Dual-band Monopole in Microstrip

Hi everybody,
I am having a little trouble in designing a dual-band monopole by microstrip at frequency 1.8 GHz and 2.6 GHz. I can't adjust the length l1 and l2 (in the figure) to responsible for these frequency. However, I can design good one with two frequency is 2.45 GHz and 5.2 GHz by this geometry. Could anyone give me idea about the length of l1 and l2 of my problem?

GEMMY, puts all sizes...!

I just get size for dual-band of 2.45 and 5.2 GHz (l1 = 26.5mm, l2 = 5.3mm). For this length the plot of S11 is very good (figure below) but i can't adjust dimension for dual-band of 1.8 and 2.6 GHz.

thank you so much for your attention!

,, put these dimensions...l1=36mm, l2=10,6mm

How can you get a plot/graph for 2 ports, when actually you have a single port dual-band antenna?

I have project designing coupled antenna by Decoupling Network for antenna at frequency 1.8 and 2.6 Ghz. So far i thought should do two antenna with two port first, then i take the decoupling network after. What do you mean, vfone? What should i do first, now?
thank you for your attention!

Simulating the two antennas separate doesn't give much information, because things will change a lot when put antennas together.
The best would be to use from the beginning a single port for both antennas and do the tuning for best dual-band performances.

Thank you so much, vfone
You must have many experiences, i now like to ask you about the "substrate" and "ground" for this antenna in my project (especially in their dimensions). I do an antenna with one-port and it have good dual-band at 1.8GHz and 2.6GHz. Now i will have to do another antenna like that and make them coupled (MIMO, and they will be symmetry). Do you think the dimensions of the substrate and the ground now will be douple of the ones when only have one antenna?
Thank you, again!

The substrate dimensions should not be affected by the number of printed antenna elements (or slots), but ground would be larger for more elements.
Hard to estimate if ground needs to be double for two elements compared to one element, could be little bit smaller. The EM simulator gives you all the information.