What is shown in the pics here are not stacking but phasing, like dual antenna's on a truck. Stacking would be mounting them horizontally but same results.
I bought one that I figured was sufficient and it only cost 130.00.. Well, I have been reading about it. Bad Choice, but I did not keep the receipt, so I will have to use it. RCA looks good on the box, but not a good unit apparently. and it is $10.00 cheaper on-line.Funny; you didn't say what rotor you have.