About the reflection of two coaxial transmission line

How to estimate the reflection of two segments of coaxial transmission line with same characteristics impedance but different size?

Beacause there is a step in the center conductor, there is a fringing electric field that looks like a capacitance from the center conductor to ground. Because the center conductor current is impeded going from the bigger to the smaller diameter, it acts like a series inductance. So the classic model for such a step in diameter is a shunt capacitor, then a series iinductor, followed by another shunt capacitor. Most microwave analysis programs have the model, or specific model component values for your dimensions can be readily found in the literature.