Recently I mounted a new network based on MT. I have worked with other companies before, and it's the first time that I have used routerboards. This is my AP configuration:

1 x Routerboard RB532A + RB502
3 x Compex WLM54AG (200mW)
3 x 17db horizontal polarized sector antennas
3 x 6m HDF-400 low loss antenna cable
Self bonding tape and PVC electical tape from 3M to isolate connectors.

Like CPE I'm using realtek 8186 based with 12db integrated panel antenna. It gave me good results with other networks so I don't want to change.

It's impossible to use 802.11g, because only two or three customers can connect at very low signal levels. In 802.11b the situation is slightly better, but signal levels still low. Two clients at less than 300m clear LOS have signal levels of -82 and -79 respectively.

Any idea?

Try disabling 2 of the three sectors and see what kind of signal you get off of just the one sector.
I beleive you have issue with the radio cards, radio isolation.

You may have to put the sectored radios on individual Router boards, and seperate metal enclosures.

I have disabled one of the sector to try this posibility, and the others two have 8 channels of separation. Noise floor are -103 and -101 in the enabled sectors. Signal levels exactly the same.

Could be your need to change antenna port setting on the AP.

This for the b/g Compex and the A/B/G card.
If your pigtail is hooked to the main port on the compex card the MT setting will be antenna B.
If your pigtail is hooked to the Aux port on the compex card the MT setting will be antenna A.

Thanks stealthwave, but I knowed it. It's only for the WLM54G23.