Dielectric waveguide having higher order mode suppression filters

A dielectric waveguide for the transmission of electromagnetic waves is provided comprising a core of polytetrafluoroethylene (PTFE), one or more layers of PTFE cladding overwrapped around the core, the core and/or cladding having mode suppression filters of an electromagnetically glossy material embedded therein, and an electromagnetic shielding layer covering the cladding. The mode suppression filters are preferably mica cards.

BACKGROUND OF THE INVENTION 
This invention relates to a dielectric waveguide for the transmission of 
electromagnetic waves. More particularly, the invention relates to a 
dielectric waveguide having higher order mode suppression filters. 
Electromagnetic fields are characterized by the presence of an electric 
field vector E orthogonal to a magnetic field vector H. The oscillation of 
these components produces a resultant wave which travels in free space at 
the velocity of light and is transverse to both of these vectors. The 
power magnitude and direction of this wave is obtained from the Poynting 
vector given by: 
EQU P=E.times.H(Watts/m.sup.2) 
Electromagnetic waves may exist in both unbounded media (free space) and 
bounded media (coaxial cable, waveguide, etc.). This invention relates to 
the behavior of electromagnetic energy in a bounded medium and, in 
particular, in a dielectric waveguide. 
For propagation of electromagnetic energy to take place in a bounded 
medium, it is necessary that Maxwell's Equations are satisfied when the 
appropriate boundary conditions are employed. 
In a conventional metal waveguide, these conditions are that the tangential 
component of the electric field, E.sub.t, is zero at the metal boundary 
and also that the normal component of the magnetic flux density, B.sub.n, 
is zero. 
The behavior of such a waveguide structure is well understood. Under 
excitation from external frequency sources, characteristic field 
distributions or modes will be set-up. These modes can be controlled by 
variation of frequency, waveguide shape and/or size. For regular shapes, 
such as rectangles, squares or circles, the well-defined boundary 
conditions mean that operation over a specific frequency band using a 
specific mode is guaranteed. This is the case with most rectangular 
waveguide systems operating in a pure TE.sub.10 mode. This is known as the 
dominant mode in that it is the first mode to be encountered as the 
frequency is increased. The TE.sub.mn type nomenclature designates the 
number of half sinusoidal field variations along the x and y axes, 
respecitvely. 
Another family of modes in standard rectangular waveguides are the 
TM.sub.mn modes, which are treated in the same way. They are 
differentiated by the fact that TE.sub.mn modes have no E.sub.z component, 
while TM.sub.mn modes have no H.sub.z component. 
The dielectric waveguide disclosed in U.S. Pat. No. 4,463,329 does not have 
such well-defined boundary conditions. In such a dielectric waveguide, 
fields will exist in the polytetrafluoroethylene (PTFE) cladding medium. 
Their magnitude will decay exponentially as a function of distance away 
from the core medium. This phenomena also means that, unlike conventional 
waveguides, numerous modes may, to some degree, be supported in the 
waveguide depending upon the difference in dielectric constant between the 
mediums, the frequency of operation and the physical dimensions involved. 
The presence of these so-called "higher order" modes is undersirable in 
that they extract energy away from the dominant mode, causing excess loss. 
They cause, in certain cases, severe amplitude ripple and they contribute 
to poor phase stability under conditions of flexure. 
A launching horn employed in conjunction with a waveguide taper performs a 
complex impedance transformation from conventional waveguide to the 
dielectric waveguide. Techniques such as the finite element method may be 
used to make this transformation as efficient as possible. However, the 
presence of any impedance discontinuity will result in the excitation of 
higher order modes. 
Having described the ways in which higher order modes may be stimulated in 
such a dielectric waveguide assembly, mode filters for suppressing their 
presence will now be disclosed. 
SUMMARY OF THE INVENTION 
A dielectric waveguide for the transmission of electromagnetic waves is 
provided comprising a core of PTFE, one or more layers PTFE cladding 
overwraped around the core, mode suppression filters of an 
electromagnetically lossy material embedded in the core and/or cladding, 
and an electromagnetic shielding layer covering the cladding. The mode 
suppression filters may be affixed to a launcher. The mode suppression 
filters are preferably mica cards. The core may be extruded, unsintered 
PTFE; extruded, sintered PTFE; expanded, unsintered, porous PTFE; or 
expanded, sintered, porous PTFE. The core may contain a filler. The 
cladding layer(s) may be extruded, unsintered PTFE; extruded, sintered 
PTFE; expanded, unsintered, porous PTFE; or expanded, sintered, porous 
PTFE. The cladding layer(s) may contain a filler. The electromagnetic 
shielding layer covering the cladding preferably is aluminized tape, and 
most preferably is aluminized Kapton.RTM. polyimide tape. The dielectric 
waveguide may be further overwrapped with a tape of carbon-filled PTFE.

DETAILED DESCRIPTION OF THE INVENTION AND PREFERRED EMBODIMENTS WITH 
REFERENCE TO THE DRAWINGS 
A dielectric waveguide for the transmission of electromagnetic waves is 
provided comprising a core of polytetrafluoroethylene (PTFE), one or more 
layers of PTFE cladding overwrapped around the core; the core and/or 
cladding having mode suppression filters of an electromagnetically lossy 
material embedded therein, and an electromagnetic shielding layer covering 
the cladding. The mode suppression filters are preferably mica cards. 
The composition of the higher order modes which are created and supported 
in the dielectric waveguide assembly have field distributions which are 
unique from the desired, fundamental mode of propagation. Subsequently, it 
is possible to filter out these unwanted modes by consideration and 
placement in the waveguide of resistive cards such as mica. Placement of 
the mica cards should be such that there is little or no interruption of 
the desired mode. 
Because the desired mode is vertically polarized, it has no component in 
the same plane as the filters. However, the presence of TE.sub.mn and 
TM.sub.mn modes, where n.noteq.O, would mean that the filtering action 
would start to take place on these modes, thus leading to their 
attenuation. Depending upon the desired effect, these cards can be 
oriented as desired. They may be of arbitrary shape, but are preferably of 
the shapes shown in the drawings described below. These shapes ensure that 
there is a smooth transition into the launcher rather than an abrupt 
discontinuity, which would mean that the incident energy would be 
reflected rather than absorbed. 
The filters may be inserted into the cladding by slitting the cladding and 
fitting them in place. Alternatively, they may be embedded in the core by 
forming a slot and inserting them or simply forcing them into the core 
material. Another method is to cast or secure them in the launching horn. 
A detailed description of the invention and preferred embodiments is best 
provided with reference to the accompanying drawings. 
FIG. 1 shows the dielectric waveguide of the invention, with parts of the 
dielectric waveguide cut away for illustration purposes. When lauancher 20 
with conventional flange 21 is connected to dielectric waveguide 10, 
electromagnetic energy enters the launcher 20. An impedance transformation 
is carried out in the taper 13 of the core 12 of waveguide 10 such that 
the energy is coupled efficiently into the core 12 of dielectric waveguide 
10. Once captured by the core 12, propagation takes place through the core 
12 which is surrounded by cladding 14. The core 12 is 
polytetrafluoroethylene and the cladding 14 is polytetrafluoroethylene, 
preferably expanded, porous polytetrafluoroethylene tape overwrapped over 
core 12. A cladding layer of polytetrafluoroethylene may be extruded over 
core 12. Propagation uses the core/cladding interface to harness the 
energy. Mode suppression filters 15 may be secured to the wall of launcher 
20. The filters 15 are of an electromagnetically lossy material. 
Preferably, the mode suppression filters 15 are mica cards. 
To prevent cross-coupling or interference from external sources, an 
electromagnetic shield 16 is provided as well as an external absorber 18. 
The shield is preferably aluminized Kapton.RTM. polyimide tape, and the 
absorber is preferably carbon-filled PTFE tape. 
FIG. 2 is an elevational view, partly in cross section, taken along line 
2--2 of FIG. 1. Within the opening 17 of launcher 20, the mode suppression 
filters 15 are secured to the launching horn 20 such that, upon insertion 
of the waveguide 10 into the horn 20, the filters 15 may or may not 
penetrate and become embedded within the cladding 14. 
FIG. 3 is a pictorial view, partly in cross section, of the waveguide 10 
according to the invention and showing the core 12 surrounded by cladding 
14, electromagnetic shield layer 16 and external absorber layer 18. In 
this embodiment, rectangular mica cards 15 are inserted into slits in the 
cladding 14 and are oriented in the horizontal plane as shown adjacent the 
core 12. 
FIG. 4 shows a pictorial view, partly in cross section, of core 12 having 
mode suppression filters 15 located adjacent thereto as shown. The 
cladding and outer coverings are omitted for clarity of illustration. 
FIG. 5 shows an alternate embodiment of core 12 having triangular shaped 
mode suppression filters 15A positioned adjacent thereto. 
FIG. 6 shows a further alternate embodiment of core 12 having triangular 
shaped mode suppression filters 15B positioned adjacent thereto in an 
inverted configuration from that of FIG. 5. The cladding and outer 
coverings are omitted from FIGS. 5 and 6 for clarity of illustration. 
While the invention has been disclosed herein in connection with certin 
embodiments and detailed descriptions, it will be clear to one skilled in 
the art that modifications or variations of such details can be made 
without deviating from the gist of this invention, and such modifications 
or variations are considered to be within the scope of the claims 
hereinbelow.