Newer amateur radio operators may read or hear some discussion of antenna gain. The gist of it usually involves how much stronger signals are with a particular type of antenna.
This concept of antenna gain can be confusing or misleading to new hams because it sort of suggests that some antennas actually amplify received or transmitted RF signals. Not so.
Like the decibel (dB) antenna ‘gain’ is always relative. So when we speak of antenna gain it refers to a performance improvement compared to a different type of antenna.
More commonly, gain is used to compare highly directional antennas (beams, dishes) to monopoles or dipoles.
While antenna gain usually emphasizes directivity it may also be used to compare two similar types with different characteristics, as efficiency is another contributing factor. For example, we can observe and measure the relative gain of a stubby “rubber duck” handheld radio antenna (pathetic) vs. a 5/8-wave whip (much better) even though they are both vertical monopoles with the same directivity.
The two types of antennas usually referenced against are isotropic and the simple dipole. When measuring performance of other antennas, they will often be compared to one of these two.
If the gain improvement is relative to the theoretical isotropic antenna, the units should specify the gain as dBi. If measured against a dipole antenna, the gain should indicate dBd. Any other comparisons should mention the reference antenna in the text.
Some good references for antenna gain are given below:
Antenna Gain Explained article
What is Antenna Gain? video by ham
Antenna Gain Wikipedia topic