I have a property which is in a bit of a hole. My driveway goes up to an elevation above my house where semi-local TV signals are present. When they were still in local business, I bought a powered antenna from Radio Shack and mounted it on a pole at the top of that driveway. It has an internal amplifier which is powered from a small (200 mA) DC source down at the house, and which is "injected" on the same coax line the signals it picks up are returned on (there is no separate power line needed to run to it).
I imagine this is simply a matter of "AC riding on DC", and that at the antenna, there is some kind of circuitry to pull off the DC without impacting the signals (I know I would likely be aghast if I opened it, since it may be something as small as 2 or 3 capacitors, when it cost around $70, but it was the SuperBowl, and I was having people over).
Fast forward to today. This driveway is wide enough for 1 car, and is semi-long and has a small curve. I'm a bit of a train signal system fan, I thought it would be neat to to put something the railroad calls a "dwarf signal" next to the road to let cars at the bottom know when a car is coming down from the top that they can't yet see so they can stop and back down. Since we're talking approx 600 feet, I'd rather not run another cable up there to power this light and any associated sensor(s).
THE QUESTION:
In such AC+DC amplified antennas, can I reasonably expect to draw off DC current for this signal system without interfering with the TV signals present? Broadcast TV uses such a wide swath of frequencies that it might be hard to interface with this antenna without creating interference or attenuation. I'm guessing I would likely need a very wide bandwidth bandstop filter.
Can anyone point me to any circuit examples for circuits like this?
Best Answer
you can get steal few milliamperes of the power that's going up the coax.
the power goes in at the bottom using a "bias tee"
but to extract power at mid-span you want one without the capacitor. or you get one and bypass the capacitor.
A suitable inductor can be made by passing a wire through both holes of a UHF/VHF balun core.
simulate this circuit – Schematic created using CircuitLab