resonant frequency and bandwith

Hi,
Why we calculate the bandwidth and resonant frequency below the -10dB range in the S11 parameter chart.

This is just a definition, you may also define your bandwidth being as -20dB... no matter.
But the standard definition -3dB one as power.

A return loss of 10 dB is considered acceptable for many applications. Specifying bandwidth where the return loss was 3 dB would generally not be a good idea, as such a return loss is unacceptable for most applications. Conversely, specifying bandwidth as the 30 dB return loss value would generally be a bad idea, as 30 dB is unnecarrily good for most applications.

What is an acceptable return loss depends on the application, but 10 dB is not an unreasonable number. Hence the reason its often specified.

Obviously for most resonate structures (e.g. antenna), the resonate frequency is important. One would be less interested in the resonate frequency of an attenuator, since that will be well outside the operating range of the device.

Additon,-10dB return loss means 90% input power will be transferred to load which could be accepted by almost applications. Of course,you can say -10dB is habit value.

Actually, a -10 dB return loss would mean the device is an amplifier such as an IMPATT diode, Gunn diode etc, returning 10x the input power.

Return loss should be a positive number of dB.

Dave

-10dB Return Loss (S11) came from the old antenna matching approach where the limit of the VSWR was 2:1
Return_Loss[dB] = -20*LOG[(VSWR-1)/(VSWR+1)].
That means 10% of the TX power is reflected back from the antenna, which most of the time is a maximum acceptable value for reflection.

Form what I read, return loss should be POSITIVE for passive antenna.