Who said anything about towers? Hiding them in attics and rooftops is precisely about making them expensive and time consuming to remove. Triangulating the precise location of such devices carries a certain cost to the authorities during a crisis. They will have limited equipment and manpower to conduct such searching as well as a need to provide security for the same.

So what if it takes them just two days for something to be removed? Those two days of comms for the activists have been highly valuable, and there should be many other units still operating and hopefully others in reserve to activate.

I think there is a place for mobile solutions, however. How about suspending equipment with enough power to operate for a week from weather balloons anchored by monofilament? Set up a sensor, such that an unauthorized drop in altitude will trigger releasing the anchor? This way, if the authorities are trying to reel the unit back in, it will at least have another 1/2 hour of transmitting time. I'd also add some switches to trigger erasure of the keys for the encrypted hard drive and overwriting memory in the even of unauthorized opening of the case.