Tech Briefs

Economical broadband junctions have potential utility in diverse microwave systems.

An improved broadband planar magic-T junction that incorporates microstrip/slotline transitions has been developed. In comparison with a prior broadband magic-T junction incorporating microstrip/slotline transitions, this junction offers superior broadband performance. In addition, because this junction is geometrically simpler and its performance is less affected by fabrication tolerances, the benefits of the improved design can be realized at lower fabrication cost. There are potential uses for junctions like this one in commercial microwave communication receivers, radar and polarimeter systems, and industrial microwave instrumentation.

The Improved Broad-Band Planar Magic-T Junction incorporates a unique microstrip ring structure,microstrip/slotline transitions, and impedance-matching λ/4 transmission-line sections.
A magic-T junction is a four-port waveguide junction consisting of a combination of an H-type and an E-type junction. An E-type junction is so named because it includes a junction arm that extends from a main waveguide in the same direction as that of the electric (E) field in the waveguide. An H-type junction is so named because it includes a junction arm parallel to the magnetic (H) field in a main waveguide. A magic-T junction includes two input ports (here labeled 1 and 2, respectively) and two output ports (here labeled E and H, respectively). In an ideal case, (1) a magic-T junction is lossless, (2) the input signals add (that is, they combine in phase with each other) at port H, and (3) the input signals subtract (that is, they combine in opposite phase) at port E.

The prior junction over which the present junction is an improvement affords in-phase-combining characterized by a broadband frequency response, and features a small slotline area to minimize in-band loss. However, with respect to isolation between ports 1 and 2 and return loss at port E, it exhibits narrow-band frequency responses. In addition, its performance is sensitive to misalignment of microstrip and slotline components: this sensitivity is attributable to a limited number of quarter- wavelength (λ/4) transmission-line sections for matching impedances among all four ports, and to strong parasitic couplings at the microstrip/slotline T junction, where four microstrip lines and a slotline are combined.

The present improved broadband magic-T junction (see figure) includes a microstrip ring structure and two microstrip-to-slotline transitions. One of the microstrip/slotline transitions is a small T junction between the ring and a slotline; the other microstrip/slotline transition effects coupling between the slotline and port E. The smallness of the T junction and the use of minimum- size slotline terminations help to minimize radiation loss. An impedance- transformation network that includes multiple quarter-wavelength sections is used to increase the operating bandwidth and minimize the parasitic coupling around the microstrip/slotline T junction. As a result, the improved junction has greater bandwidth and lower phase imbalance at the sum and difference ports than did the prior junction.

The upper portion of the ring between ports 1 and 2, consisting of two λ/4 transmission-line sections that have a characteristic impedance of Z1 and meet at port H, serves as an in-phase combiner. The portion of the ring below ports 1 and 2 consists of two pairs of λ/4 transmission-line sections having characteristic impedances of Z2 and Z3 connected in series and meeting at the T junction. These sections are used to transform between the microstrip and the slotline, which has a characteristic impedance of Zsl. The slotline is terminated at both ends with stepped circular rings to provide a broadband virtual open circuit. Finally, the slotline output is transformed to a microstrip output at port E by use of a microstrip-slotline transition.

An experimental version of this junction, optimized for operation at a nominal frequency of 10 GHz, was built and tested. The experimental results show that this junction functions over the frequency band from 6.6 to 13.6 GHz (as defined by falloff of 1 dB) with an insertion loss of less than 0.6 dB, phase imbalance of less 1°, and amplitude imbalance of less than 0.25 dB.

This work was done by Kongpop U-yen, Edward J Wollack, and Terence Doiron of Goddard Space Flight Center.


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