Read this, and inviting expert comments:
"why you should have the antenna vertical.
As a fully qualified radio amateur that transmits in the 2.4ghz band I think I know what I'm talking about.
Our little radios only radiate a very small amount of rf power it's quite simple if your transmitter has its antenna vertical then so should the receiver.
Basically if your tx is vertical and you rx is horizontal your system is out of phase technically you rx is receiving a weaker signal .
With the frequency being very high a weak signal is something to be avoided at all costs.
Many people prefer to have the antenna curled up in the radio box all I can say to those people is you do it at your own risk !"