What range can I get from an antenna?
This question is not an easy one to answer!! This is because there are many factors that go into determining the range of operation of an antenna, and even then there are variables that will affect the range possible from an antenna. The Friis transmission formula indicates that in free space, the power received by an antenna sent by the transmitting antenna depends on the gain of the two antennas, the frequency of operation and the distance between the antennas. Other factors like the radiation efficiencies of the two antennas, the mismatch occurring between each antenna and its feed transmission line, and the polarization mismatch loss also have an effect on the power received by an antenna. As you can see, the list is long and by no means exhaustive! So the range available from an antenna depends, conversely, on all the factors mentioned (except on the distance between the antennas obviously), as well as on the minimum power required for the receiving antenna to function reliably.
The Friis formula mentioned in the previous paragraph assumes that the transmitting and receiving antenna are in free space, but in today’s world, it’s a little hard to have both antennas sitting in free space with no obstacles in between! Reflections occurring from buildings and other obstacles in the way and absorption of some energy by them will change the range. Add to this the fact that the medium carrying the waves can introduce loss and absorption as well can really complicate matters. These are the things that make accurately predicting the range that can be obtained from an antenna a very difficult task.