I don't really know much about radio theory so I am not the best person to help you. I just did some reading and from what I saw, circular polarised antennae are generally used for FPV. According to this web page
http://blog.oscarliang.net/linear-circular-polarized-antenna-fpv/ , linear antennae actually have better range. Polarised antennae are used mainly because they reduce multipath interference (when the radio waves bounce off objects) since circular waves will just change their polarisation when they strike objects. The problem is that if left-hand polarised waves are received by right-hand polarised antennae (or vice-versa), there is a 30 dB power loss. I think it is best to use a transmitter with a long range module such as the EzUHF The website sums it up like this:
When to use circular polarization:
your receiver is close to large obstacles such as trees, or buildings (within 30meters)
acrobatic flying (constantly changing the position of the plane)
low altitude flying
flying very high altitude (you need a directional antenna)
When to use linear polarization:
flying out in the open field with no obstacles
very stable fly, does not roll or pitch a lot
long distance flying
standing on top of a hill
have no space for large antenna