Noise Figure

Mar/1  2016

suddenly i ran into a puzzle about the Noise figure. a simple LC is a filter, it has attenuation, so base on the theory on everywhere, NF is increased the S/N decrease.  

but, i had this experience:  remove the LC selection from a simple superhet Radio, the Noise increased dramatically. this mean the S/N is increased very much because adding this LC selection circuits.  and i know exactly why, because this LC remove so much unwanted signal and wind-band noise. 

ooh, why book say NF will increase when it pass through the LC filter? and, i look out the book and i know why, because thermal noise can not be less than -174dBm/Hz, so signal been attenuated, so NF increase, elegent. 

now, why? why LC does increase the S/N? 

actually, this is a conception mass of me.  the book say signal "pass through" the device, not with or without it. actually happen is, the wanted signal's S/N is decreased, but, compare to no LC filter,  S/N definitely increased. that's all i messed up.