Player Radars come in the form of a bell. They can be purchased from the vote shop, located at portal "castle", for 10 research writs (100 vote tokens). See Portals for more information about Portals.
Player Radars, as the name suggests, is like a radar that detects players. If a player is within 200 blocks of the radar (the bell), then the player will be detected. The radar does not show names, however it shows if the player is in your clan, is a rival, or is a neutral/no clan player.
There is a one minute cooldown between use, so the radar cannot be spammed. It does not cost anything to move or break. Anyone can ring the bell if they are trusted in your claim or if the bell is not in a claim.