# How to design drone power tether system? (high power with ultra light cable)

There are drones that are powered with power tether from the ground station, and such drones can be airborne 24/7, not only 20 minutes when using batteries.

I'm very interested to reverse-engineer how such power tethers could be constructed. Such system needs to usually be able to transfer around 500 W and 30 meters (100 feet) in length.

Main restriction for designing such system is weight of tether cable which should be as light as possible, ideally less then 1 kg (2.2 pounds).

Which approach is better - high-voltage DC voltage or high-voltage AC?

Also by using high voltage we have two drawbacks - operator safety and cable insulation weight increase.

When using AC we can also choose to go with very high frequencies, is there any benefit in using high frequency and get better power transfer?

So what would be ideal combination of voltage to transfer high power on light and thin cable?

is there any special type of cable that would allow for high power transfer but keep weight low?

• I would not rule out DC so quickly, all be it a high voltage DC. It's not like your are going to strap a 500W power transformer on the back of your little drone. Nov 21 '17 at 20:03
• If you are serious about this, you need to figure out what kind of two core wire you can get that is 100ft long and weighs 1kG or less. Once you know that you can figure out the resistance of the wire and calculate what voltage you need to attach to one end of it. Then check the wire will withstand that voltage. If not.. repeat.. Nov 21 '17 at 20:05
• Off-the-wall solution: high-frequency AC and a Goubau line en.wikipedia.org/wiki/Goubau_line Nov 21 '17 at 20:15
• Would using litz wire (en.wikipedia.org/wiki/Litz_wire) and high frequency AC maybe be a way to go? Nov 21 '17 at 20:24
• Consider aluminum instead of copper wiring. A bare wire of aluminum weighs half as much as a bare wire of copper that has the same electrical resistance. Nov 21 '17 at 20:54

Let's check out their website:

They seem to use several solutions:

• High voltage DC with motors in series (that's kinda crappy)
• High voltage DC with voltage "regulators"
• High voltage high-frequency AC

This last one is interesting. They use 7 kHz AC high voltage on the cable, and a tiny transformer (due to the high frequency) in the drone.

The choice between DC and AC would mostly depend on which results in the minimum weight converter in the drone to get the desired power.

So, AC on the cable allows to use just a transformer and rectifiers in the drone, but you can get skin effect problems on the cable. I guess that's why they chose 7 kHz and not higher. It would result in a rather small transformer...

DC would require a converter in the drone. The lightest would probably be a resonant one. Some weight can be saved by downsizing the input caps.

I wonder why they didn't just use a balloon though.

• Really great answer. But to answer your question, we have tried using balloon for our wifi router that is used in crisis events (www.meshpoint.me). We had same expectations like you - that balloon would be much easier to use to lift and keep in air out wifi router. But it turned out not to be the case. Balloon that lifts 800g of weight needs 2 cubic meters of helium, if you want to lifth 1200g then you need over 3 cubic meters of helium. Bottles for storing helium are very big and very heavy. Also keeping balloon in place with tethered strings is very frustrating because wind rotates it! Nov 21 '17 at 21:13
• Balloons leaks helium Jul 23 '19 at 18:40
• "I wonder why they didn't just use a balloon though." Balloon positioning and altitude are badly affected by wind. Since a balloon has large cross-section vs weight, it doesn't take much wind to provide significant force. When the lateral force equals the net lift, the tether will move to a 45 degree angle, and the altitude will drop by 30%. A drone can tilt to give stationary position and altitude regardless of wind (within limits). Dec 22 '19 at 3:01

I had a similar (identical) project: Physically lightest topology for SMPS?

Step 1 seems to be shave every gram you can off the drone itself. De-pin the flight controller. Use a smaller flight controller, and de-pin that one. Use smaller gauge wire. Use higher-voltage components. Turn the motors on a lathe. Drill holes in the carbon fiber.

From there, use the highest voltage you have access to to get power up to the drone. If you have single-phase power, use bare-conductor 24+awg for the neutral, and PTFE/silicone wire for the hot phase, twisted together with a Nylon/Kevlar strand for support. The wires are going to get hot, and you need something else to hold them up so they don't sag.

Use a small rectifier and large-value voltage divider to feed voltage from the primary side directly into your flight controller's battery monitor, and use telemetry to get that value (among others) back down to the ground. Adjust the ground voltage to compensate for huge losses along the wires -- you don't have the weight budget for sense lines.