What is antenna gain？
With any antenna, the apparent increase in signal is not an amplification of signal, but it is the act of redistribution of available Radio Frequency (RF) signal into a preferred direction. So basically, antennas only divert, direct, or concentrate radio energy in some direction, they don’t create it.
The increase in signal using an antenna is called gain and is measured in dBi. The base for dBi measurement is an isotropic radiator (idealistic model that has the same value when measured in different directions) at radio frequencies. As new RF signal is not generated, the stronger signal is achieved at the expense of most other directions.
Some people think that a higher gain antenna will give them the strongest signal and highest quality connection. This is true in some cases, but in certain applications too much gain can be a bad thing. The truth is, the answer to whether you need a higher or a lower gain antenna lies in your application.
Which antenna and gain do I need?
If you want to focus all of the signal to direct it to a distant target, then the high gain antenna is definitely the best choice. High gain antennas need to be pointed in a preferred direction to send RF signal so that limited signal can be intensified in desired location, as illustrated below.
However, if you want to broadcast evenly to a whole room (or give omni-directional access to your wireless signal), you do not want much gain (or it’s directivity). Remember, "gain" is simply stealing radiated energy from some directions to intensify others. The higher the dBi number of the antenna, the higher the gain, but less of a broad field pattern, meaning that the signal strength will go further but in a narrower direction, as illustrated in the diagram below.
Antenna Application Example
Single vs MIMO
Back in the days of 3G, an antenna with a single output was fine. Nowadays with 4G coverage pretty much everywhere, you should ensure the antenna you buy is a MIMO (Multi Input, Multi Output) type. This is reflected in the fact that in 2019 pretty much all antennas for sale are MIMO.
Most 4G routers will use either SMA or TS9 ports, with SMA probably the more common. Just be sure to check which ports your router has before you buy.
As mentioned above, most routers use SMA connectors. However, don’t worry if find an antenna you like the look but it has a different style of connectors to that of your router, as SMA to TS9 convertors are available.