Transmission line and method for manufacturing transmission line

A transmission line comprising:

• an insulating substrate;
• a first ground conductor and a second ground conductor spaced at a distance from each other in a thickness direction of the insulating substrate;
• a first signal conductor and a second signal conductor side by side in a width direction of the substrate and between the first ground conductor and the second ground conductor in the thickness direction without any other conductor pattern interposed between the first ground conductor and the second ground conductor, the first signal conductor and the second signal conductor each being combined with the first ground conductor and the second ground conductor and defining a strip line;
• and
• a third ground conductor between the first signal conductor and the second signal conductor in the width direction;
• wherein
• the substrate includes:
• a first insulating layer;
• and
• a second insulating layer having a second relative permittivity lower than a first relative permittivity of the first insulating layer;
• and
• the first insulating layer and the second insulating layer are laminated in the thickness direction of the substrate and are in contact with each other;
• the first signal conductor, the second signal conductor, and the third ground conductor are on an interface at which the first insulating layer and the second insulating layer are in contact with each other;
• the first signal conductor, the second signal conductor, and the third ground conductor each include a surface in contact with the second insulating layer and a surface in contact with the first insulating layer;
• and
• the surface in contact with the second insulating layer is larger than the surface in contact with the first insulating layer.