The most important part of how to position your antennas is in the plane. If the Rxer and Txer antennas are cross-polarized, there is a 30 db signal loss. This is about a 95%+ loss of signal. You need to ensure the Txer and the Rxers antenna have a similar polarization (i.e. if the Rxer is vertically polarized, then the most effective polarization for the transmitter is vertical.) Also, vertical is better than horizontal as the ground tends to absorb horizontal signals. How do you achieve constant vertical polarization? Point the Txer's antenna straight up and the three Rxers in the fuse with each of their antennas polarized differently. That way you have at least one antenna vertically polarized at all times. In jets and Giants, this is important as we fly them way out there. In parkflyers this is not nearly as critical.
That being said, not much signal is lost at 45 degrees off from vertical. But point the antenna directly at the plane and you are presenting the Rxer's with worst possible case. By the way, I think the DX18 and for sure the DX9 has two antennas. One vertical and there is a horizontal one in the carrying handle.
Use Flight log data to determine your transmitter's coverage area. You start seeing frame losses and you know you are getting close to the limit. Use real-time telemetry with an alarm on frame losses and your planes will never fly away from you.