMENTS
and Propegation, volume 19, No. 5, September 1971 Attention is now called to FIG. 1 which illustrates a
"Reflection Properties of Periodic Surfaces of Loaded 30 typical system in which embodiments of the present in-
Dipoles." The devices disclosed in the foregoing and vention can be advantageously employed. The system
other papers do exhibit appropriate dichroism and have of FIG. 1 includes a source of X band signals 10 which
proved satisfactory in certain applications. However, radiates X band energy through a dichroic plate 12.
the need for dichroic plates exhibiting lower power loss The X band energy strikes a subreflector 14 and is re-
has been recognized for certain applications. 35 fleeted toward a Cassegrainian reflector 16.
OBJECTS AND SUMMARY OF THE ,NVENT,ON
Accordingly, an object of the present invention is the reflects and focuses the S band energy in front of the
provision of a dichroic plate capable of passing a signal dichroic plate 12. By correctly orienting the tilt angle
of selected frequency with negligible power loss. 40 of the dichroic plate, the S band signals are reflected by
Another object of the present invention is the provi- it in line with the X band signals toward the subreflec-
sion of a dichroic plate which is essentially insensitive tor 14. The subreflector 14 reflects both the X and S
to the variations in the tilt angle of the plate relative to band energy to the reflector 16 which in turn transmits
the direction of signal radiation. . both the X and S band signals along a common bore-
A more particular object of the invention is the provi- sight.
sion of a dichroic plate capable of passing a polarized The function of the dichroic plate 12 is to reflect the
wave form and in which dipole elements are oriented S band energy and to pass the X band energy with little
so as to cancel cross polarization effects. degredation in either power or radiation pattern. These
The foregoing and other objects of the invention are requirements necessitate that the dichroic plate have a
achieved in a preferred embodiment of the invention low loss characteristic and be relatively insensitive to
by providing a thin conductive plate having a matrix of tilt angle, i.e., the angle of incidence of the X band en-
slots formed therein, each.slot defining a dipole ele- ergy on the plate 12.
ment. Each slot is shaped to include an intermediate The present invention is directed to dichroic plate
loop portion which loads the dipole element and embodiments useful in systems of the type depicted in
thereby decreases the sensitivity of the plate's resonant FIG. 1 for passing linearly and circularly polarized radi-
frequency to tilt angle variations. In a linearly polarized ation of a selected frequency.
embodiment of the invention, the matrix of slots is Attention is now called to FIG. 2 which schematically
comprised of parallel rows in which the orientation of illustrates a known type of dichroic plate for passing
the slots in each row is alternated. As a consequence of ,. linearly polarized radiation within a selected frequency
the interaction between adjacent slots, either in the band. More particularly, FIG. 2 illustrates a plurality of
same or adjacent rows, the cross polarization effects loaded dipole elements 30 arranged in parallel rows 32,
produced by individual slots is compensated for result- 34, 36. The dipole elements 30 may be formed, for ex-
ing in a nulled net cross polarization. In a circularly po- ample, by slots formed within a conductive panel such
larized embodiment of the invention, the plate is pro- „ as a thin sheet of copper or aluminum. A dichroic plate
vided with parallel columns extending orthogonal to of the type represented in FIG. 2 is discussed in the
the rows with the slots in each column also being alter- aforementioned paper appearing in the IEEE transac-
nated. tions.
3,769,623
3 4
"* •*Experimentation with a slotted dichroic plate of the FIGS. 4-6 exhibits low transmission loss. A model con-
type illustrated in FIG. 2 was conducted including the structed in accordance with the arrangement illustrated
measurement of transmission loss through the plate for in FIG. 4 was built and tested and its measured cross
various tilt angles. A high transmission loss characteris- . polarization was at least 40 dB down for angles of inci-
lic was revealed in certain orientations. In the case of 5 dence up to 60°.
perpendicular polarization, transmission loss increased It should be understood that the slots 44 formed in
with angle of incidence 0 to 2dB at 0 = 60°. On the panel 40 should be packed with as great a density as is
other hand, losses were less than 0.5 dB for angles of reasonable. The dimensions of each slot are of course,
incidence up to 60° in the case of parallel polarization. dependent upon the desired band pass frequency. More
The difference in behavior of these two polarizations is 10 particularly, the integrated length of each slot should
attributable to cross polarization effects and can be ex- be approximately one half wave length. Thus, for a di-
plained with reference to FIG. 3. FIG. 3 illustrates a chroic plate intended to pass 8.0 gigahertz radiation,
single, short, loaded dipole element. Normally, cross the length of each slot should be approximately 0.7
polarization excited in the legs of the loop of the dipole inches. In the fabrication of a dichroic plate in accor-
element is equal and opposite. When the dichroic plate IS dance with the invention, it is important that the di-
is tilted about the X axis in FIG. 3, corresponding to the mensions of the slots be uniform and thus very small
case of parallel polarization, cross polarization in the tolerances, e.g., ±0.001 inches, are essential. The slots
legs of the dipole element loop remains equal and op- 44 can be formed within the plate 40 by various fabri-
posite and thus no net cross polarization is produced. cation techniques such as precision machining or
However, when the dichroic plate is tilted about the Y 20 chemical etching.
axis of FIG. 3, corresponding to the case of perpendicu- The technique employed in the linearly polarized di-
lar polarization, the net cross polarization increases chroic plate of FIG. 4 to cancel cross polarization ef-
with angle of incidence because of the increased phas- fects is also applicable to the provision of a circularly
ing between the legs of the dipole element loop. polarized dichroic plate. More particularly, attention is
The present invention is directed to embodiments of 25
 nOw called to FIG; 7 which schematically illustrates a
a dichroic plate which exhibits negligible transmission first embodiment of a circularly polarized dichroic
loss regardless of the angle of incidence. Transmission plate in accordance with the invention. The dichroic
loss is minimized in accordance with the present inven- plate of FIG. 7 consists of parallel rows of .dipole ele-
tion by arranging the dipole elements so that adjacent ments 60, 62, and 64 as well as parallel columns of di-
elements effectively interract to cancel cross polariza- 30 pole elements 66, 68, 70, and 72. It will be noted in
tion effects. FIG. 7 that the orientation of the successive slots is al-
Attention is now called to FIGS. 4 and 5 which illus- ternated along each row or column. As a consequence,
trate a preferred embodiment of a linearly polarized di- cross polarization effects are cancelled as is repre-
chroic plate in accordance with the present invention. sented in FIG. 7.
The dichroic plate of FIGS. 4 and 5 consists of a thin 35 It should, of course, be recognized that arrangements
panel 40 of conductive material such as copper or alu- of dipole elements other than those shown herein can
minum. In a typical application, the panel 40 would be be utilized in accordance with the present invention to
square, on the order of 32 inches along each side and achieve the net cancellation of cross polarization ef-
have a thickness of about 0.016 inches. fects. These various arrangements are characterized by
A plurality of dipole elements 42 are defined in the 40 alternating the orientation of the elements along the
panel 40 by forming slots 44 extending therethrough. rows and columns.
As shown, the slots 44 are arranged in parallel rows 46, For example, one further arrangement of dipole ele-
48, 50, 52, and 54. Each of the slots 44 is illustrated as ments in accordance with the invention is illustrated in
including a pair of base portions 44a and 44* and a FIG. 8 for forming a circularly polarized dichroic plate,
loop portion including legs 44c and 444. Note that in FIG. 8 the orientation of the dipole ele-
As previously pointed out, dipole elements formed by ments is alternated along each of the rows and columns
slots with loops have been previously discussed in the but, however, the orientation of the elements from col-
literature, as for example, in the aforementioned IEEE umn to column or row to row is not alternated. It is in
article. The presence of the loop portion within a slot this latter respect that the arrangement of FIG. 8 differs
effectively electrically loads the dipole element and de- from the arrangement of FIG. 7.
creases the sensitivity of the plate's resonant frequency FIG. 8 illustrates dipole rows 80, 82, 84 and dipole
to tilt angle. columns 86, 88, 90, 92. Coupling between adjacent di-
In order to null the net cross polarization effects dis- pole elements, as 94, 96 produces a cross polarization
cussed in connection with FIG. 3, the orientation of the . component to the left along column 90. However, cou-
slots are alternated in accordance with the present in- pling between elements 98,100 produces an oppositely
vention as shown in FIG. 4. Thus, proceeding from left directed cross polarization component to the right
to right along row 52 of FIG. 4, for example, it will be along column 88. As a consequence, the net cross po-
noted that the first slot has a loop pointing down, the larization effect is nulled.
second slot pointing up, the third slot point down, etc. ,„ It has been found that dichroic plates built in accor-
In the adjacent row 54, the first slot has a loop pointing dance with the teachings of FIGS. 7 and 8 exhibit very
up, the next slot pointing down, etc. As a consequence low transmission losses for angles of incidence up to
of the arrangement illustrated in FIG. 4, each slot proj- 60°. Even more significantly, however, such plates ex-
ects toward an adjacent slot as is better shown in FIG. hibit almost no pattern degradation which might other-
6. The two slots illustrated in FIG. 6 produce opposite
 65 wise lead to signal losses.
cross polarization effects which tend to cancel or null From the foregoing, it should now be appreciated
the net cross polarization. As a consequence, a dichroic that several embodiments of low loss dichroic plates
plate constructed in accordance with the teachings of have been disclosed herein for passing linearly or circu-
3,769,623
farly polarized radiation within a selected frequency
band.
What is claimed is:
1. Dichroic means for passing radiation within a se-
lected frequency band and reflecting radiation outside
of that band, said dichroic means comprising:
means defining a plurality of dipole elements each
comprised of an elongated path including a first
and second spaced base portions and a substan-
tially U-shaped central portion interconnecting
said first and second base portions, said elongated
path having a length proportional to the wave-
length at the center of said selected frequency
band; and
means supporting said plurality of dipole elements in
a plurality of parallel rows with the orientation of
elements in each row being alternated.
2. The dichroic means of claim 1 wherein said sup-
porting means supports some of said elements in a plu-
10
15
a thin panel of electrically conductive material;
a first plurality of slots extending through said panel
arranged in a plurality of parallel rows, each of said
slots including first and second aligned and spaced
base portions and a U-shaped central portion ex-
tending orthogonal to said base portions;
said slots in each row being arranged with the U-
shaped central portions in adjacent slots extending
in opposite directions.
5. The dichroic plate of claim 4 wherein each slot has
a.length substantially equal to one half wavelength at
the center frequency of said first frequency band.
6. The dichroic plate of claim 4 wherein aligned slots
in adjacent rows are oppositely oriented.
7. The dichroic plate of claim 4 including a second
plurality of slots extending through said panel arranged
in a plurality of columns interlaced with and extending
orthogonal to said parallel rows, said slots of second
rality of parallel columns interlaced with and extending 20 Plurality bein8 shaPed and dimensioned substantially
orthogonal to said rows with the orientation of ele- identical to said slots of said first plurality and arranged
ments in each row being alternated.
3. The dichroic means of claim 1 wherein said means
defining said dipole elements includes a thin panel of
electrically conductive material having a plurality of 25
slots extending therethrough, each slot forming said
elongated path.
4. A low loss dichroic plate suitable for use in an an-
tenna feed system for passing polarized signals within
a first frequency band and reflecting signals outside of 30 the center frequency of said first frequency band,
said f i r s t frequency band, said plate comprising: * * * * *
along each of said columns with the U-shaped central
portions in adjacent slots extending in opposite direc-
tions.
8. The dichroic plate of claim 7 wherein aligned slots
in adjacent rows are oppositely oriented and aligned
slots in adjacent columns are oppositely oriented.
9. The dichroic plate of claim 8 wherein each slot has
a length substantially equal to one half wavelength at
35
40
45
50
55
60
65



Low loss dichroic plate

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