diegog asks:
"Is there an external antenna one can buy to solve this?"
No.
What you CAN buy is a 3rd-party USBa adapter/dongle. Numerous ones are available. Not all of them seem compatible with Macs, read the reviews carefully. There are also threads here on how to get a 3rd-party bt adapter up-and-running. Some terminal commands may be needed.
If you get a 3rd-party adapter, you should also get a 3'-6' USB "extension" cable, like this:
Amazon.com: usb extension cable 3ft
www.amazon.com
Plug the adapter into the female end.
Plug the other end into an available USBa port.
Now, "loop it around" so that the adapter is closer to the bt devices you'll be using.
This "gets it away from" the metallic (case) and electrical interference at the back of the Mac...