A shelter's greatest strength — mass and earth around you — is also what blocks radio and cell signals from reaching you. Concrete, steel, and soil are very effective at stopping the signals you need. Boosters and external antennas are how you pull a usable connection down into the ground where you actually are.
The shielding problem
Every foot of earth and every inch of reinforced concrete attenuates signal. A phone that works fine on the surface may show nothing underground. The universal solution is the same for cell, radio, and satellite: get an antenna outside and run the cable in.
Cell signal boosters
If there is any cell signal at the surface, a booster can bring it inside. A weBoost Cell Signal Booster system uses an external antenna to capture the weak signal, amplifies it, and rebroadcasts it inside the shelter — turning "no bars" into a usable connection.
External antennas for radio
The same principle applies to your radios. A handheld deep in a shelter is nearly deaf; the same radio connected to an outside Roll-Up J-Pole Antenna or a mounted antenna can reach for miles. For any HAM setup, running coax to a well-placed external antenna is the difference between silence and solid contacts.
Plan antenna runs during construction
The time to plan your antenna cabling is while you are building. Run conduit for coax from the surface into the shelter before you backfill — retrofitting cable runs into a finished, buried structure is far harder. Sketch your entry and cable paths in the Builder.
Satellite needs sky, not boosters
One caveat: satellite devices need a clear view of the sky and cannot be "boosted" from underground. Their antenna simply has to be at the surface. Plan a way to deploy your satellite messenger outside or at an opening when you need to send.