Where power lines come within 15 feet of any portion of the mast and antenna in any direction. On uneven ground where the vehicle is not reasonably level. Where the mast will not be completely visible as it is extended. Where your vehicle is a traffic hazard. Look up and live Always remember to look above your vehicle for power lines and other obstructions.
Tree branches can hide power lines from view, so be on the lookout.
Many states have a High Voltage Safety Act or similar law. Your local electric power utility will be happy to provide you with information applicable to your area. Here in Georgia, the law covers any activity near high voltage lines. If you're anywhere near a power line the law applies. Under this law it is a criminal offense to do any work within 10 feet of a power line without first meeting stringent requirements.