1. Field of the Invention
This invention relates to directional transmission line couplers, and more particularly, to a compensated directional coupler for improved directivity of the suspended substrate or stripline type.
2. Description of Prior Art
Directional couplers have been used in transmission lines and in microwave receivers and in power sources for communications and radar in the forms known as "stripline", "suspended substrate" and "microstrip". In general, the type of coupler under consideration relies on "even" and "odd" modes (waves) of energy propagation. With the proper even and odd mode impedances, the coupler maintains an impedance match and a high directivity over a broad bandwidth when the even and odd mode velocities are identical. If the even and odd mode velocities are not identical then the coupler performance is poor. Unequal mode velocities can be due to: (1) using transmission line types that utilize only partially filled dielectric configurations (e.g., microstrip and suspended substrate) and (2) an anisotropic dielectric (i.e., a dielectric with a dielectric constant dependent upon the direction of the RF electric fields). In either case the even and odd mode electric fields "see" different effective dielectric constants and hence different effective mode velocities. It is necessary to compensate for this difference in wave velocity if directivity and an impedance match are to be maintained over a large frequency range.
A number of attempts have been made in the past to overcome this problem of phase velocity difference. One technique for overcoming the problem is the use of lumped capacitances. This technique has the disadvantage of limiting bandwidth of the coupler.
Other techniques, that may be broadband, have been developed for the case where the even mode velocity is less than the odd mode velocity (i.e., v.sub.e &lt;v.sub.o). Microstrip is a type of transmission line that results in v.sub.e &lt;v.sub.o. The techniques disclosed in U.S. Pat. No. 3,629,733 issued Dec. 21, 1971 to Podell; U.S. Pat. No. 3,980,972 issued Sept. 14, 1976 to Podell et al; and U.S. Pat. No. 4,027,254 issued May 31, 1977 to Gunton et al are for the microstrip case with v.sub.e &lt;v.sub.o. The Podell and Podell et al patents describe a coupler having two conductors printed on the surface of a dielectric substrate having periodically indented confronting edges positioned with respect to each other so that the spacing between the confronting edges of the conductors remains uniform. The even mode conductors are at the same RF potential and the even mode velocity is not appreciably altered by the indentations. However, the odd mode is greatly altered by the indentations since it effectively travels along the gap and "sees" a longer effective length (or equivalently a smaller velocity). Thus the velocity difference has been compensated. The Gunton et al patent utilizes coupled fingers to compensate for the unequal mode velocities.
The technique disclosed in U.S. Pat. No. 3,508,170 issued Apr. 21, 1970 to Poulter is to compensate for "end effects". The original main coupled region is composed of straight conductors in air (with equal mode velocities). The end conductors are curved and produce a variable coupling or mismatch. The compensation alters the main line mode velocities in order to correct for the errors at each end.
Another technique is disclosed in U.S. Pat. No. 4,178,568 issued Dec. 11, 1979 to Gunton. This patent utilizes a long coupler with a variable coupling to achieve a large bandwidth with warped modes.