Problem with patch antenna design

I was assigned the project of designing a patch antenna with the following specs:reasonant freq of 2.4 GHz and 500 MHz Bandwidth(Τhe S11 should be under -5 Db).I have calculated the patch and the feeder line dimensions and designed the layout in Momentum.When i simulate the Antenna without the Groundplane i get the desired results.How is this possible?I mean you need a Groundplane to get your Antenna working.If i put a Groundplane in the layout the system won't work.Moreover are there any tips on how i can choose the dimensions and the position of the grounplane?

I don't quite have an answer for your antenna working without a ground plane. However, the ground plane mainly affects the radiation pattern of your antenna. If it is a very small ground plane (same size as patch), you will get fields diffracting around it producing an almost omnidirectional pattern as opposed to a directional pattern. If you make the ground sufficiently large, the fields will be reflected to produce a more directional pattern. However, if the plane is too large, you may begin to get 2 main lobes (with minimum variation) in your radiation pattern. This might be due to the fields cancelling out due to superposition. All the best!

With only the side ground (no ground plane below) it is more like a monopole antenna.

If you have tuned the patch to resonance without the ground plane (field mostly in air) it is too large for patch antenna operation (field mostly in substrate). So check the patch dimensions. For the ground plane, just make it larger than the patch.

In fact in a standard patch antenna (with ground plane) the radiation occurs from along side edges L and W, so the surface metal conductor does not form the radiating element, as it does for example in a monopole or dipole antenna.
Removing the ground plane under the patch, it will change the EM operation mode of the antenna, and then, the radiation happen mainly do to the radiation of the conductor element (as in a basic monopole antenna).

Somewhere i came across a paper suggesting that the groundplane dimensions should be the same as the patch's plus 6*h,where h is the hight of the substrate.I tried it and it gave me pretty good results.Any opinion on that?

Sounds fine. Of course you could make it larger, but 6*h is enough so that the patch antenna does not "see" the limited substrate/ground size.

ok,i have another question now,how do you know how far from the patch should you position the Groundplane?

Not sure if I understand your question. Patch antennas have the ground plane under the patch (no offset). That ground must be larger than the patch, as we just discussed above.

Well I figured out the question was wrong from the beginning .The setup was was wrong. I had not mapped the layers plus I used the cond2 as groundplane while at the same time i hadn't deactivate Momentum's default groundplane.When I will have correct results I'll post them here.