I suspect that the drop chance for broken/non-broken antennas is similar to how the containment suit drops work - the more you hit the drone in the fight, the more the chance that it will break.
Or at least my small biased dataset leads me to think that - 10 combats in a row where I did three fairly powerful ranged attacks and got 10 damaged antennas. Then did a couple fights where I just did a single move and got intact antennas right away.
25,30 dam
23,35,28 dam
21/25 dam
30 dam
24/23 dam
23,36,58 dam
27/28 heli
29/29 dam
20/40 dam
24/38 dam
37 dam
35 dam
31 dam
34/48/21 heli
29/46/37 heli
20/45/42 heli
14/44 heli
28/24/37 dam
6/4/6/10/12 heli
6/5/4/7/7/6 heli
6/4/6/6/5/7 heli
4/5/4/7/7/6 dam
4/7/6/5/ /4/8 dam
1/2/7/9/8/2/7 dam
1/1/1/1/2/2/2/1/1/ /1/1/2/2/2/1/2/ /1/8 dam
/ = round break
, = pass break