Note: Descriptions are shown in the official language in which they were submitted.
WO 92/15125 PCT/GB92/00260
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ELLIPTICAL WAVEGUIDE
TECHNICAL FIELD OF THE INVENTION
This invention relates to the manufacture of flexible
or flexible and twistable elliptical waveguide.
BACKGROUND
Waveguides are widely used for the transmission of
electromagnetic energy in the RF, microwave and
millimetric wavelength range. Such transmission can
take place within the confines of the conductive wall
of the waveguidE~, with a minimum of loss, reflection
and distortion of the signal.
Standard waveguides are formed of extruded rigid metal
tubing which is available in approximately 4.5 metre
straight lengths which can then be bent or twisted to
shape using specialist manufacturing equipment. Longer
lengths are attained by joining shorter lengths
together using atandard or custom made flanges. In
microwave systems it is common for the waveguide runs
to require several bends and twists to enable the
waveguide to avoid various obstacles in its path. The
use of standard :rigid waveguides therefore requires the
production of a series of detailed manufacturing
drawings to accurately define the shape of the required
waveguide. This adds to the cost of the system, as
does the operation of forming the waveguide into shape
SUBSTITUTc SHEET
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and the subsequent inspection and fitting of the
waveguide. Ret ro-fitting and replacement of rigid
waveguides is also a potentially expensive operation,
especially in more complex systems.
An important tape of waveguide is elliptical cross
section waveguide. This has a lower microwave loss when
operating in the a TE 11 mode than the equivalent
comparable standard rectangular waveguide. Elliptical
waveguide is also capable of transmitting higher
microwave power than the equivalent rectangular
waveguide operating in the H 10 mode. Since the wall of
elliptical wavec~uide is convexly curved in all areas,
the external compressive strength of such waveguide is
greater than that of rectangular or double ridged
waveguide desigr.~ed to operate within the same frequency
range. This increased resistance to deformation is of
importance in adverse mechanical or environmental
conditions.
Flexible elliptical waveguide is currently manufactured
in two halves, each of which is pressed to form a series
of transversely extending shallow corrugations. The two
halves are then axially soldered, brazed or welded to
form the compleite waveguide. In general however, this
form of waveguide is very stiff to bend by hand and is
not, to any significant extent, twistable. In addition,
such elliptical waveguide is expensive to manufacture,
and can only be manufactured in certain fixed sizes for
which tools are available, and in fixed lengths.
SUMMARY OF THE ~LNVENTION
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According to the present invention there is provided
tubular waveguide having a wall which, when viewed in
longitudinal section, is formed in a series of external
ribs separated by grooves, in which said wall comprises
an electrically conductive metal strip formed in a
generally helical configuration with adjacent edge
portions of adjacent turns of the strip mechanically
interlocked by being folded over each other, and in which
said interlocked edge portions are disposed on the crest
of said ribs, wherein when viewed in transverse cross
section said waveguide is of substantially elliptical
shape without straight regions, and when viewed in
longitudinal section said interlocked edge portions
comprise straight regions extending substantially
parallel to the longitudinal direction of the waveguide
for a major part of the length of said edge portions, and
the bottom of the grooves are also substantially straight
extending substanti<~lly parallel to the longitudinal
direction of the waveguide.
It should be understood that the term "elliptical" as
used herein is intended to cover other non-circular and
substantially ellip;~e-like shapes such as ovals.
An electrically conductive element is preferably
interposed between t:he folded edges to reduce
electromagnetic leakage. In flexible and twistable
waveguide there will usually be just a non-rigid
mechanical interlock between adjacent turns, but flexible
and non-twistable waveguides may be manufactured by
rigidly securing they adjacent turns to each other, e.g.
by soldering, brazing or welding.
CA 02103570 2001-10-09
3A.
There is also disclosed a method of manufacturing tubular
waveguide of substantially elliptical cross section, said
waveguide having a wall which, when viewed in
longitudinal section, is formed in a series of ribs
separated by intervening grooves, said method comprising:
- continuously winding a pre-formed electrically
conductive metal strip about a rotating mandrel of
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substantially elliptical cross section in a generally
helical configuration such that adjacent edge portions
of adjacent turns of the strip are interengaged, the
strip being wound onto the mandrel together with a
support element: for supporting the interengaged edge
portions of the strip, and
- performing a secondary forming operation on said edge
portions, whilst said edge portions are supported by the
support element on the mandrel, so that said edge
portions become folded over each other and form a
mechanical interlock between said adjacent turns with
said interlocked edge portions extending along the crest
of said ribs.
BRIEF DESCRIPTION OF THE DRAWINGS
The invention is exemplified in the accompanying
drawings, in which:
Fi ure 1 is a perspective view of waveguide of
the invention in the process of manufacture,
FiQUre la is an end view of the waveguide of
Fig. 1 whilst wound on the mandrel,
Fi ure 2 is a longitudinal section through a
wall portion of the waveguide,
Fi ure 3 is a further longitudinal section
through a wall portion of a different
waveguide of the invention
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FiQUre: 4 is a side view of a joint between two
lengths of waveguide of the invention,
FiQUrea 5 and 6 show an adaptor connected to
an end of the waveguide in perspective view
and longitudinal section respectively, and
FiQUrea 7 to 9 are longitudinal sections
through different forms of the adaptor.
DETAILED DESCRIPTION OF THE DRAWINGS
Hy way of example, the waveguide of the present
invention may be manufactured from a thin (e. g. 0.106
mm) strip of brass, metal-plated brass (e.g. plated with
a 3 to 4 um layer of silver, tin, gold, nickel or
palladium nickel), or other metallic or conductive
material (e. g. solid silver). The strip may typically
be 5.715 mm wide:.
Referring to Fi.g. 1, the strip 1 is passed through
formers 2 (e.g. rollers) which form the strip into the
required cross-;sectional profile at 3. The profiled
strip 3 is then fed through a forming tool 4 which feeds
the strip aroundl a rotating elliptical arbour (mandrel)
5 of the required major and minor dimensions (e. g. 14.71
mm by 8.29 mm) such that adjacent turns 6 of the
resulting helix 7 form a mechanical interlock with each
other. Hy pre;ssing against the profiled strip, the
forming tool 4 performs a final forming operation on the
strip 3 so that 'the f final waveguide includes a series of
angular helical ribs 6 of substantially square or
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rectangular cross section separated by intervening
grooves. As thE~ waveguide is formed it is pushed along
the arbour 5 in direction 12 so that a continuous length
of waveguide passes off the end of the arbour 5. The
cross sectional size of the waveguide can easily be
changed simply by changing the size of the arbour.
Figure 2 shows a mechanical interlock 8 which can be
used to form flexible and twistable waveguide. The
profiled strip 3 is wound onto the arbour 5 together
with a support element 9 which supports the ribs 6 to
prevent their collapse and also supports the interlock
8 during final engagement. The adjacent edges of the
turns are folded back upon each other by the forming
tool 4 to compleae the mechanical interlock 8, which is
supported by th.e element 9 during manufacture of the
waveguide. The element 9 may be an aluminium wire or a
nylon filament, having a diameter of 0.91 mm for
example.
The mechanical :interlock 8 of Fig. 2 also incorporates
an electrically conductive seaming wire 10 which is
wound onto the elliptical arbour 5 together with the
profiled strip 3 and is interposed between the folded-
back edges of adjacent turns 6. This seaming wire 10
may for example be of 0.21 mm thick copper wire plated
with tin or silver. The seaming wire 10 enhances the
mechanical performance of the waveguide and reduces
microwave leakage from the waveguide.
The mechanical :interlock 8 which is shown in Fig. 3 is
similar to that of Fi,g. 2 but is suitable for flexible
and non-twistable waveguide. A support element 9 is
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again included, but the seaming wire 10 is replaced by
a length of solder wire 11 of e.g. 0.23 mm diameter.
After formation of the helical waveguide as described,
the waveguide is heated to melt the solder 11 which is
then allowed to cool so that the adjacent turns 6 become
rigidly bonded l.ogether in the region of the interlock
8.
Long lengths of the waveguide can be manufactured by
joining shorter lengths using a jointing flange 15 ( Fig.
4) which is soldered, brazed or welded to the adjacent
ends of the two lengths of waveguide. Another method is
to join a new si.rip 3 onto the end of a previous strip
3 prior to application to the mandrel, by soldering
brazing or welding or by other suitable means, so that
the resulting waveguide is of a substantially continuous
form with less-obvious joints.
The helix 7 ma5r be covered by a protective layer of
elastomer, heat-shrunk polymer, organic or metallic
braid, or the hike, to provide added environmental and
mechanical protection. Flexible and twistable versions
can have a further covering of an electrically
conductive elasitomer for example, in order to provide
improved electromagnetic screening and isolation
properties.
Another version of the waveguide may be manufactured so
that the screening effectiveness is deliberately very
low, thus formir.~g a "leaky feeder" waveguide. This can
be achieved by forming the waveguide with a loose, non-
rigid interlocH: between adjacent turns, and/or by
omitting the seeuning wire 10.
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In order to electrically connect and match the
elliptical waweguide to a different microwave
transmission link, e.g. a standard rectangular
waveguide, a spe~~ial elliptical-to-rectangular waveguide
flanged adaptor is required. This can be soldered,
clamped or attached by other suitable means onto the end
of the elliptical waveguide. A suitable adaptor 20 is
shown in Fig s 5 and 6, and comprises a rectangular-
section waveguide part 21 which terminates in a flange
interface 22. The opposite end of the rectangular part
21 is joined to an elliptical section 23, the internal
dimensions of which are similar to those of the
elliptical wavec~uide 7. The free end of the elliptical
section 23 is provided with an internal annular recess
24 to receive the elliptical waveguide, which is
soldered, mechanically fixed, adhesively bonded or
otherwise secured therein. An internal step 25 is
formed between 'the rectangular and elliptical sections
21 and 23. A set of tuning screws 26 are inserted
through the wall of the rectangular and elliptical
sections, and these screws are placed one eighth of a
wavelength apart: so that by screwing them in and out of
the adaptor theft can be tuned to match the rectangular
and elliptical sections.
Instead of the step 25 the elliptical section of the
adaptor 20 can t>e tapered, as shown in Fig. 7. In each
case the tuning screws can be located in both of the
wider walls of t:he waveguide or omitted altogether.
The adaptors of Fig. s 5 to 7 may be of brass, copper,
aluminium, stainless steel, titanium, alloy or polymer.
The inner surface oflthe recess 24 may be plated with
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silver, tin or cold for example, to aid soldering to the
elliptical wave~~uide.
The waveguide o:E the invention could also be interfaced
to other forms of transmission line via a similar
matching adaptor of suitable design, for example
circular, double ridge, single ridge, and quad ridged
dielectric wave;guides, or elliptical waveguide of a
different size or orientation. Fig. 8 shows an adaptor
for matching :into coaxial transmission lines, the
adaptor including a launching probe 28 and tuning screws
29.
The waveguide can also be coupled to surface propagating
microwave lines, such as strip line, microstrip line and
finline. Fig. 9 shows an adaptor for matching into
microstrip line 30, incorporating a matching waveguide
ridge 31 and turning screws 32.
The waveguide is used to conduct electromagnetic wave
energy from an electromagnetic generator. The
performance of the waveguide of the invention is
superior to that of flexible rectangular waveguide
designed to operate at the same frequency. For example,
the microwave attenuation of rectangular flexible
waveguide type 'WG19 from 16 to 20 GHz is 0.9 dH/metre
whereas the microwave attenuation of the equivalent
waveguide of thE: invention is less than 0.4 dB/metre.
The maximum microwave power handling of the waveguide of
the invention i:; also superior to standard waveguides.
For example, from 16 to 20 GHz the maximum power
handling capability of rectangular flexible waveguide
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type WG19 is 0.21 KW whereas the figure for the
equivalent waveguide of the invention is 0.5 KW, with
the waveguides filled with air at ambient temperature
and pressure, and dry.
The return loss of the waveguide of the invention to
elliptical transitions at each end depends upon its
length and size. By way of example, a one metre length
of the waveguide described above has a return loss of
better than 27d13 within its operating band.
The minimum bend radius in the E plane for the waveguide
of the invention is better than for other forms of
elliptical waveguide. For a typical waveguide of the
kind described., the minimum bend radius whilst
maintaining the microwave specification is 25 mm in the
E plane and 62 mm in the H plane. The minimum bend
radius for other kinds of flexible elliptical waveguide
is typically 150 mm in the E plane and 380 mm in the H
plane.
A further advantage of the flexible and twistable form
of waveguide of the present invention is the high degree
of twisting which can be achieved without degrading the
performance of the waveguide beyond the permitted
specification. The maximum amount of twist is typically
360o per metre. The maximum twist for other forms of
flexible elliptical waveguide is typically only 6o per
metre.
A given wavegu:ide of the present invention operates
effectively within a relatively small frequency band,
for example 15 t.o 20 GHz. However, a range of different
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sizes can be manufactured to cover a typical range of,
but not limited to, 0.50 GHz to 50GHz.
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