UWB circular antenna: input impedance (Zo= 50 Ω)
An impedance of 117 Ohms sounds pretty reasonable for a particular point in the band, it translates to a reflection coefficient of -8 dB or so. I don't think you should be optimising it further yet.

UWB antennas are a compromise, intended to work over a very wide range of frequencies.

To judge its performance you need to analyse it over its full range of frequencies, 2 to 10 GHz or so, and look at the reflection coefficient over the whole band. Generally these shape of antennas work well enough directly at 50 Ohms without a matching network. Using a simple transformer like a quarter wave line of a higher impedance, will help one frequency but hurt others.

To tune the antenna, try adjusting the shape and size of the blob, especially its distance from the groundplane and its curvature near the feedpoint.

If you're able to change the shape - there are many design papers published about these, try searching for UWB in the IEEE APS transactions, or just on google.

For example, my first hit is this paper which shows the following antenna and its reflection coefficient:
   

Which indicates that the impedance in your simulation is pretty typical of UWB antennas of this type.