Can you get your antenna onto the roof of the apartment building? In my experience a bottle of whiskey gifted to the super helps here! Even if you have to run 10 storeys up a riser, a preamp will overcome this loss easily. Our ham radio club had our repeater on top of a 35 storey building using the above method and we had a ten year run before the old guy retired. Long before then our one member had moved out of the building so we had to vacate.