If you search for 'Mine', 'Minefield', or 'Buoy' you'll be able to find more information, but here's a quick design guide for a Sensor buoy:
- Zero engines
- Missiles blow up if they run out of fuel, without engines they don't burn any.
- No fuel
- Without an engine we have no need of fuel
- No Agility
- We don't need to hit anything
- No warhead
- We don't need to do damage
- Sensors
- Normally a Sensor Buoy needs to have a good chance of detecting any 'enemy' ship that crosses a jump point or into the controlled space. Maximum jump deviation is 2M km with capped out tech.
Since we're talking a Comms Buoy I'd probably go with a combination of a very large resolution active sensor and an EM sensor large enough to detect the active from other jump points in the system. (As missile sensors currently have a resolution cap of ~21,000,000, the buoy can be rather small)