Capacitive coupling of 2 microstrip lines separated by air(special geometry)

Hi,
i am trying to capacitevely couple the maximum of power from a first microstrip line to a second one.these lines are not parallel so they do not lie on the same substrate (picture below).

These 2 lines have special geometry and are separated from each other by an airgap.

Each transmission line is constituted by discontinuous conductors disposed on opposit sides of the substrate layer.(picture below)
A second substrate separe Each line and the aluminium ground.
Each line has 2 ports:
- port 1 and Port 2 are related to the first line.
- Port 3 and Port 4 are related to the second line.
Before coupling the 2 lines, i succeeded to obtain an S21= -0.1 dB ( for the first line) and an S43=-0.10 dB (for the second line) which are very good.

The problem is that when i tried to couple the lines, my maximum S31 or S41 is - 4.9 dB which is not good at all :-( , since i am trying to couple all the power from the forst line to the second (-1dB<S31 & S41<0 dB)
What are the boundaries to use?
Can you help me please to have an S31 or an S41 less than -1.5 dB :roll:?
https://obrazki.elektroda.pl/18_1332170289.png
https://obrazki.elektroda.pl/90_1332170338.png
https://obrazki.elektroda.pl/5_1332170357.png
https://obrazki.elektroda.pl/38_1332170374.png

Decrease the gap between line1 and line2 may improve, but there is limit for that.

Thank you for answering tony_lth,
in fact normally the airgap shoud be between 2mm < airgap < 3 mm, i've decreasing it till 1.1 mm though and nothing
i think that the problem is due to Ports dimensions. even if i've obtained -0.1 dB for each line separatly.
Normally the two grounds are connected the one to the other, should i connect them in my HFSS DESIGN too ?
can you give me your mail so that i can send you the design ?

The simulation should be at most the same as the actual situations. Connect them ,and add more vias, just as in the real life.

Thanks,
okay i will try to put some vias to connect the ground of "Line1" to the ground of "Line2"

---------- Post added at 12:46 ---------- Previous post was at 12:45 ----------

and i will use a gap 2mm instead of the 1.1 mm one.

Maybe you need some bunding wires. Otherwise, it seems impossible to get S21 < S31.

tony_lth said:

Maybe you need some bunding wires. Otherwise, it seems impossible to get S21 < S31.

Click to expand...

What's bunding wires Tony?

Assuming that i want to transmit maximum power from line 1 to line 2, i think that It's normal to have an S21 < -20 dB.
and and -1 dB<S31<0 dB

I've been using a driven modal waveport excitation port i don't think that it's necessary to use Driven terminal one

do you want me to send you the design so you could have a look?

Ivan here is the design :