Problem with directivity in 30 db backward directional coupler

Hi, I contruct stripline 30 dB Backward Directional coupler with substrate Air - PCB(Er=2.3) - Air.
I have problem with very small or zero directivity.

Coupled port 13
Directivity 32 is very bad

1 ------------------------------------------- 2
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| |
| |
3 4

Fig. DC 30 dB

When I construct this DC in dielektrikum. Directivity was better than 12dB.
I think that my problem is in difference velocity Even and Odd mode.
Is some way haw easy sonstruct DC 30dB at stripline with thin substrate and air around?

THX