Note: Descriptions are shown in the official language in which they were submitted.
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COMPACT WIDEBAI~~ WAVEGUIDE TWIST TRANSITION
BACKGROUND OF THE INVENTION
The present invention relates to a junction placed between hollow
waveguides intended for transmission of electromagnetic waves.
Many different applications of electromagnetic energy transmission,
especially at the microwave and higher frequency ranges, make use of hollow
waveguides. It is often desirable to be able to twist the direction of the
electrical
field vector associated with energy traveling through such a waveguide. For
example, it may be necessary to change the polarization of directional
transmission
paths between relay link sections to impart polarization differences between
signals
fed to different antenna elements, or packaging constraints may dictate that
the field
vector be rotated.
Wherever two sections of rectangular waveguide meet which are rotated on
an axis relative to each other, for example at 90° with respect to one
another, an
intermediate waveguide transition section is typically required. This
waveguide
transition section or "twist" provides a low return loss for the transition.
In fact,
without any such transition section, in the case where the two waveguides are
rotated exactly 90° relative to each other, 100% energy reflection will
occur at the
junction for all radio frequencies.
One common way of ensuring low reflection is to insert a section of actual
twisted waveguide between the two main waveguides. This section is twisted
about
the axis of the waveguide in such a manner as to maintain a given cross-
section
along its length. This type of waveguide twisted section is a standard product
offered by most waveguide component manufacturers. The section of twisted
waveguide must normally be kept sufficiently long to provide a smooth, gradual
transition to ensure that the desired propagating waveguide mode undergoes
negligible reflection.
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However, there are certain disadvantages to using this sort of waveguide
twist, it being relatively expensive to manufacture, and it occupying
substantially
large space which may be unacceptable for certain applications. For example,
to
maintain low return loss over the full desired bandwidth, a minimum length of
several guide wave-lengths is typically required.
Transition sections are also available which can accomplish the
90° twist
function in a substantially reduced length. For example, there is a so-called
single
section quarter-wave step-twist transition. This type of transition section
consists of
a one-quarter wavelength long waveguide having a rectangular cross-section
which
is normally identical to that of the rectangular waveguides to be joined. The
short
length of waveguide is oriented at a 45° angle relative to each of the
two main
waveguides. In practice, this type of single section quarter-wave step
transition is
not normally fabricated from a length of actual waveguide, but is instead
machined
from a solid metal block. It may also include an integral flange with threads
or
through-holes to enable screws to fasten it to the flanges of each of the two
waveguides. This single section quarter-wave step-twist transition with
rectangular
cross-section can provide low reflection transition, but typically over only a
narrow
bandwidth which is a fraction of the full operating capability of the
waveguides
themselves.
So-called mufti-section step twists find application where wider bandwidth is
required. These consist of two or more such quarter-wave sections having
identical
rectangular cross-section with each section rotated about an integral multiple
fraction of 90°. Disadvantages of a mufti-section step twist
implementation are (1) a
high cost of fabrication due to the need to machine complex parts at precise
angles
and (2) the fact that the multiple section step twist, while shorter than a
twisted
waveguide section, is still longer than a single section device. Consequently,
the
step twist transition may still be larger than is desirable for certain
intended end
uses.
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SUMMARY OF THE INVENTION
What is needed is a configuration to accomplish a waveguide twist function
through a significant angular twist of the electric field, such as 90°,
while
maintaining a low return loss over a full operating bandwidth of the waveguide
sections. The solution should at the same time be of a minimum physical length
equal to approximately one-quarter of a waveguide wavelength referenced at the
low
frequency end of the waveguide band. Furthermore, the device should be easy to
manufacture on standard machining equipment.
Briefly, the present invention accomplishes these objectives by the use of a
single section, quarter-wave step-twist transition having a precisely defined
complex
cross-sectional interior shape. The shape is generally of the same rectangular
shape
as the corresponding waveguides. However, the interior opening is accentuated
in
one or more ways. For example, the corners of the opening are positioned
outboard
of the corners of the corresponding standard waveguide. In addition, the
longer pair
of opposing sides are indented slightly inward at a center vertex. The
characteristic
shape of the opening thus resembles a bow tie.
The center axis of the precisely defined shape is rotated with respect to both
waveguides. For example, in the case of two rectangular waveguides having
their
axes rotated 90° with respect to one another, an axis of symmetry of
the complex
cross-section waveguide is rotated at 45 ° relative to the two main
orthogonal
waveguides.
In an optional configuration, other outwardly extending vertices may be
added to the short opposing sides of the opening to elongate the shape and
further
provide advantageous electrical performance.
BRIEF DESCRIPTION OF THE DRAWINGS
The foregoing and other objects, features and advantages of the invention
will be apparent from the following more particular description of preferred
embodiments of the invention, as illustrated in the accompanying drawings in
which
like reference characters refer to the same parts throughout the different
views. The
drawings are not necessarily to scale, emphasis instead being placed upon
illustrating the principles of the invention.
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Fig. 1 is an isometric view of a waveguide junction and associated
waveguides according to the invention.
Fig. 2 is a front view of a preferred embodiment of a waveguide junction
according to the invention.
Fig. 3 is a chart illustrating a calculated performance for the design of Fig.
2.
Fig. 4 illustrates measured electrical performance from actual hardware
fabricated to the design of Fig. 2.
Fig. 5 illustrates a design variation in which an extra pair of vertices are
added to the outer short ends.
Fig. 6 is a chart of calculated performance for the embodiment of Fig. 5.
DETAILED DESCRIPTION OF THE INVENTION
Fig. 1 is an isometric representation of how two electromagnetic waveguides
are coupled with a waveguide junction according to the invention. The first
waveguide 10 consisting of a main rectangular body 11 and flange 12. A second
waveguide section 16 having a main body 17 and flange 18 is coupled to the
first
waveguide 10 by a waveguide twist junction 20. Waveguide sections 10 and 16
may, for example, be WR-28 industry standard rectangular waveguides having
normal inside dimensions of 0.140 by 0.280 inches and a standard operating
bandwidth of 26.5 GigaHertz (GHz) through 40 GHz. It should be understood that
the invention may be used with other standard rectangular waveguide
specifications
ranging from the bands WR-42 through WR-04. For larger waveguides, the concept
may be impractical because of the increased mass of the waveguide twist plate
20.
In any event, the waveguides 10 and 16 are connected to one another through
the expediency of having threaded holes 14 drilled in the flanges respective
flanges
12 and 18. The holes 14 are conveniently located at specified positions such
as in
the corners of the waveguide flanges. The two waveguide sections 10 and 16 are
fastened to one another using screws or other fasteners 19 placed through the
holes
14 in the flanges 12 and 18 as well as through holes 21 formed in the
periphery of
the waveguide twist 20.
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The waveguide twist 20 consists of a section of material which is typically of
a thickness, L, of one-quarter wavelength referenced at the low end of the
standard
operating frequency band of operation.
The slot or internal cross-section 22 of the waveguide twist 20 is a precisely
$ defined shape similar to a bow tie. The major axis .A1 of the bow tie is
rotated 4$°
relative to the major axes A2 and A3 of the respective waveguides 10 and 16.
This
major axis A1 of symmetry of the bow tie shaped slot 22 is thus rotated in a
manner
similar to previously known single step transition twists.
Fig. 2 is a more detailed view of the twist plate 20 as implemented for two
WR-28 waveguide sections disposed at a 90° angle. The drawing of
Fig. 2 in
particular shows the bow tie shape of the slot 22 in more detail. As can be
seen, the
bow tie shaped slot 22 is symmetrical about the axis A1. The characteristic
bow tie
shape is generally defined with respect to a rotated outline 28 of the
corresponding
waveguide. In particular, the corners 32 of the bow tie are located outboard
of
1$ where the standard waveguide corners would be located. In addition, the two
longer
opposing sides are indented at a vertex 30.
The bow tie shape is precisely defined by the following table of offsets
referenced to the center point 34 for the numbered points in the drawing of
Fig. 2 for
application again with the WR-28 waveguide:
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POINT "X" AXIS "Y" AXIS
1 +.045 -.045
2 +.150 +.003
3 +.159 +,053
4 +.053 +.159
5 +,003 +.150
6 -.045 +.045
7 -.150 -.003
8 -.159 -.053
9 -.053 -.1.59
10 -.003 -.1.50
11 +.045 -.045
Table 1
The dimensions of Table 1 are specified with respect to the center point 34.
The twist plate 20 is typically machined from a solid block of material. The
material used for the twist plate 20 may be brass or other suitable metal used
in
fabricating waveguide components. The thickness T of the block may, for
example,
be 0.172 inches for the WR-28 waveguide. The major dimensions W and H are both
0.75 inches.
The outline of the bow tie 22 is thus made up of only straight lines and four
circular arcs all having the same diameter to allow for simple machining. The
corners 32 of the rectangle are preferably filleted by the same amount, at a
radius of
about 0.031 inches, to further expedite machining.
In other embodiments, the corners 32 might be square; in such an instance,
the vertices 30 might need to be in a slightly different position to achieve
the same
performance.
Fig. 3 illustrates a calculated performance for the particular design shown in
Fig. 2, and Fig. 4 shows actual measured performance from hardware fabricated
to
this design. The measured response of Fig. 4 illustrates both insertion loss
(at 0.5
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dB per division scale) on the upper trace as well as return loss (at a ~ dB
per division
scale) on the lower trace. The device exhibits better than a 23 decibel (dB)
return
loss over all but the very high end of the 26.5 GHz through 40 GHz band. The
measured response does not show deeper nulls in the return loss indicated in
the
calculated response. This is probably a result of minor misalignments which
are
present in the actual measurement setup and of imperfections in the machining.
In
the particular bow tie twist design of Fig. 2, even the calculated return loss
is greater
than 20 dB at 40 GHz, which is the very high end of the WR-28 band.
Fig. S illustrates a design variation developed for improved full waveguide
band response. This implementation is a more general version of a bow tie
shape
having an extra pair of vertices 40 added at the distal end points along the
major axis
Al. The ttvo additional vertices 40 are shaped to the same radius as the four
corners
32, again to simplify the machining process.
Although a model has not been fabricated to this design, its calculated
performance is shown in Fig. 6. This plot shows that for the full WR-28
waveguide
band from 26.5 GHz to 40 GHz and even above that band, a better than 30 dB
return
loss is indicated. Somewhat less performance would be expected from a working
model, because as the calculated return loss becomes smaller, mechanical
precision
becomes more and more critical to achieve the desired result. The expectation
is
that from 25 to 27 dB worst case across the full waveguide band is a realistic
possibility.
A linear scaling of dimensions allows application of this design to other
waveguide sizes, provided that the waveguide of interest has a width and
height in a
precise 2 to 1 ratio. This is the case for most standard rectangular
waveguides but
other standard bands such as WR-42 and WR-90 are known to depart from the 2 to
1
waveguide size ratio. For ratios other than precisely 2 to 1, an optimal bow
tie
cross-sectional shape would be slightly different from what would be obtained
from
a simple linear scaling of the designs presented here. However, the optimal
dimensions could be determined through same calculation procedures.
Similarly, dimensions could be calculated for twist angles other than
90°.
However, 90° is typically the most commonly used angle for a waveguide
twist and
is also the most demanding angle for which to achieve lower return loss over
the full
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operating bandwidth, because it represents the angle for which there is
greatest
overall change in the waveguide mode from one waveguide 10 to the other 16.
While an exact analytical description of the operating device 20 is not known
precisely, it is observed however that the bow tie slot 22 is relatively large
in area
and therefore supports multiple propagating modes at the higher end of the
waveguide band. This over-moded characteristic most likely plays an important
role
in the devices ability to achieve the desired performance.
While this invention has been particularly shown and described with
references to preferred embodiments thereof, it will be understood by those
skilled
10 in the art that various changes in form and details may be made therein
without
departing from the scope of the invention encompassed by the appended claims.
