improve signal strength on 9300 ? nokia ad-14 ?

I have bad or no reception at home when my 9300 is open and horizontal. (reception seems better with my treo 600 though).
Could someone give me a tip on how to improve my signal strength ? May be an accessory ? Could I use the nokia ad-14 with or without an antenna ? or is it too big and must it only fit in his car holder ?
Thanks !