Electronics > Projects, Designs, and Technical Stuff
Power transfer from power lines using induction
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IanB:
When I drive around I frequently see isolated monitoring stations next to highways mounted on a pole with a solar panel and a box of electronics. I am almost certain these are free standing, self-powered devices with a radio link to some base station somewhere. So I think the device you are looking for may already have been invented and you just have to adapt/re-purpose it somehow?
ejeffrey:
I think those stations use GSM radios and probably only power up for a few minutes a day.  So the power budget for an always on wifi repeater would be higher.
Marco:
Capacitive can light fluorescent tubes with comparitively small electrodes ... the available power between a small "antenna" and the grounding conductor on the pole might be enough.
ejeffrey:
By my estimate, the available power for a "small" magnetic pickup coil near a power line (but not encircling it) is:

\$P_{max} = \frac{V \mu I_0^2 f}{8 \pi d^2}\$

Where V is the volume of the core, \$\mu\$ is the permeability of the core, Io is the current in the transmission line and d is the distance to the power line.

I'm not sure what the current is in your transmission line or what you consider a safe distance, but putting in my guesses, it doesn't look good for getting 10+ watts.
Siwastaja:
If you start aiming for <100mW average consumption, things suddenly start getting feasible. I don't know offhand if leaching 100mW from the power line is feasible, but it would be in the right ballpark. But even if it didn't work out, a modern 20W solar panel would be fairly small & cheap, and should be able to support 100mW average, even during long-term bad weather, excluding the completely dark winter in the arctic area. For storing 100mW average over 4 days, for example, the required battery is just 10Wh, or a single $3 18650 cell.

I see no reason to spend watts of power, except to simplify design by using general-purpose networking/computing parts. But I think you need to go beyond that, and optimize for the power. Your links are fixed and directional, so little actual power is needed for radio communication, even assuming less-than-ideal modulation schemes. It all boils down to being efficient and using the right RF tools for the job; custom if necessary. I know this isn't the easy road.
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