I assume you actually mean wireless broadband which is different from wifi.
You first issue is does the dongle model you have even have a place to connect a antenna.
You next problem will be getting the proper antenna. You will need to find the radio frequency range, antenna are optimized to for each.
A final note in some countries it is illegal to use antenna on cellular equipment.
A old trick used on WiFi before outdoor stuff got cheap was to run a long USB cable and place the dongle outside (someplace dry). You can also try a old satellite dish with the USB at the focal point..it is tricky to get aligned is since the dish is not directly pointed at the tower. Note this only works when the radio is running in the microwave range. If it is in 700mhz of something you would need a different dish.