You may have noticed recently — as I did walking toward the Castro — that one of the antenna towers on Sutro Tower looks like it broke off. But it's actually just getting replaced with a new one with higher-tech antennas inside.
Hoodline got the word after (of course) some Redditors did some quick sleuthing, and the company Dialectric appears to be behind the work — which involves adding more broadband antennas within the "stack," and moving local ABC affiliate from VHF channel 7 to VHF channel 12. (It seems like it will be still be ABC 7, but it will be on "virtual" channel 7, and meanwhile KRON-TV, currently on "virtual" channel 4, will be moving from UHF to VHF channel 7.)
As Dialectric explains, "This is one of our most complex repack antenna designs, due in large part to the strict earthquake code that applies to antennas that are installed in the Bay Area." The work expected to be done by the end of the year, and the third "candelabra" arm of the tower will get back to its normal height.
The Sutro Tower newsletter reported on the work in July, noting, "When the repack project is complete, 30% of the radio frequency spectrum previously allotted to television broadcasting will be converted to wireless purposes, allowing the rollout of 5G cellular technology nationwide."
Below is apparently an image of what the new candelabra arm looks like.