U.S. patent number 3,758,879 [Application Number 05/284,821] was granted by the patent office on 1973-09-11 for variable directional coupler.
This patent grant is currently assigned to International Standard Electric Corporation. Invention is credited to Daniel E. Beguin, Bernard Chiron, Michel P. G. Cuo.
United States Patent |
3,758,879 |
Beguin , et al. |
September 11, 1973 |
**Please see images for:
( Certificate of Correction ) ** |
VARIABLE DIRECTIONAL COUPLER
Abstract
This invention relates to a microwave directional coupler,
having an easily adjustable coupling coefficient. In a short-slot
hybrid junction comprising two adjacent waveguide sections having a
common sidewall with a coupling slot, two dielectric slabs having
their planes parallel to the common wall plane are disposed, in
front of the slot, respectively in the two waveguide sections and
symmetrically with respect to the common wall. Variations of the
coupling coefficient are obtained by moving symmetrically the two
slabs in a direction perpendicular to the common wall. This
solution preserves the decoupling and matching characteristics of
the coupler.
Inventors: |
Beguin; Daniel E. (Saint-Prix,
FR), Chiron; Bernard (Nanterre, FR), Cuo;
Michel P. G. (Chilly-Mazarin, FR) |
Assignee: |
International Standard Electric
Corporation (New York, NY)
|
Family
ID: |
9082371 |
Appl.
No.: |
05/284,821 |
Filed: |
August 30, 1972 |
Foreign Application Priority Data
|
|
|
|
|
Aug 31, 1971 [FR] |
|
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7131466 |
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Current U.S.
Class: |
333/111 |
Current CPC
Class: |
H01P
5/182 (20130101); H01P 5/04 (20130101) |
Current International
Class: |
H01P
5/04 (20060101); H01p 005/12 (); H01p 005/14 () |
Field of
Search: |
;333/10,11,98R |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Rolinec; Rudolph V.
Assistant Examiner: Nussbaum; Marvin
Claims
We claim:
1. An improved directional sidewall coupler of the type wherein
first and second waveguide sections having a sidewall as a common
wall are provided with a coupling slot, wherein the improvement
comprises:
first and second dielectric slabs positioned within said first and
second waveguide sections, respectively, symmetrically with respect
to said common wall in front of said coupling slot, and having
planes which are parallel to said common wall; and
means for positioning the plane positions of said slabs with
respect to said common wall.
2. A coupler according to claim 1, wherein said first and second
slabs are trapezoid-shaped.
3. A coupler according to claim 2, wherein said positioning means
include plastic feet fixed to waveguide section walls between which
said first and second slabs are forced.
4. A coupler according to claim 1, further including means for
enabling said first and second slabs to be moved symmetrically
along an axis normal to said common wall.
5. A coupler according to claim 1, wherein the transverse thickness
of said slabs is controlled so as to produce a desired coupling
coefficient.
Description
BACKGROUND OF THE INVENTION
This invention relates to a directional coupler for microwaves
which permits the coupling coefficient to be easily adjusted.
In numerous microwave applications, where it is required to split
microwave energy, as for example in feeding antenna arrays, it is
very useful to have available directional couplers having different
coupling coefficients that are adjusted to have different
predetermined values.
Several solutions are known. A first consists of varying the
coupling hole sizes so as to vary the coupling coefficient.
However, this process is relatively expensive and difficult to deal
with because it requires specific machining for each coupling
coefficient value.
A second solution has been recommended for couplers, hereafter
referred to as topwall or broad wall couplers, having a coupling
slot which is small with respect to wave length, described for
example in the article "Directive couplers in wave guides" by M.
Surdin, issued in the "Journal I.E.E." review, vol. 93, 1946, pages
725-735. In such couplers comprising two waveguide sections coupled
through their broad walls, the electric field intensity is varied
at the coupling slot by inserting into one or both waveguide
sections dielectric slabs normal to the common broad wall including
the coupling slot. The dielectric slabs are movable along a
direction parallel to that broad wall and normal to the
corresponding waveguide portion axis so as to vary the electric
field intensity on the coupling slot and thus change the coupling
coefficient. However, such couplers produce only relatively low
couplings (coupling coefficient having a value higher than 10 dB).
In addition, this technique cannot be applied to so-called sidewall
couplers with two waveguide sections coupled through their
sidewalls.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a sidewall
coupler having an easily adjustable coupling coefficient.
It is a further object of the present invention to provide such a
coupler of the "short-slot hybrid junction" type with a coupling
coefficient adjustable to about 3 dB.
According to a broad aspect of the invention, there is provided an
improved directional sidewall coupler of the type wherein first and
second waveguide sections having a sidewall as a common wall are
provided with a coupling slot, wherein the improvement comprises
first and second dielectric slabs positioned within said first and
second waveguide sections, respectively, symmetrically with respect
to said common wall in front of said coupling slot, and having
planes which are parallel to said common wall, and means for
positioning the plane positions of said slabs with respect to said
common wall.
Other advantages and features of the invention will appear more
clearly from the following description of the invention, said
description being made in conjunction with the accompanying
drawings, in which:
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic view of the directional coupler according to
the invention;
FIGS. 2a, 2b, 3a, and 3b are diagrams to explain coupler operation;
and
FIG. 4 shows curves illustrating variations of coupling coefficient
as a function of dielectric slab position.
DESCRIPTION OF THE PREFERRED EMBODIMENT
FIG. 1 illustrates a directional coupler of the short-slot hybrid
junction type as described, for example, in the article "The
short-slot hybrid junction" by H. J. Riblet, issued in the review
"Proceedings of the I.R.E." of February 1952, pages 180-184. Such a
coupler is a sidewall coupler comprising two waveguide sections
having cross-sectional diemsnions a and b and having a sidewall as
a common wall 9, said common wall 9 being provided with a coupling
slot 5 of length L. Such a coupler has four ports, two, 1 and 3, on
the main waveguide, and two, 2 and 4, on the auxiliary
waveguide.
According to the invention, there are provided in each waveguide
section, in front of coupling slot 5 and symmetrically with respect
to the plane of common wall 9, a dielectric material slab, either 6
or 7, respectively, of thickness e whose plane is parallel to the
plane of common wall 9. The distance from each plate to the
external side wall of each waveguide section is indicated by p. By
changing distance p, the coupling coefficient is varied in the
coupler. As shown in FIG. 1, dielectric slabs are fixed with feet 8
which are themselves fixed to the waveguide broad walls, slabs 6
and 7 being forced between feet 8. Of course, the slabs may also be
directly fixed to waveguide walls. Operation of the coupler
according to the invention will be described with reference to
FIGS. 2 and 3 which make it possible to analyze coupler operation
by means of two distinct modes A and B. Parts (a) in FIGS. 2 and 3
illustrate electric field configurations in various coupler
cross-sections corresponding to ports 1 and 2, to slot 5 and to
ports 3 and 4, respectively. Parts (b) of FIGS. 2 and 3 illustrate
electric field amplitude patterns in those same cross-sections. In
mode A, FIG. 2, it is assumed that two phased transverse-electric
modes H.sub.10 are excited in both waveguide sections on ports 1
and 2 in such a manner that electric field amplitudes are given
by:
E'.sub.1 = 1/2
e'.sub.2 = 1/2 .
in front of the coupling slot and assuming that dielectric slabs
have been removed, it results a waveguide with a double broad wall
2a wherein a mode H.sub.10 is propagated. Thus, on ports 3 and 4
electric fields will be:
E'.sub.3 = e.sup.-.sup.j.sup..beta. L /2
e'.sub.4 = e.sup.-.sup.j.sup..beta. L //2
where .beta..sub.1 is the propagation constant for mode H.sub.10 in
waveguide of dimension 2a and disregarding propagation through
waveguide sections of width a that is performed in an identical
manner in operation modes A and B.
In operation mode B, FIG. 3, assumption is made that two opposite
phase modes H.sub.10 are excited in the two ports 1 and 2, such
that:
E".sub.1 = 1/2
e".sub.2 = - 1/2 .
in front of coupling slot 5 in waveguide of dimension 2a
(dielectric slabs having been removed), a mode H.sub.20 will be
propagated and, in a same manner as previously, it results in ports
3 and 4:
E".sub.3 = e.sup.-.sup.J.sup..beta. L /2
e".sub.4 = - e.sup.-.sup.J.sup..beta. L /2
where .beta..sub.2 is the propagation constant for the mode
H.sub.20 in the waveguide of width 2a.
If modes A and B are superimposed, it results
E.sub.1 = E'.sub.1 + E".sub.1 = 1
e.sub.2 = e'.sub.2 + e".sub.2 = 0
e.sub.3 = e.sup.-.sup.j.sup..beta. L + e.sup.-.sup.j.sup..beta. L
/2 = e.sup.-.sup.j (.sup..beta. .sup.+ .sup..beta. ) L/2 . cos
(.beta..sub.1 - .beta..sub.2) L/2
e.sub.4 = e.sup.-.sup.j.sup..beta. L - e.sup.-.sup.j.sup..beta. L
/2 = -j e.sup.-.sup.j (.sup..beta. .sup.+ .sup..beta. ) L/2 . sin
(.beta..sub.1 -.beta..sub.2) L/2.
Thus it appears that, when microwave energy is applied to port 1,
port 2 is entirely decoupled and that energy is coupled to port 4,
the coupling coefficient being equal to:
C = (E.sub.1).sup.2 /(E.sub.4).sup.2 = 1/sin.sup.2 (.beta..sub.1
-.beta..sub.2) L/2.
According to the invention, use is made of dielectric slabs
inserted parallel to the common wall into the waveguide of width
2a, which causes the propagation constants .beta..sub.1 and
.beta..sub.2 to vary as a function of slab position in the
waveguide and, this, in a different manner for modes H.sub.10 and
H.sub.20. Thus, the amount .beta..sub.1 - .beta..sub.2 is varied
and, as a consequence, the coupling coefficient C.
A good matching is maintained for the coupler by shaping the slabs
as an isosceles trapezium. A good decoupling is maintained for the
coupler by locating the two dielectric plates in symmetric
positions with respect to the common wall 9. The relative phase
shift between the two outputs 3 and 4 remains equal to .pi./2.
Phase shifts between inputs and outputs vary with the coupling
coefficient. Indeed, when p is varied, it causes the amount
(.beta..sub.1 +.beta..sub.2) to vary. The low selectivity
advantages of the initial coupler, without dielectric slabs, still
exist in the coupler according to the invention. In a preferred
embodiment, the dimension a is so selected that, in the waveguide
of width 2a in front of the coupling slot, there exists only the
modes H.sub.10 and H.sub.20.
In a typical embodiment, waveguide sections of width a = 21.15 mm
were used for a frequency range centered on 9.500 MHz. The length L
was so selected that, the dielectric slabs being removed, a
coupling coefficient of 3 dB 1 reached at 9.500 MHz, i.e.
(.beta..sub.1 -.beta..sub.2) L/2=.pi./4 and L = 30 mm.
The dielectric slabs used were made of polytetrafluoroethylene,
known under TEFLON (trademark) of dielectric constant 2.1. Each
trapezoid-shaped slab has a broad base of 4 cm, a narrow base of 2
cm and a thickness e = 3 mm. Each slab was fixed by feet made of a
copolymer of styrene, known under REXOLITE (trademark) of
dielectric constant 2.54, which may be easily fixed to metal
waveguide walls.
The full-line curve of FIG. 4 represents measured coupling
coefficient variations as a function of the distance p for the so
designed coupler. Dotted lines of FIG. 4 are theoretical curves for
various values of thicknesses e, as shown in FIG. 4. Those
theoretical values have been calculated from the theory given by R.
Seckelmann in the article entitled "Propagation of the TE modes in
dielectric loaded waveguides" issued in the IEEE Transactions on
Microwave Theory and Techniques, vol. MTT-14 No. 11, November,
1966, pages 518-527.
It is to be noted that C may be continuously varied between 3.1 and
5 dB. Differences from theoretical values to measured values may
mainly result from approximations introduced into the
above-mentioned theory as well as, among other, from the
discontinuity influence produced by the coupling slot for wave of
mode H.sub.10 which passes from a width-a waveguide to a width-2a
waveguide.
The desired coupling may be achieved with an accuracy of .+-. 0.05
dB, provided that dielectric slabs are positioned with an accuracy
of .+-. 1/10 mm.
The coupler, illustrated in FIG. 1, has been made with dielectric
material slabs which are secured to waveguide section broad walls.
But obviously, mobile slabs, movable by any known means, may also
be used and, for instance moved by mechanical means, provided that
however slab moves be always symmetrical with respect to common
wall 9.
Of course, the described embodiment does not limit the scope of the
invention and the principle thereof could also apply to other types
of couplers with long or short slots. The principle of the
invention may also apply to broad wall coupler with large coupling
slot. But this would raise important problems about electric field
distorsion and matching which does not exist for sidewall
couplers.
* * * * *