A Broadband Planar Magic

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A Broadband Planar Magic-T Using Microstrip-Slotline Transitions

Abstract:

In this term paper we have presented an improved broadband planar magic-T junction that incorporates microstrip/slotline transitions. In comparison with a prior broadband magic-T junction, this junction offers superior broadband performance, low phase and amplitude imbalances. Its geometrically simpler and its performance is less affected by fabrication tolerances. We have also implemented this device in ADS(Advance Design System)

Introduction:

A magic-T junction is a four-port waveguide junction two input ports (labelled 1 and 2, respectively) and two output ports (labelled E and H). The junction consists 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.

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.

There are potential uses for junctions like this one in commercial microwave communication receivers, radar and polarimeter systems, and industrial microwave instrumentation.

CIRCUIT CONFIGURATION:

The improved broadband magic-T junction 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 H and E ports

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.

ODD Mode:

Intheoddmode,the signalsfromports1and2are out-of-phase. This creates a microstrip virtual ground plane along the -axis of the magic-T and at port H .

ODD Mode:

In the even mode, the signals from ports 1 and 2 are in-phase, thus creating a microstrip virtual open along the -axis of the magic-T.

ADS Simulation:

Circuit parameters VALUE:

conclusion:

The presented Magic T structure shows significant improvement in bandwidth. The magic-T also has less parasitic around the tee junction, which makes it less sensitive to fabrication misalignment. This results in a much smaller phase imbalance in this design.

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