When looking for a long-range WiFi antenna there are a few things to know.
The first is that simply because an antenna is bigger such as a 15 or 20dBi does not necessarily make it is better.
Here is a image of the range antennas are most effective, compared with the length or dBi.
A 2 dBi antenna is very effective at short range while a 9 dBi antenna is not effective at short range, but works very well picking up signals from a distance.
If you simply want range to cover a home a 2-5 dBi antenna will be plenty enough. If there are block walls or other interference a larger antenna will not make the signal better. If you live in a home that has many WiFi dead spots then a wifi booster would be a much better option, or upgrading to a better long-range router.
If you do need to get distance from your WiFi connection, such as a detached garage or to a neighbor a longer antenna can be a good option. Getting a 7 or 9 dBi antenna would be much more better in this case.
Another thing to keep in mind is with the new dual band routers, Is do you need an antenna that works in both the 2.4 GHz and 5 GHz frequencies?
The 5 GHz frequency is still new but becoming more important as time goes on. The 5 GHz frequency is not good at distance so while it does have a faster transfer speed it does not work as well as the old 2.4 GHz range.
The 2.4 GHz frequency is the best for distance, while the 5 GHz frequency is the best for high-speed but only in a close range.
Long-Range WiFi Antennas
NextG USB-Yagi Plug & Play 11N Long Range WiFi antenna 2200mW
Yagi antennas are well known and good at getting a signal from a distance. In fact It is widely used in the Pen Tester community for looking to check a wireless routers security.
This big of antenna will be overkill for many, but is ideal for RV travelers, boaters, detached buildings, or any place trying to get the best distance from a WiFi signal.
TP-Link 2.4GHz 24dBi Directional Grid Parabolic Antenna, N Female Connector, Weather Resistant (TL-ANT2424B)
Like the Yagi antenna Parabolic antennas are often used a lot for getting the best distance. Parabolic antennas are the definition of a Non-Omni-Directional antenna.
The difference is a parabolic antenna can be better when it is pointed directly at the source, but not good when not pointed at the source.
Getting the best signal is done by using an App and watching the signal strength. The antenna is than turned until the best signal possible is being received.
This type of antenna is not very portable and is used in the same way the Yagi is used.
SilverStone Technology High Gains Dual band WIFI antenna 5GHz 9dBi, 2.4GHz 5dBi with RP-SMA plug WAD21
This is a 9 dBi antenna that is cheap and is much more commonly used to upgrade a router or USB WiFi adapter for better distance.
It uses a common screw on SMA interface that is widely used in the WiFi industry. Some router and USB adapters use proprietary interfaces so if you go this route be sure what type you have.
There are many similar 9 dBi antennas that are cheap and can be a big improvement over a 3-4 dBi antenna that many WiFi electronics come with.
There are many good antennas that can improve a wireless signals range.
Personally I carry a 5 DBI and a 9 dBi antennas which I use with an Alfa USB adapter and my laptop. They are easy to screw on and unscrew when needed. I use Alpha USB adapters that make this very easy to do.
So for example if I am very close to a router takes me 10 seconds screw on 5 dBi antenna and get much better coverage. But if I am at a location with my laptop and the WiFi signal is from a distance I simply screw on the 9 dBi antenna and get better reception.
Hopefully you are not on the go as much as I am, and have a fixed location so you can dial in the correct antenna and not worry about it.
There are more expensive antennas that have a low dBi and high dBi side that can be bought and would be very good for a wide coverage area, near and far.
If you are very serious about your WiFi signal and want the best of both worlds buying one of these would be your best option.