Yes, unfortunately you have to buy the GXM30 but it can be bought for around $180 at one place I saw on a thread here in one of these forums. The GXM30 has it's own ID which you use to activate the device which is really the radio plus antenna in one package. No way around it!!!
That's why I didn't buy the 2820. Didn't need the Blue Tooth option and the GXM30 didn't come with it.