End launched microstrip or stripline to waveguide transition with cavity backed slot fed by offset microstrip line usable in a missile

A low profile, compact microstrip-to-waveguide transition which utilizes electromagnetic coupling instead of direct coupling. The end of the waveguide is terminated in a cavity backed slot defined in a groundplane formed on a dielectric substrate. The slot is excited by a microstrip line defined on the opposite side of the substrate, offset from the slot centerline. A cavity covers the substrate on the microstrip side, and is sized so that no cavity modes resonate in the frequency band of operation. The transition is matched by appropriate selection of the length of the slot and the length and position of the microstrip.

TECHNICAL FIELD 
This invention relates to transitions between a waveguide and a microstrip 
line or stripline. 
RELATED APPLICATION 
This application is related to commonly assigned application Ser. No. 
08/247,732, filed May 23, 1994, "END LAUNCHED MICROSTRIP OR STRIPLINE TO 
WAVEGUIDE TRANSITION WITH CAVITY BACKED SLOT FED BY T-SHAPED MICROSTRIP 
LINE OR STRIPLINE USABLE WITH A MISSILE," by P. K. Park and E. Holzman. 
BACKGROUND OF THE INVENTION 
Microstrip-to-waveguide transitions are needed often in microwave 
applications, e.g., radar seekers. Modern millimeter wave radars and 
phased arrays have a need for a compact, easy to fabricate high 
performance transition. Usually, the antenna and its feed are built from 
rectangular waveguide, and the transmitter and receiver circuitry employ 
planar transmission lines such as microstrip line or stripline. The 
microstrip-to-waveguide transition plays a critical role in that it must 
smoothly (i.e., with minimal RF energy loss) transfer the energy between 
the transmitter or receiver and the antenna. Traditional 
microstrip-to-waveguide transitions are bulky, and they require that the 
microstrip line directly couple with the waveguide by penetrating its 
broadwall; such transitions are not very compatible with the thin planar 
structures of state-of-the-art radars. 
The conventional microstrip-to-waveguide transition employs a microstrip 
probe, and is difficult to fabricate because the microstrip probe must be 
inserted into the middle of the waveguide. A hole must be cut in the 
waveguide wall for the probe to penetrate. A backshort must be positioned 
precisely behind the probe, about one-quarter wavelength. Fabricating the 
transition with the backshort placed accurately is difficult. Furthermore, 
the transition does not provide a hermetic seal, and it is difficult to 
separate the waveguide structure which leads to the antenna and the 
microstrip. A separate set of flanges must be built into the antenna to 
allow separation of the antenna and transmitter/receiver. 
Another type of transition is the end launched microstrip loop transition. 
This transition is difficult to fabricate because the end of the loop must 
be attached physically to the waveguide broadwall. It is difficult to 
position the substrate precisely and to hold it in place securely. There 
is no hermetic seal, and also to separate the waveguide and microstrip 
line requires breaking the microstrip line for this transition. Further, 
the substrate is aligned parallel to the waveguide axis instead of 
perpendicular; such a configuration does not lend itself well to 
constructing compact layered phased arrays. 
SUMMARY OF THE INVENTION 
A low profile, compact microstrip to waveguide transition, employing 
electromagnetic coupling is described. The transition includes a 
termination for terminating an end of said waveguide, comprising a 
dielectric substrate having opposed first and second surfaces, wherein a 
layer of conductive material is defined on a first surface thereof facing 
the interior of the waveguide. The conductive layer has an open slot 
defined therein characterized by a slot centerline. A microstrip conductor 
is defined on the second opposed surface disposed transversely relative to 
the slot and offset from its centerline. In an exemplary embodiment, the 
conductor terminates in an open-circuited end located one-quarter 
wavelength past the slot centerline. A conductive cavity is defined behind 
the second substrate side. Dimensions of the cavity are such that no 
cavity modes resonate in the frequency band of operation of the 
transition. 
Dimensions and placement of the slot and placement of the microstrip 
conductor are preferably selected to match the waveguide and microstrip 
transmission line characteristic impedances.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
This invention introduces a low profile, compact microstrip-to-waveguide 
transition which utilizes electromagnetic coupling instead of direct 
coupling. An exemplary embodiment of a transition 50 for transitioning 
between a rectangular waveguide 52 and a microstrip line 58 is shown in 
FIG. 1. The end 54 of the waveguide 52 is terminated in a cavity backed 
slot 56 which is excited by a microstrip line 58 offset from the slot 
centerline 60. The slot 56 and microstrip line 58 are etched on the 
opposite sides of a dielectric substrate 62, fabricated of a dielectric 
material such as quartz. Thus, in the conventional manner, the opposite 
sides of the substrate 62 are initially covered with a thin film of 
conductive material such as copper. Using conventional thin-film 
photolithographic etching techniques, the dimensions of the slot and 
microstrip and their positions can be fabricated precisely, easily and 
inexpensively. The slot 56 is defined by removing the thin copper layer 64 
within the slot outline. To define the microstrip conductor 58, the thin 
copper layer is removed everywhere except for the material defining the 
microstrip conductor. Thus, the substrate 62 and line 58 define a 
conventional microstrip transmission line, except for the slot defined in 
the groundplane layer 64. A backshort placed one-quarter wavelength behind 
the microstrip line (required in conventional transitions) is not required 
in this transition. 
In this embodiment, the slot 56 is centered on the end 54 of the waveguide 
52, in that the longitudinal centerline or axis 68 of the slot is 
coincident with a center line extending parallel to the long dimension of 
the waveguide end, thus centering the slot along the short dimension of 
the waveguide; and the slot is also centered along the long dimension of 
the waveguide as well. This placement will depend on the type of waveguide 
for which the particular transition is designed. For example, the slot 
will be centered at the end of a circular waveguide. The microstrip line 
58 is disposed transversely to the slot longitudinal centerline 68 and 
offset from the transverse centerline or axis 60. 
In the typical application, the substrate 62 comprises a portion of a 
larger substrate, in turn comprising a larger microwave circuit comprising 
a plurality of microstrip lines defined on the substrate, and with other 
waveguides having their own transition in the same manner as illustrated 
for waveguide 52 and transition 50. 
When the microstrip line 58 is excited, currents flow on the line 58 and 
the ground plane 64 directly below it. If a slot is cut in the ground 
plane in the path of the microstrip, e.g., slot 56, the microstrip current 
(indicated by the arrow in FIG. 2) is disturbed, and an electric field is 
exited in the slot 56, as shown in FIG. 2. If the end of a rectangular or 
circular waveguide is placed adjacent to the slot, as shown in FIG. 1, the 
microstrip energy will couple to the slot electric field and into the 
waveguide. The transition 50 exploits this energy transfer property. 
The slot 56 also can couple the microstrip energy to unwanted modes such as 
the parallel-plate and dielectric surface wave modes; such energy would be 
wasted in that it does not couple to the waveguide and increases the 
transition energy loss. Moreover, in the event the transition is used in a 
larger, more complex circuit employing a plurality of similar microstrip 
to waveguide transitions, there can be interference between transitions. 
To eliminate the coupling to these unwanted modes, a rectangular cavity 70 
(see FIGS. 1 and 2) can be used to cover the transition on the side of the 
microstrip line 58. The cavity 70 is essentially a four sided electrically 
conductive enclosure, having a closed end parallel to the substrate 62 of 
FIG. 1. The cavity 70 includes a small opening 72 (see FIG. 1) defined 
about the microstrip transmission line to permit the line to exit the 
cavity without shorting to the cavity walls. If the opening maintains a 
spacing from the line equal to about three times the width of the line, 
typically no capacitive loading will occur. Smaller openings may require 
use of known measures to adjust for the effects of the capacitance. The 
cavity dimensions must be chosen so that no cavity modes resonate in the 
transition's frequency band of operation. The selection of cavity 
dimensions to accomplish this function is well known to those skilled in 
the art. 
To maximize the amount of energy transferred from the microstrip line 58 to 
the waveguide 52, the transition 50 is matched by appropriate selection of 
the length of the slot and the position and length of the microstrip line 
58. Typical waveguide characteristic impedances are of the order of 100 to 
350 ohms depending on the waveguide height. On the other hand, the 
characteristic impedance of the microstrip line is usually 50 ohms for 
most applications. One way to match these impedances is to use quarter 
wavelength impedance transformers on either the microstrip side or the 
waveguide side or both. These transitions add length and complexity to the 
transition. This invention eliminates the need for these transformers by 
taking advantage of the natural transforming characteristics of the slot. 
FIG. 2 shows the electric field profile of the slot when its length is 
resonant. The slot length is resonant when the input impedance seen at the 
slot centerline 68 is pure real valued. This resonant behavior is well 
understood: the voltage profile along the slot is sinusoidal, while the 
current remains constant. Thus, the impedance seen by a microstrip line 
placed at the center of the slot is maximum, while the impedance decreases 
as the microstrip is offset toward the slot edge; if the microstrip is 
moved all the way to the edge, it sees a zero ohm impedance. Thus, as the 
microstrip is offset toward the edge, it will eventually see a 50 ohm 
impedance. Further, by extending the open-circuited end 58A of the 
microstrip line 58 one-quarter wavelength (L14) past the slot centerline, 
as shown in FIG. 2, maximum current will excite the slot 56 and give the 
best match. 
The transition can be constructed without the cavity 70 backing the slot, 
and it can still be matched to the waveguide and operate well. However, if 
the transition is part of a more complex assembly including a plurality of 
transitions, then energy from one transition can interfere with energy 
from another transition. If, however, such isolation is not required in a 
particular application, the transition can omit the cavity 70. 
FIG. 3 is a simplified line drawing of an embodiment of a Ka-band 
waveguide-to-microstrip transition 100 in accordance with the invention. 
The waveguide 102 has a rectangular cross-sectional configuration which is 
140 by 280 mils. The quartz substrate 112 is 200 by 186 mils, with a 
thickness of 10 mils. The slot 106 is centered within the end of the 
waveguide, and is 124 mils in length by 20 mils in width. The microstrip 
conductor 108 is 21.4 mils in width, and is offset 59 mils from the center 
of the slot, with the open circuit end 108A extending 52 mils above the 
slot centerline. The cavity 120 has a depth of 50 mils. A channel 122 is 
provided for the microstrip line, and is 79 mils high, by 135 mils deep, 
and 65 mils wide in this exemplary embodiment. 
FIG. 4 shows a waveguide to stripline transition 150 for transitioning 
between a rectangular waveguide 152 and a stripline, employing a cavity 
(172) backed slot 166 in accordance with the invention. This transition is 
similar to the microstrip to waveguide transition 50 of FIG. 1, except 
that the stripline conductor 156 is sandwiched between two layers of 
dielectric. As in the transition 50, a dielectric substrate 160 is 
disposed at the end 154 of the waveguide 152. The substrate surface facing 
the interior of the waveguide is covered with a conductive layer 164, in 
which the slot 166 is defined by selectively removing the conductive layer 
within the slot outlines. On the opposite surface of the substrate 160, 
the stripline conductor 156 is defined by selectively removing the 
conductive layer covering the surface 168. In contrast to the waveguide to 
microstrip transition 50, the transition 150 includes a layer of 
dielectric 162 adjacent the conductor surface 168 of the first substrate 
160, so that conductor surface 168 is sandwiched between substrate 160 and 
dielectric layer 162. 
One particular application to which the invention can be put to use is in 
the RF processor of a missile, e.g., an air-to-air missile having a seeker 
head to guide the missile to a target. One such missile 200 is shown in 
simplified form in FIG. 5. The missile includes an antenna section 202, a 
transmitter section 204, a receiver module 210 including an RF processor, 
and a seeker/servo section 206. The receiver module is shown in further 
detail in FIG. 6, and includes a module chassis 212 which supports several 
active devices including low noise amplifiers 214. The module includes an 
LO input port 216 and a receive signal port 218. The LO and receive 
signals are delivered to the respective ports via waveguides (not shown) 
connected at the back side of the housing. A quartz substrate (not shown) 
carries microstrip or stripline circuitry (not shown in FIG. 6) used to 
define the waveguide to microstrip transition or waveguide to stripline 
transition in accordance with the invention. The cavity backing the 
transition is defined by sides of the chassis channel 217 and 219 and the 
module cover 220. In this example, the microstrip or stripline conductor 
leading away from the LO port 216 is connected to a mixer/control circuit 
located in area 222 of the chassis, and the microstrip or stripline 
conductor leading away from the receive signal port 218 is connected to 
the low noise amplifiers 214. The receiver module 210 is sealed 
hermetically at the two input ports 216 and 218 by the quartz substrate 
covering the port openings and being sealed to the chassis around the 
perimeter of the openings. The particulars of the waveguide to 
microstrip-line or stripline transitions are as shown in FIG. 1 and FIG. 
4. 
Current trends in RF seeker design emphasize the reduction of cost and 
volume while achieving high performance. For millimeter wave radars and 
phased radars, the packaging of the seeker is a significant problem. In 
some cases, although the components can be designed and built, they all 
cannot be placed physically within the seeker envelope. To integrate the 
antenna with the transmitter/receiver circuitry is a difficult task with 
conventional, bulky microstrip-to-waveguide transitions. A typical active 
phased array can easily require hundreds of these transitions. This 
invention provides tremendous cost savings and volume reduction and can 
make presently unrealizeable radar designs feasible. 
This invention provides a low profile end launched microstrip-to-waveguide 
transition which has the following advantages compared to existing 
microstrip-to-waveguide transitions: 
1. A microstrip line does not have to penetrate the waveguide. 
2. A backshort does not have to be placed one-quarter wavelength behind the 
microstrip line. 
3. The transition is compact and easy to fabricate from a single piece of 
dielectric substrate. 
4. The transition is compatible with the planar structure of standard 
transmitter and receiver modules used in phased arrays. 
5. Often, to physically separate the antenna and transmitter or receiver 
assemblies is necessary for testing of the components. Performing this 
separation with conventional transitions usually requires that one break 
the microstrip line. This transition provides a natural flat surface (the 
substrate 58 with the slot in FIG. 1) to easily separate the assemblies 
without breaking any circuitry. 
6. The transition substrate automatically creates a hermetic seal for the 
transmitter and receiver assemblies, typically located on a microstrip or 
stripline circuit board. In particular, the receiver typically has 
delicate wire bonding and active semiconductor elements which need the 
protective hermetic seal against corrosion. 
It is understood that the above-described embodiments are merely 
illustrative of the possible specific embodiments which may represent 
principles of the present invention. Other arrangements may readily be 
devised in accordance with these principles by those skilled in the art 
without departing from the scope and spirit of the invention.