Airdrop not working on iMac later 2011

i have iMac later 2011 with Yosemite and have 2 iPhone 6 plus with iOS 8.1 , my 2 iPhones can see each other's airdrop and transfer with no problems, but none of iPhone can see my iMac nor my iMac see anything or give any options and i saw on youtube that airdrop in finder the round signal beside the word airdrop it moves like fan but mine doesn't, it looks like it's not active to me, i know nothing about it and i can't find any help online, please help ; BTW yes its all on same wifi and bluetooth enabled also logged in iCloud.