Dbi explained

We all want to get the max out of our drones, and sometimes there is a very good reason to upgrade your antenna since not every drone has a decent antenna.

But what antenna do you choose?
There is a lot to tell about antennas and radiation and such, here I just want to tell the bare basics to help you understand what all these things mean.

Apart from the model or shape of the antenna it’s important to know what you want with it.
There are directional antennas and omni directional antennas, and I have explained all of that here.
There is also another important thing to take in consideration as well:

This stands for: dB(isotropic) – the forward gain of an antenna compared with the hypothetical isotropic antenna, which uniformly distributes energy in all directions. Linear polarization of the EM field is assumed unless noted otherwise.

In simple words:
A higher dbi gives more range but less coverage. This means that the drone will fly further or higher from you, but the area that is covered by the signal is more narrow. You will get better results, but only of you keep the controller pointed to the drone.

A lower dbi gives less range, but the drone can be controlled over a wider area.
This means that you can control your drone without having the controller pointed to it.
Standard a drone has an omnidirectional antenna with a low dbi. this makes the drone safer to control as the chance that you loose connection is reduced due to the wide area it works in.

This image shows what I mean

As you can see, lower dbi has less range, but more coverage.
in a 3d view it looks like this

This is something you should take in consideration before upgrading your antenna.
a high gain antenna is nice, but it would be strange to lose signal when your drone is far away and out of reach of the antenna signals.

I would advise to choose something that has the best of both, a somewhat higher gain for distance, but not too much so you can still have room to control it in.