I visited this site one day and sent a link to a family member that does communications work and used those rotors - back in the day and his first comment was wow - so you can still purchase a U 100.
So I replied and asked if he ever used them before. This is what he had to say - "I had a couple that I used on the CB antenna.... The U-100 was just too light for the antenna size. The wind would back turn the motor through the gears.. I rigged up a solenoid and a lever that would "lock" the motor when it was not turning... the main gear was made of pot metal. The wind loading just cracked it in half."
That got me to thinking and I looked again on the site and saw that the factory did the same thing for higher end models - for about $600 - $1000.00