Hello,
Just find some method to shift the resonance frequency of the antenna to lower level, it can be done by 2 ways, the first one is by using proper scaling like if you want to shift the resonant frequency from 3GHz to 1GHz then just simply calculated the scaling factor by 3GHz/1GHz = 3, that we have to enlarge the dimension of the antenna to approximately 3 time larger than the original size and we will get the same bandwidth at lower resonant frequency of 1GHz. Although this might not a best method but it is the fastest way to tune the resonant freq to low level but not so accurate from my little experience.
The second method that i found is by using equation from book, that is "speed of light = resonant frequency x lambda", just plug in the value for the C and resonant frequency, then we will able to get the value for lambda and the width of the antenna will be equal to lambda/2, then from here we can get our dimension for the low freq antenna.
Currently, the s11 results that i'm getting now is quite wide in bandwidth (as in high freq model) but still not very good in impedance matching i think, thus make the return loss value still less than 10dB, so might have to spend more time for tuning, hopefully can achieve a return loss of 30dB and over.
I have try to use the "optimization" function located in the transient time solver, but on the whole still not so sure how to use it, anybody can show some detail steps on using that function? say as an example optimize a parameter called "length" and achieve our target results in s11 plot say 30dB.