Why accepted power larger than incident power in HFSS?

[kae_jolie]

shouldn't it be the other way around?

 

[tallface65]

You give no information...what kind of model? Complex port impedance? Is it a small difference (numerical residual) or large difference? more info please.

 

[kae_jolie]

Complex port impedance. I set the source power to 2W. I matched the lumped port source impedance with 73-j11 ohm

and the readings are:

Power Incident:1.9747 W
Power Accepted: 2.0181 W
Power Radiated: 1.9379 W


Thanks.

 

[ermai]

I have a problem like this....I always get simulation result which the radiated power is more than accepted power.....
Can someone tell me why and how HFSS does....

 

[kae_jolie]

I have a problem like this....I always get simulation result which the radiated power is more than accepted power.....
Can someone tell me why and how HFSS does....

This is different. When you get radiated power more than accepted power, which means radiation efficiency larger than 1, this means HFSS did not really do a good job measuring power radiated because your radiation box is not meshed as well as your source. To fix, you need to manually mesh your radiation box to lambda/6. Simulation will take longer to run, though.

 

[tallface65]

Your power issue is due to the complex load you have on the port. It is of note that HFSS, along with all other field solvers, consider a matched S parameter to be matching to the real part of the impedance only. Circuit solvers consider a match to be the complex conjugate of the load. This difference in terminology will cause your S parameters to be mathematically correct, based upon the convention used, but the Return Loss can be larger than one due to this definition. If the return loss is > 1, then the incident power, defined to be 1 Watt, will be increased as the solver 'sees' a gain.

Consider defining your own output variable that includes the complex conjugate definition for a match.

Have Fun.

 

[kae_jolie]

Consider defining your own output variable that includes the complex conjugate definition for a match.

How is that going to make a difference? Please elaborate. Thanks.

Also, I don't understand how S11 would be larger than 1 due to the different definitions? I'm confused.

Thanks.

 

[tallface65]

My suggestion alluded to setting the port impedance to, say, 50 Ohms and create an output variable for your S parameter based upon the complex conjugate definition of a match. Then your radiated power will be fine (though not correct for a complex load ) If you want the correct accepted and radiated power, then you can create the output variables based upon the complex impedance matching and an input of 1W...no problem. Many people do this, particularly for RFID (matching to a complex IC load)

S11 can indeed be larger than one if the reference impedance is complex but the S parameter definition only considers a real match. When this occurs, S Parameters are no longer linked to energy conservation...I have attached a brief description.

Have Fun.

 

[kae_jolie]

Thanks tallface. How do you create an output variable for S parameter based upon complex conjugate definition of a match? If you could give an example or step-by-step procedure in HFSS, I would really appreciate it.

 

[tallface65]

No time for either...I suggest you tinker with it or contact someone familiar with RFID and complex conjugate matching. The output variables are not difficult...I encourage you to at least make an honest effort to figure it out...

 

[kae_jolie]

Please see question I posted at https://www.edaboard.com/threads/208350/#post879673

I meant to post it here.

Thanks.