When multiple quads are racing around a course with pilots at the start-finish line, FPV video channels interfere with each other. Pilots loose their video as another quad races past.
There are 8 channels within the allowed 5.8GHz frequency band. The transmitters are not of the highest quality and adjacent channels interfere, especially when an adjacent transmitter is much closer to your receiver.
High power transmitters will just make the problem worse. Lower power transmitters, channel allocation, antenna selection and course layout etc. should help.
If the pilot position is set back from the course, you will benefit from the relatively short range of adjacent channels.
The problem will be worse when the model passes close by a pilot on an adjacent channel. If all models are always some distance from the pilots, the relative signal levels received will be more consistent over a smaller range. Receivers will be more selective and not swamped by powerful nearby signals.
The 5.8GHz band is quite wide and typically antennas are not at their best over such a wide bandwidth.
I think that due to the short wavelength at 5.8GHz, antenna dimensions and construction are quite critical when looking for best performance. Mass produced off-the-shelf antennas may be centred on the band but possibly not that good.
Although making these antennas is a PITA, if made for the lowest channel, may be less responsive to the higher channels etc.
The original antennas I made were dimensioned for the channel I was using at the time; the lowest frequency. These are the RH antennas I tested mated with the LH Hobbyking low-cost types, and found that mixing LH and RH didn’t work. Something that can be used to an advantage.