signal is very low

i have atheros 5213 chipset based card (Tp link wn 651g 100 MW) & 12 dbi 90 degree sector antenna but my client have very low signal distance clint to Ap only 5 to 800 meters with clear LOS.
& client side using 10 dbi directional panel antenna
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Tplink is a very weak card, try something else.

Software Specification
Standards

IEEE 802.11g, IEEE 802.11b
Wireless Signal Rates With Automatic Fallback

11g: Up to 108Mbps(dynamic)
11b: Up to 11Mbps(dynamic)
Frequency Range

2.4-2.4835GHz
Wireless Transmit Power

20dBm(MAX EIRP)
Modulation Type

1M DBPSK, 2M DQPSK, 5.5M/11M CCK, 6M/9M/12M/18M/24M/36M/48M/54M OFDM
Receiver Sensitivity

108M: -68dBm@10% PER
54M: -68dBm@10% PER
11M: -85dBm@8% PER
6M: -88dBm@10% PER
1M: -90dBm@8% PER
256K: -105dBm@8% PER
Work Mode

max 20 dbm support but mikrotik show card rate txpower is only 17 dbm can i upto txpower to 20 dbm?

Sure you can, but after that, replace it with a new card.

anyway to check radio power coz in tp link web show the 20 dbm max but i have secter antenna 12 dbi & clear LOS but my signal is very low any one have idea?