Aren’t antenna’s supposed to have ground plane. These seem to work without it. How?
Whether an antenna needs a ground plane or not depends on how long it is .
The most common antenna is usually a 1/4 wave vertical which does need a ground plane , but other types dont .
The GSM antenna is most likley an end fed 1/2 wave with an additional loading coil to make the antenna look like a 5/8 wave which has a more optimum radiation pattern.
Aren't antenna's supposed to have ground plane. These seem to work without it. How?
Notice the base of the antenna in the picture. It contains a magnet to attach the antenna to a steel plate of some kind, such as an automobile roof. That metal plate is your ground plane.