Microstrip Antenna gain problem

hello everyone
i am trying to simulate MSA in HFSS where i got results at specific points ;
but now if i change dimension even in small amount gain suddenly decrease to negative from 5 dbi
how?
help me regarding the subject

Hard to say without more information.

What sort of MSA? What dimension are you changing? What frequencies are you simulating?

what dimension you changed?

i am simulating my antenna on LTE/WLAN(1.8,2.3,5.2) bands wit inverted F design
antenna volume of 180x120
substrate fr4 in one design & other one is RTduroid 5880
i am changing 2 radiation arms of F

Check what definition of antenna gain you used:

(a) radiated power / incident power
or
(b) radiated power / (incident power - reflected power)

What we usually call antenna gain is (a) and this does not change rapidly when you tweak the radiator length. But you might have looked at (b) where mismatch is included.

Shouldn't it be the other way around? The standard antenna efficiency definition is represented by expression b):
accepted = incident - reflected

Gain is usually understood as the product of efficiency and directivity

Yes, of course, my bad!

It should be this way: "What we usually call antenna gain is (b) and this does not change rapidly when you tweak the radiator length. But you might have looked at (a) where mismatch reduces the gain."

@Electron3

I would expect the gain of an IFA to be somewhere around 2dBi or less, so the 5dBi figure seems fairly good anyway(i.e. better than a dipole).

What does the gain pattern do? What does the total_radiated_power say? Is it just that gain has moved somewhere else on the polar plot?

When you change the dimensions you would expect the frequencies shift slightly. I presume you are doing a fine enough sweep to see this?

What happens to the return loss (S11 or SWR)?

yes i placed 30 passes for my design.
gain pattern is the major problem i seen ere it comes into picture of 2 beams radiation instead of directive single one.
retunrn loss quite good about -20 to -25 db



i am working for GSM900/1800 for FR4 substrate material
i won't able to achieve my return loss bands till now in proper way on patch antenna line feed
what are the geometrical modification can i deal with
suggest the same ...
My dimensions of patch are 80x100mm
Ground plane 140x180

Patch or inverted F?



Do you know what antenna you are designing?

yes i am designing inverted F design with LTE/WLAN bands & GSM Bands which will cover all bands regarding Mobile base stations
i want to design a single antenna which will cover needs of 4G @ mobile base stations