Confused - Microstrip Antenna / IE3D

Hi.. I'm using Zeland's IE3D software to simulate a microstrip patch antenna.

Size of patch is about 8mm x 6.3mm
substrate thickness = 0.381mm
dielectric constant = 2.33
Shud operate at a frequency of about 15 GHz

now, i'm confused abt this..

While taking simulations...
1. Can i do like this (why/why not) ? - i keep the patch size and substrate thickness constant But change the dielectric constant of the substrate.

Or if i change the dielectric constant - i should change the patch size also .. Is this mandatory ?
if so.. why ?

Now...
Say, supposingly, in my simulations, i'm not too concerned about operation at 15GHz.. . then in this scenario.. can i change the dielectric constant (and keep patch size and substrate thickness constant) for different simulations.

I wud be grateful if someone can help me out in this..

it depends on your application. If you are looking for parametric analysis, where u r studying the effect of dielectric, u can change the dielectric constant without changing the geometry.

But if u are going to actually change the substrate and still want the antenna to work at 15GHz, then u have to change the geometry when u change the dielectric constant

thanks... yes.. i'm trying to study the effects of parameter change such as dielectric constant (and so loss tangent) without changing the geometry..

I will 1st be using IE3D software to get various results.. and then later on try to fabricate..

Since i will be keeping the geometry same and only change the dielectric constant (and loss tangent simultaneously) during simulations from IE3D, i think the centre frequency will change.. right ?

So while setting up the software for simulation in IE3D .. what should i enter in this field - "Highest Operating Frequency Used "

FOr the original case shown at the start of the thread, i used this Operating Frequency setting as 16GHz. BUt that was for 1 set of dielectric constant, width, length of patch etc..
but now.. since the dielectric constant will be changed, i assume "The Highest Operating Frequency" value should also change.. Because by changing the dielectric constant, i dont know for which frequency range (or centre frequency) will this new set of antenna paramters work.. Or is there any formula to calculate this ? OR i just randomly put 20GHz and simulate and see if the S11 curve goes below the -10dB value.. but this will be too tedious..

Thanks