Dish satellite receiver signal
I'm sure it is not just DC but there is a (tiny) RF signal superimposed. 

The DC is there for power supply of the LNA (power going to the LNA).
The RF is the received Signal (coming from the LNA).

Both components can easily be separated by a high pass filter (capacitor)/low pass filter (choke coil).







simulate this circuit &ndash; Schematic created using CircuitLab

A similar technique is used in the audio range for supplying power to preamplifiers that are integrated in microphones. It's called phantom power or phantom feed.