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 !"

Hello Sir,
If we go via Omini antenna radiation pattern the radiation in max perpendicular to the antenna and then we have the radiation lobe around it.
> If we keep Antenna pointing towards the RX then it will be a null zone and this will result in range reduction.

> If we keep antenna vertical ( ref. to ground) then we are utilizing only half of the lobe ( neglect the reflected signal etc.) there is null zone right on top of head.

> I feel ideally antenna should be kept horizontal to ground so that radiation lobe covers maximum flight envelop and always has the strongest part pointing towards the model. ( since we will be always moving the TX in the direction of the model)

I presume you mean the Rx antenna kept horizontal. And the Tx antenna? It can rotate fully z-theta. So should that be horizontal too? Pointing towards the pilot or at 90 degrees to him?

I am talking about the TX antenna only.

RX antenna polarization or Vertical / horizontal does not matter  ( we are talking about 2.4 GHz signal and not the Video signals). Since the model can be in any orientation...

to save from this we should place the receiver antenna RX1,RX2 @ 90 degree to each other.

And where should the Tx antenna point towards? The plane, the pilot or along the width of the Tx?

Along the width of the TX.

This way we get best ( strong ) signal right from top of our head till ground ( 90degree). see pictorial illustration above.

Well reasoned 

Useful stuff. Thanks Saurabh

Also to add, its a large misconception that the whole wire is the antenna like in 72mhz RX, In 2.4 ghz RX only the exposed top 3cm is the antenna and that should be straight for max gain and rest of the cable is coaxial extension.