The present disclosure relates to coupled slow-wave transmission lines. In this regard, a transmission line structure is provided. The transmission line structure includes a first undulating signal path formed from first loop structures. The transmission line structure also includes a second undulating signal path formed from second loop structures. The second undulating signal path is disposed alongside of the first undulating signal path. Further, a first ground structure is disposed above or below either one or both of the first undulating signal path and the second undulating signal path.