[borissk]

It will be a lot less than the percentage a radio antenna increases the fuel consumption of a car.

Modern planes like the 787 are made to a big degree from composites and the antenna can be installed inside the fuselage - leading to 0% increase in air resistance.

[Pasorrijer]

This is not accurate. I don't know of a single installation within a fuselage, because the composite has enough metal in or around it to cause interference.

The fuel concern is a significant one, as well as performance. Enough fuel or performance impacts and you can restrict the airports flown with these antennas. It's not as big a problem on widebodies, but it's the reason you haven't seen WiFi on small regional or the smallest narrow bodies until recently.

[londons_explore]

The pictures on the site show a roof mounted one... It's raised like 4 inches off the body, so it's gotta cause quite some drag at 500 mph