Rectangular waveguide elbow bent across the broad side of the waveguide with corner flattening and a transverse bar

A rectangular waveguide elbow (E-elbow) bent across the broad side of the waveguide with an outer corner symmetrically flattened by conductive flattening or smoothing plane which provides for elimination of undesirable reflections by providing a cross cylindrical bar at the median between the inner corner and the center of the flattening or smoothing plane and wherein the cylindrical bar has an enlarged portion at its center which extends a portion length of the bar. A second embodiment provides a bar which does not have an enlarged portion but wherein the diameter of the bar ratio to the length of the shorter side of the waveguide is at least 0.258.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates in general to rectangular waveguide with elbows 
(E-elbow) which are bent across the broad side of the waveguide with the 
outer corners symmetrically flattened by conductive flattening or 
smoothing planes. 
2. Description of the Prior Art 
Such elbows are described, for example, in "Taschenbuch der 
Hochfrequenztechnik", by H. Meinke and F. W. Fundlach, Springer Verlag, 
2nd Edition, 1962, pages 401 and 402. Such elbows are utilized in various 
microwave circuits with rectangular waveguides. By using angled waveguides 
a more compact structure is achieved as compared to a comparable 
low-refraction circular arc elbow, particularly for use as waveguide 
shunts are filters of different types, such as for example fixed frequency 
shunts or filters, polarization shunts or filters or wave mode shunts or 
filters. Using waveguides with a rectangular cross-section having a side 
ratio of a:b equal to 2:1 are most often utilized. Such waveguides can be 
used in the relative frequency range with a maximum bandwidth of f.sub.o 
and f.sub.u =2:1 for the TE10 wave. Also, as discussed in the above 
reference publication "Taschenbuch der Hochfrequenztechnik" the reflection 
of an E-elbow can be reduced if, as shown in FIG. 1a, the exterior corner 
of the elbow is symmetrically flattened or smoothed with a conducting 
plane. FIG. 1b illustrates the standing wave ratio s at frequencies f for 
E-elbows as shown in FIG. 1a with corner flattening planes of varying 
degrees. The optimum cathetus measurement x.sub.o is shown in the lowest 
curve wherein the ratio of x.sub.o /a=0.395 wherein a is the width of the 
long side of the waveguide has been derived and described in the above 
referenced publication. With such ratio, the reflection of an E-elbow will 
remain under r=5% in the frequency range of a waveguide which will usually 
be 1.25 fcTE10 through 1.9 fcTE10. Only in partial frequency bands of this 
frequency range can smaller reflections be achieved and for this purpose, 
the cathetus dimension can be changed somewhat with respect to x.sub.o 
depending upon the position of the partial frequency band within the full 
frequency band of the waveguide. 
Utilizing a side ratio of the rectangular waveguide a:b=2:1, FIG. 1b 
illustrates in detail how the respective SWRs of E-elbows change in a 
waveguide over a frequency range for an E-elbow with a bend angle of 
90.degree. and a few selected ratios x/a of the corner flattening or 
smoothing plane. Without corner flattening wherein the ratio x/a=0, an 
E-elbow represents an inducive disturbance with respect to a cross-section 
plane lying in the median line of the bend which inductive disruption 
increases greatly from the lower toward the top of the frequency range of 
a rectangular waveguide as shown. With increasing corner flattening or 
smoothing, in other words, increasing the quotient x/a, the inductive 
disruption becomes less and less. When the corner flattening or smoothing 
reaches a point where the ratio x/a=0.395, equal disruptions of r=5% will 
remain at the lower and upper frequency limits of the waveguide frequency 
range. These disruptions will have opposite phase angles and therefore 
such reflections will not fall below this value using a corner flattening 
method of compensation. Such reflections still represent a significant 
disruption in many utilizations which are standard today can be attributed 
to the fact that the compensation measure of corner flattening or 
smoothing alone is not precisely complementary to the disruption which is 
to be compensated over the entire frequency range of a rectangular 
waveguide. 
For further reduction of the reflection factor over a relatively broad 
frequency band, it has already been proposed to provide a conductive 
cylindrical cross-bar in the area of the geometrical median of the bend, 
with the cross-bar being aligned parallel to the broad sides of the 
waveguide and extending between the narrow sides of the waveguide which 
lie opposite each other and to provide the flattening or smoothing plane 
with a conductive means as for example, a metal cylinder which projects 
into the interior space of the waveguide in the area of its diagonal point 
of intersection. An E-elbow compensated in these manners have very low 
reflection over an entire frequency band of the waveguide, but, however, 
the cost of manufacturing such devices is expensive because of the three 
different compensation features. 
SUMMARY OF THE INVENTION 
The object of the present invention is to provide an improved E-elbow which 
has a very low reflection factor over a relatively broad frequency band 
and which is relatively inexpensive to construct. 
This object is achieved by modifying the prior art E-elbows by providing a 
cylindrical conductive cross-bar which extends parallel to the broad sides 
of the waveguide between the narrow sides of the waveguide which are 
opposite one another and which is located with the ends at the geometrical 
median of the bend of the elbow. The conductive cross-bar has an enlarged 
portion at the center of the bar which has a diameter d.sub.Q larger than 
the remaining portion of the bar outside the center area. 
An additional structural embodiment of the invention provides that the 
enlargement of the diameter of the cross-bar can be eliminated in a 
rectangular waveguide elbow having a bend angle of 90.degree. and a 
waveguide side ratio of a:b=2:1 when the ratio of x/a where x is the 
distance from the non-angled elbow to the point on the angled elbow where 
the flattening plane meets the broad side of the waveguide and where a is 
equal to the length of the broad side of the waveguide. In this invention, 
this ratio of x/a is selected to be approximately 0.352 and the conductive 
cross-bar is attached at a point comprising the mean height between the 
inner bend of the waveguide and the flattening plane. The diameter d.sub.Q 
of the cross-bar is selected to be at least approximately d.sub.Q /b=0.258 
where b is equal to the length of the narrow sides of the waveguide. 
Other objects, features and advantages of the invention will be readily 
apparent from the following description of certain preferred embodiments 
thereof taken in conjunction with the accompanying drawings, although 
variations and modifications may be effected without departing from the 
spirit and scope of the novel concepts of the disclosure and in which:

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
This invention relates to my co-pending application entitled "Rectangular 
Waveguide Elbow Bent Across the Narrow Side with Capacitive Loading", U.S. 
patent application Ser. No. 101,577, filed Dec. 10, 1979. 
FIG. 2 illustrates a waveguide elbow bent across the broad side a of the 
waveguide which is designated as an E-elbow and wherein the bend angle 
.alpha.=90.degree. and in which the waveguide side ratio is selected to be 
a:b=2:1 and wherein the flattening plane or smoothing is given by the 
relationship x/a of the cathetus distance x to the broad side a of the 
waveguide. The distance x is equal to the distance from the untruncated 
elbow apex to the position where the symmetrical smoothing plane 2 joins 
with the broad side a of the waveguide as indicated in FIG. 2. In the 
present invention, the E-elbow is provided with a round conductive 
cross-bar 1 which extends parallel to the broad sides of the waveguide and 
extends between the narrow sides of the waveguide in the region of the 
bend and wherein the centerline of the cross-bar 1 is located at the 
median point between the inner bend K of the elbow and the plane 2 and is 
located on athe bisector w of the corner of the untruncated waveguide. 
Thus, the distances of the cross-bar 1 from the inner bend K and to the 
plane 2 are equal. The diameter d.sub.Q of the cross-bar 1 is selected to 
have a ratio to the narrow side b of the waveguide of 0.275. To provide 
additional compensation in the waveguide, a portion 3 having a length 1 
with a diameter larger than the cross-bar 1 is mounted over the cross-bar 
1 and electrically connected to the cross-bar 1 at its center portion. For 
example, the member 3 could be a collar which is slipped over the 
cross-bar 1. In a sample embodiment according to this invention, the ratio 
1/a was chosen to be 0.17. 
FIG. 3 illustrates a further embodiment of the invention which is 
compensated by corner flattening or smoothing wherein the ratio x/a=0.352. 
A cross-bar 1 which is electrically conductive is mounted between the 
narrow walls of the waveguide parallel to the broad walls of the waveguide 
at the median angle of the bend as in FIG. 2, but the enlarged portion 3 
is eliminated in the embodiment of FIG. 3 due to the fact that the 
diameter d.sub.O of the cross-bar is selected so that the ratio d.sub.Q /b 
of the cross-bar will have a value of 0.258. In this embodiment, the 
cross-bar has a constant diameter which results in the cost of the device 
being cheaper than the one formed with an enlarged portion on the 
cross-bar. In the embodiment of FIG. 3, the waveguide side ratio a:b is 
equal to 2:1, however, when the dimensions of the waveguide side ratio and 
the bend angle .alpha. are known and vary from 2:1 and 90.degree. 
corresponding values of x/a and d.sub.Q /b can be simply derived so as to 
provide compensation without the enlargement 3. 
FIG. 4 comprises a plot of a measured curve for the standing wave ratio in 
the sample embodiment according to FIG. 3 plotted as a function of 
frequency. 
As can be seen from FIG. 4, the E-elbow compensated according to the 
invention has reflection factors which are below 1% in the frequency range 
of 1.13 fcTE10 &lt;f&lt;1.95 fcTE10. Thus, according to the invention, an 
E-elbow compensated only with corner flattening or smoothing and having an 
x.sub.o /a ratio of 0.395 can be improved by at least a factor of 5 with 
respect to the reflection factor by utilizing the teachings of the present 
invention wherein the cross-bar 1 is utilized either with the enlarged 
portion 3 or wherein the cross-bar 1 has the diameter specified above 
relative to the FIG. 3 embodiment. 
Although the invention has been described with respect to preferred 
embodiments, it is not to be so limited as changes and modifications may 
be made which are within the full intended scope of the invention as 
defined by the appended claims.