Relay Tower OSC-527

After extending the satellite dishes, you'll get three radio signals:

Miller Family Radio Signal

The source is the Miller family bunker in Big John's Salvage. The bunker itself is located in an overturned blue train car in the middle of the location but you need to activate a circuit breaker first before you can gain access to that bunker.

Distress Signal

This is coming from the secret safe storage chamber in the basement of Fallon's Dept Store

Supermutant Radio Broadcast

This is coming from Shaw High School, probably from a captured female prisoner forced to read the poorly written script by supermutants.