nulled return loss

Hi all,

What is the First Null Return Loss (FNRL)? I think it's applications are in Low pass filters.
Does any one know anything about this? Is it a criteria for LP filters?

Thanks.

Null in other words is zero response, means it can be for any filter or antenna.
If it is LPF then the RL at corner freq is the FNRL.

Many Thanks for the help.
Could you please tell me the amount of FNRL for the filter I've attached?

The (3-dB)cut off frequency is about 4.5 GHz.
The red plot is S21 and the blue one is S11.
What's the FNRL? (-40dB @ 4.2GHz or -15dB@3.8GHz)

I am confused with the result. It shows two results of RL. One may be from schematic and the other from sonnet EM simulator.

From the graph, I estimate that( as it is not table)
FNRL from one case is it is at 4.8GHz and 3.6GH in other case. Both are at -40dB.

Thanks for the help. I got it.