What do the radiation patterns of ADS-B transmissions on large airliners tend to look like?
In airliners, like in most aircraft, the ADS-B transmission come from the ATC radar transponder. The transponders in aircraft with an Maximum Takeoff Mass exceeding 5700kg /12500 lbs are required to have antenna diversity. This means that the aircraft will have a top and a bottom antenna connected to the transponder. 

ADS-B transmissions are sent out of either the top or bottom antenna, the selection of which should be random with a 50-50 distribution.

Radar and TCAS replies are sent out the antenna on which the incoming interrogation was strongest.

The antennas are vertically polarised and have an approximately omnidirectional radiation pattern in the horizontal plane. In the vertical plane, most radiation energy will be focused 45 degrees around the horizon. They have a cone of silence.

The aircraft structure blocks and reflects part of the radiated energy. This is causing the far field pattern to be not uniform and varies from aircraft to aircraft. On the lower antenna the main effect is of the fuselage and the engines. On the top antenna the fuselage, the tail and the wings/winglets are the main obstacles.

On small aircraft this is more of a problem, especially when the transponder antenna is between large fixed gear struts. 

This study contains an antenna pattern of the top antenna of an aircraft