Although I spent many years in the electrical field, I specialized in Electrical Metrology (a long time ago). Bearing this in mind, I really know very little about satellite television system theory.
As we recently lost our ability to acquire certain transponders used by our satellite television receiver, I began looking into the basics of this area of RF theory … and, in short, I'm intrigued by the idea of sending a DC signal over the same transmission line use to transmit RF signals. I'm certainly not clear about it at this point, but, based on what I've read (thus far), this combination of DC and RF is commonly referred to as "Phantom Power" by the satellite people.
Having used many power splitters and directional couplers in my day — mainly for calibration testing of Precision Measurement Equipment (PME) — I'm especially puzzled by a component of this area of electronics known as "DC POWER PASS". Apparently, the idea is to pass a DC voltage over the same coax used to transmit down-converted RF satellite signals. As this seems to run contrary to the division of power I was [once] accustomed to, I would really love to read a lucid explanation of this method of controlling satellite television system circuitry.
I hope that I've been clear enough.
Best Answer
Basically, it's AC superimposed on DC, with a filter to separate them out at the other end
Satellite TV line-power schemes (and other power-over-coax setups) work by simple superimposition; they add the AC signal atop the DC supply voltage on one end using a coupling capacitor and RF choke to create a summing node, and then use a filter (coupling capacitor and RF choke) to separate them out at the other end, as follows (although the values certainly aren't right, and OA1 is really a generic amplifier block):
simulate this circuit – Schematic created using CircuitLab
Note that this is not the same as true "phantom power" as applied to balanced lines such as pro-audio mics and PoE; these setups have the DC voltage set as a common-mode voltage from both signal lines to ground, or between two pairs of signal lines for that matter.