This increased coverage and connectivity to your ISP.
Unscrew and open your WiMAX modem
Make a small hole in the rear panel for RP-SMA connector
Products for external antenna project for WiMAX modem:
2.5-2.7GHz 18 dBi WiMAX Outdoor Directional Panel Antenna
Step 1: Connect External Antenna
Connect cable to your external antenna
Secure end with waterproof tape
Step 2: Connect to Modem
Connect the other end of the connector to RP-SMA port
Step 3: Find Right Spot
Find the location that gives the best signal
Mount external antenna in that spot
Step 4: Very Simple, Very Easy
Here's a simple YouTube video which shows you the steps
Make certain you know what frequencies your service provider is broadcasting on when choosing an antenna.