John Hancock Center nearing completion showing the antenna mounts.
From my personal archives. Original image source unknown.
An original model of the John Hancock Center. Note the stubby mounts. This would be a "later" model because the structural cross bracing shown is in the final design configuration.
From my personal archives. Original image source unknown.
I believe the JHC and Sears Tower both had structural tubes integrated into their original design. These later served as the mounting bases for the antennas. Large antennas were planned from the start. It's the point SOM tried to argue in defending the Sears Tower's tallest title from the Petronas Towers in the article I posted above. The antennas can be dismantled and removed, but the mounting tubes cannot. They are a permanent part of the upper structure. Obviously The Council on Tall Buildings didn't buy it.
Sear Tower after completion showing some communication equipment affixed to the tubes.
From my personal archives. Original image source unknown.