why is the get not good if it land in the basket on top of the pole in rishut hayachid?
The Gemara in Gitin (79a) says that if a husband throws a get into a basket which is on top of a pole in a Reshus ha'Yachid belonging to his wife, she is not divorced. Rashi (DH "Avul") explains that this is because the top of the pole (where the basket is) is taller than the walls surrounding the poles, which make this area a Reshus ha'Yachid. A get is only acquired by the Reshus ha'Yachid of the woman when the walls making up the Reshus are above the get. This is because the acquisition can only be made through a "Chatzer ha'Mishtameres" -- "guarded courtyard," meaning that the item being acquired is guarded by the Chatzer. If the walls of the Chatzer are not guarding the item, such as in our case where the get is taller than the walls, and might be blown over the walls by a wind, the get is not considered guarded and therefore not acquired by the woman.