Ok, I tested the same Antenna booster over a super long distance (10km+)
My drone is on FCC mode, 5.8Ghz Auto (dual band but it loves to stay in 5.8GHz).
Without the booster, I notice that at various distances between 5 and 8km on the way out, that sometimes the signal will drop to 2 bars. At the same time if I maintain my RC at the drone (more or less), and attach the booster it will add at most 1bar of signal. Sometimes for heck of it I dismount the booster and usually I see the signal does drop by 1bar. Between 4km and 10km I may have dismounted the booster like 4 times, just to see the effect, but mostly kept it on for safety. That said, in my flight on the return at roughly 9km I could get 3 bars (Orange) without the booster. On the way out it could have dropped to 3 bars with the booster at the same distance.
This is also same for the return flight. But generally speaking I think return flight signal was stronger with or without the booster compared to the flight outwards.
For me, I consider this money well spent and it works, just don't expect a miracle. 1 bar of signal is handy when you are at long distance or poor signal strength.
I paid US$6 for it...I will keep this with me wherever / whenever I can.