There are a few ways to match the impedance of a coplanar Vivaldi antenna. One way is to use a tapered slot. The tapered slot is a gradual reduction in the width of the slot, which helps to match the impedance of the antenna to the feed line. Another way to match the impedance is to use a stub. A stub is a short piece of transmission line that is connected to the feed line. The stub can be adjusted to provide the desired impedance match.

Here are some specific methods for impedance matching of coplanar Vivaldi antennas:

• Tapered slot: This is the most common method for impedance matching of coplanar Vivaldi antennas. The taper can be either linear or exponential. The linear taper is easier to design, but the exponential taper provides a better impedance match over a wider frequency range.
• Stub: A stub is a short piece of transmission line that is connected to the feed line. The stub is adjusted to provide the desired impedance match. The stub can be either inductive or capacitive. The inductive stub is used to match the antenna to a high impedance feed line, while the capacitive stub is used to match the antenna to a low impedance feed line.
• Combination of tapered slot and stub: This method can be used to achieve a good impedance match over a wide frequency range. The tapered slot is used to provide a broad impedance match, while the stub is used to fine-tune the match.
• Multi-feed: This method uses multiple feed points to improve the impedance match. The multiple feed points are spaced along the antenna, and each feed point is matched to the feed line.

The best method for impedance matching of a coplanar Vivaldi antenna will depend on the specific design of the antenna and the desired operating frequency range.

Here are some additional tips for impedance matching of coplanar Vivaldi antennas:

• Use a high quality substrate with low loss.
• Make sure the antenna is properly grounded.
• Use a simulation software to optimize the design.
• Experiment with different methods of impedance matching to find the best one for your application.

Murtadha Shukur thank you for your answer. Do you have any guidelines/books for calculating the Vivaldi antenna parameter?

1- Characterization of Printed Podal Vivaldi Antenna (8–18 GHz) on RT Duroid with Single and Double Cavity.)

2-An improved wideband Vivaldi antenna design (utrgv.edu) (https://scholarworks.utrgv.edu/cgi/viewcontent.cgi?article=1170&context=leg_etd)

3-https://www.mdpi.com/2072-666X/14/7/1329