What is the radio frequency of Airdrop transmission? I know the Airdrop uses WiFi direct to send the packet, but what is the radio frequency of it so I can capture the traffic. 

Is the WiFi 2.4Ghz, 5Ghz or both?
The protocol is called AWDL, trying googling that.  Here's a starter

https://stackoverflow.com/questions/19587701/what-is-awdl-apple-wireless-direct-link-and-how-does-it-work