
OK , if 80 is open, it is open, if 40 , if 20 and so on, I have been a ham for near 40 years and never seen any propagation that would have one small segment open and aother segment in the same band dead with the exception of maybe 10 meters as that band is 2 MhZ wide and it might be possible that the MUF is splitting it but I would still find that to be very rare. I think when you hear what sounds like dead segments is due to just no one is on the band. Example, you can hear some fair amount of SSB on 40 in the day, usually regional stuff, less then 500 miles but go down to the lower CW part and it sounds pretty dead down below, that does not mean the band it dead just that so many CW ops like to get on 40 in the evenings when the band tends to go much longer and signals are stronger, also a lot of folks are working and don't get on the bands until they get home. Activity is your real culprit, or lack of, in the 70s 40 was raging 24/7, lots of ops on, same goes for the other bands when we had good sun spots, these days it is a different story, we are in the era of computers, cell phones and many other things competing for our time so many times even when the bands are in good shape, you may still hear many parts within the same band that seems dead. Another example is contest, signals come out of no where wall to wall, as soon as the contest ends, like someone flipped a switch, bands can go real quiet very fast, the band did not change, the activity did,
Just my 2 cents, 73 Jim N0XE