In the field of microwave and millimeter wave energy transmission, such as commercial automotive radar systems (e.g. DE/Delphi's 77 GHz Forward Looking Radar), a myriad of microwave or millimeter wave components are involved, including millimeter integrated circuits (MMICs), diodes, printed circuits, antennas, and possibly waveguide components such as voltage-controlled oscillators (VCOs) and isolators. Most of the components utilized are typically mounted on planar microstrip transmission line circuits since this method is extremely low cost. However some components, such as antennas, may be more preferably compatible with waveguide transmission lines instead of microstrip transmission lines. Therefore, when microstrip transmission lines are used in conjunction with waveguide transmission lines, there is a need for an effective way to transfer transmitted wave energy between the microstrip transmission line and the waveguide transmission line without serious return loss and insertion loss degradation.
One method of designing microstrip to waveguide transitions is to use probes to couple energy to and from the waveguide. However, at very high frequencies (such as 77 GHz) probes are very tiny and difficult to handle in a high volume manufacturing environment. Manufacturing tolerance errors can cause serious return loss and insertion loss degradation.
For example, one prior art coupling technique is known as a probe launch. A circuit board (e.g., a DUROID.TM. board) is cut back so that a tab having a microstrip transmission line which runs to the end of the tab, is inserted into the waveguide. The typical circuit board ground plane is cut away below the microstrip transmission line protruding into the waveguide so that the insulator portion of the board supports the "stick out" tab portion of the microstrip transmission line as a probe. The cutaway circuit board is placed into a waveguide opening, thereby creating a probe launch into the waveguide. However, the difficulty with such an approach is the ability to manufacture and assemble the components in a high volume manufacturing environment. It is somewhat difficult to cut the circuit board to make the microstrip probe and then slip the cut board into the waveguide structure such that there is good contact between the ground of the circuit board and the waveguide wall. Also, it should be noted that the waveguide opening where the circuit board is inserted must be carefully controlled so that the probe does not short circuit against the waveguide wall. As such, those skilled in the art can appreciate that the whole manufacturing and assembly procedure involved with providing a mechanically and electrically stable microstrip probe end launch is not straightforward.
Another similar probe launch technique also involves a microstrip transmission line on a circuit board (e.g. a DUROID.TM. board), where at an end point along the microstrip transmission line there are a series of vias in a rectangular pattern around the end point and through the circuit board and connecting with the typical circuit board ground plane. The rectangular pattern of vias conduct all the way to the ground plane. A waveguide back short then connects with the vias at the ground plane and waveguide walls are formed perpendicular to the duroid board at the end point so that a microstrip to waveguide transition is formed. This approach allows such end launching to be formed in the middle of a board rather than at the end as described previously with the cut board and "stick out" tab probe. This approach also requires having a sizeable opening in the waveguide which can produce unwanted leakage radiation. While this latter approach may be somewhat simpler to accomplish than the former cut board approach, similar manufacturing control problems as previously described still exist.
There is therefore still a need for an efficient, cost effective method and apparatus for coupling microwave or millimeter wave frequency range energy from a microstrip transmission line to a waveguide transmission line. The present invention provides such a microstrip to waveguide transition whose simple assembly makes it ideal for high volume manufacturing.