Omni-directional antennas are used extensively with drones since they radiate and receive in all directions. The omni antennas were the only choice for unmanned systems as the drones are constantly changing directions and orientations due to their motion.
Only an omni antenna ensures that the drone’s ground station will be constantly under the antenna uniform coverage, regardless of the drone’s flight or ground track. Omni antennas might offer wide spatial coverage, but this comes at a cost. The main disadvantage of the omni antennas is their reduced gain, since the radiated power is distributed uniformly in all directions. Only a fraction of the radiated power is exchanged between the drone and its ground station and the biggest part gets wasted in all other directions. As a result, the radiated efficiency is usually less than 5% and that’s the main reason the data link between the drone and its ground station becomes unstable, weak, and finally breaks at ranges beyond 1 or 2 km with the current technology antennas. This is the cost for being able to cover all directions around the drone or the ground station.
It is an axiom that if we wish to increase the antenna’s radiated power (known as antenna gain), we have to reduce its radiation beam angle. As the beam angle decreases, the gain increases. This provides a greater coverage distance, but with a reduced coverage angle. If for example, we use a directional antenna on board the drone, it is not possible to keep the directional beam constantly pointed to the ground station as the drone moves arbitrarily. As soon as the drone’s antenna beam points away from the ground station, the link will break.