By far, this the largest of the subreddits dedicated to amateur radio. It has over 50,000 members. In their own words:
Welcome to Reddit's own amateur (ham) radio club. If you are wondering what Amateur Radio is about, it's basically a two way radio service where licensed operators throughout the world experiment and communicate with each other on frequencies reserved for license holders. All topics relating to the hobby are welcome here, from purchasing and building equipment, to operating techniques and activities, and everything in between.