Question regarding Gain & Radiation Efficiency?

[imtiaz369]

Hello, Good day. I have a question about gain and radiation efficiency in the interest of frequency region. According to S11, my interest in frequency bands ranges from 3.24 GHz to 8.74 GHz. However, if you look at the gain vs frequency graph, it drops after 3.24 GHz and then rises. The same thing happened in the radiation efficiency graph.


I'm not sure why this is happening; do you have any ideas about the graph's behavior?


Thank you in advance.

 

[vfone]

The reason could be that there is something (structure, resonator, etc.) which is coupled to the antenna.
If you post a picture of the antenna may give more information.

 

[imtiaz369]

The reason could be that there is something (structure, resonator, etc.) which is coupled to the antenna.
If you post a picture of the antenna may give more information.

Here is the picture of the antenna. It's a Vivaldi antenna; the backside is a microstrip feedline. Also, please explain more details if possible.

 

[vfone]

I got a similar structure and I was able to replicate the notch in the characteristic.
Seems that the notch is affected by the feed microstrip transmission line. Using a radial stub at the end of the feed line, may mitigate the notch frequency outside of the band of interest.