hfss return loss i characterstic help

hi,i simulate patch antenna on hfss.it show. its is showing return loss of -28 db at 6.61ghz and -37 at 18 ghz.i want to know why it is showing on 6.61ghz not on other frequency.what is reason.plz help

I am not very much clear about your question. What I get is that you are getting S11 of -28 dB at 6.61GHz and -37dB at 18 GHz.
And you are asking that why it is not same?
If this is the question then answer is very simple that S11 which you have referred as return loss is frequency dependent parameter so it will change with the change of frequency there must be no surprise in it
Have a look at
http://www.antenna-theory.com/defini...parameters.php
If problem is different then let me know

i simulate a siepreski gasket antenna on hfss,with height 19.05mm and side of triangle are 22mm.i used fr4 substrate (thickness =1.5mm).my solution frequency was 30ghz. i got S11 of -28 dB at 6.61GHz and -37dB at 18 GHz.i want to know why it shows result on 6ghz and 18ghz.if there is any rule or formula to calculate that on which frequency antenna show return loss

ok Bit clear and bit confusion.
Since I work more on RF cavities so as far as I know HFSS returns S11 parameter value at the solution frequency and which is always single value. If you are getting two values it means there must be two structures defined in solution domain.
Which solver you are using? can you share pic of your result.
As honestly I am not getting any logical interpretation of your problem. So I want to be more clear about the problem. one thing I know is HFSS solves the model at solution frequency and returns S parameters on that frequency. May be your geometry and solution frequency are very different. Try to simulate the model on some different solution frequency and see what you get

i made sipreski triangle fractal antenna.i use solution frequency 18 ghz.now i am getting return loss on 6.6ghz ang 18 ghz.i want to know why it showing return loss on 6ghz.it has any any reason or formula?
return loss on 18ghz are due to solution frequency 18ghz.am i right

Good so one problem is solved. Yes 18GHz is due to solution frequency.
Since you are getting two S11 values that may be due to reason of two structures in solution domain having different S11.
Are you sure that you added all your parts properly with Boolean commands to make them single model.
I think there should be some issue there and that second model has different S11.
Make sure that you have one structure also define solution frequency according to geometry and not randomly