DC power is very lossy; over-extending the coax feeding the unit power may result in marginal operation and ultimately smoke the unit as it tries to compensate for the loss.
It is a function of the ohmic resistance of the cable and the voltage applied. Meaning: even if you added a power injector with a higher current output, it wouldn't change the power level at the roof-mounted unit (Current = voltage / resistance).
Coax would be the better choice for power any serious distance, the larger conductor (larger than Cat-rated UTP) should have less loss and, if installed properly, accept and radiate less noise.
The major drawback to a copper connection to the roof is lightning. If you run a solid copper link from your closet / data center to the roof, you're inviting the lightning into your data center.
IMO, you'd do better to get a power source close to the roof-mount location (with commercial power-overload protection), and bring the Ethernet into the data center over fiber.At the least, install a short a segment of fiber in the path, just to reduce the chances of a direct hit coming to visit your data center (a very, very bad thing ... I've seen it).
Doing it this way will ensure that the power to the unit is good, and you keep the environmental hazards outside where they belong.
There are code and insurance issues as well that you may want to investigate.
It complicates the install somewhat, it boosts the upfront costs some, but in the long run, you'll save money and have have a more stable, reliable system.
FWIW
Scott
