18db Yagi

Is it normal for a switch from a 13db yagi to a 18db yagi to only increase the strength by 1-2dbm?

Depends on the quality of the antenna. If you had a high quality 13dB and went to a low quality 18dB you will see little change. In the end, the only thing that matters is if the client stays registered or not.

The other thing to consider is the improvement in performance due to off-axis rejection. An 18dB antenna is narrower than a 13dB antenna, so the background noise the SM sees is reduced, improving the sensitivity of the SM.

Example: If the SM was seeing a noise floor of -81dB with the 13dB, and sees -84dB with the 18dB that’s a +3dB improvment. With the 18dB you get 2dB more signal level, thus the net improvement is +5dB. for a subscriber right on the edge of staying rregistered, that might be all you need.