Miniaturized loose coupler

As the coupling becomes looser than approximately 30dB the layout of meandered parallel-coupled line structure becomes impossible. To solve this problem, we have proposed split grounded strips to place in the middle of two coupled

microstrip lines so that the coupling electromagnetic field is blocked and the coupled-line spacing is reduced. To place the proposed split ground strips make it possible to add modal velocity-compensation interdigital capacitors to improve the directivity. The design procedures of the proposed coupler have been described in detail. Two couplers with 30- and 40-dB loose coupling have been designed at center frequency of 2 GHz as examples. The measured performances have shown good agreement with the simulated one. Therefore, the proposed method has been proven to be a feasible way to implement a high directivity loose coupler with compact size.

6.3 Review and the future works

From the view point of physical dimensions, the proposed meandered parallel coupled line owns an asymmetric outline. That makes the circuit will be always with two kinds of propagation modes whether even- or odd-mode signal is excited.

Although the mode conversion exists, it is not strong enough to disturb our analysis based on even- and odd-modes. It, however, can be treated more rigorously in the future. Therefore the analysis model should be modified and a modified equivalent circuit for the proposed structure is needed. That will make the analysis result more accurate.

In the future work, the proposed meandered parallel coupled structure can be applied to the miniaturized microstrip coupler which is inserted a grounded strip. Base on this idea, a loose coupler can not only reduce the coupled-line spacing by grounded strip but also shorten the coupled-line length and get high directivity by meandering.

It is a new topic what we will go. In addition, the coupler with TEM-like behavior can be extended to a Machand type balun with broadband and good balance. In band pass filter applications, a filter can be design to free off 2fo and 4fo based on the proposed mechanism. About the 3fo, other method should be induced to suppress or remove it.

These are some future works of this research.

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