problem with epi-3601s

i have pci card 600mw epi-3601s Atheros AR5213 + computer p4 + antenna 15dbi omni
my siganl is very low and there is no stable
please help me
i would like to covered 8 km
when amp 1wat was on my network i covered this area

i would like to covered by this card

Available Transmit Power (Typical)
2.412~2.472G(IEEE802.11g)
26dBm @6 ~ 24Mbps
24dBm@36Mbps
23 dBm@48Mbps
22dBm@54Mbps

this card dont have 600mW in g…
you should reach 8km even with 100mW card, what do you use at the client?

i am just used b
2.412~2.472G(IEEE802.11b)
27 dBm@1, 2, 5.5 and 11Mbps

i have one client on 8km used airlive wl-5460ap v2
and one on 2200 m used the airlive wl-5460ap v2
and all of other used tp-link 501 access & 551g card

antenna hight is 27 m