The last time I checked ASIMO's battery pack only had enough juice for something like 30 minutes of continuous operation & then it has to plug in & recharge which isn't really that useful.

I'd like my free range robots to have a little more up-time, somewhat less down-time & not have to worry so much about staying close to a suitable power socket.

So taking a page out of (the original box set) Gamma World's book I'm thinking of broadcast power.

The requirements

  • The transmission must be omnidirectional.

  • Transmitting antennas of (at most) similar size to a commercial radio station antenna.

  • Receiving antennas of 1 foot or less.

  • Safe for people to live inside the area served (so microwave frequencies are probably out).

  • Sufficient power to charge at the very least a car battery over a period of hours.

Assuming a suitable frequency is given over to this & ignoring existing international agreements & national laws regarding radio transmissions.

  • Could a suitably small* Rectenna (assuming a suitable broadcast facility within range) provide suitable power levels to recharge a humanoid robot like ASIMO.

A subsidiary question would be what sort of range would a transmitter have within which adequate power would be received.

*We want some space for a battery as well so up to half the size of ASIMO's current battery.

  • $\begingroup$ (1) What's a "suitable" frequency? In hertz please. (2) How much power is "adequate" power? In watts please. (3) Do you want humans to be able to walk about unharmed within range of the transmitter? $\endgroup$
    – AlexP
    Commented Jan 4, 2019 at 19:08
  • 1
    $\begingroup$ Without getting into hard science (or hard engineering), this may work with directed microwave beaming. However, there are still many issues with that to become practical. I would rather suggest that your robots' area has a number of near-field wireless charging stations (which are proven to be effective). $\endgroup$
    – Alexander
    Commented Jan 4, 2019 at 19:17
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    $\begingroup$ @Pelinore "Unidirectional" in the sense of "not omnidirectional". You'll need to have a set of articulated microwave emitters that would follow your robots. $\endgroup$
    – Alexander
    Commented Jan 4, 2019 at 19:32
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    $\begingroup$ @Pelinore in that case I'm sorry, that won't be practical. You'd be wasting a lot of energy and the area would not be safe for humans. $\endgroup$
    – Alexander
    Commented Jan 4, 2019 at 19:39
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    $\begingroup$ @Aethenosity Andrea Williams' comment was originally an answer, and that was my response to it. I didn't realise it had been converted to a comment. I'll delete mine as it's no longer relevant. $\endgroup$
    – F1Krazy
    Commented Jan 12, 2019 at 10:49

7 Answers 7


You can, conditionally

This is something us humans are exploring today, however instead of radio waves we move along the electromagnetic spectrum to microwaves. Using microwaves you can remotely passalong power with efficiencies up to 95% (which is pretty amazing, all things considered). However this comes with a few complications.

  1. This method is dependent on line-of-sight.
  2. Typically this method requires massive antennas; practical examples include a 1-kilometre-diameter (0.62 mi) transmitting antenna and a 10-kilometre-diameter (6.2 mi) receiving rectenna. (See this paper)
  3. While it actually is blocked very little by atmosphere, many other types of matter will absorb the energy and produce large amounts of heat. Microwave towers to transmit a single (far lower powered than a power-transmitter would require) are known to cook bird that rest on the dish, for example.

I probably wouldn't advise it for your robots.

  • $\begingroup$ I've not opened & read it yet (because most pdf's don't translate properly for my PC) but I think the paper your using is (probably?) talking about beamed power over large distances for the purposes of transferring power generated off-world to planet side substations, so not a useful comparison for this use? $\endgroup$
    – Pelinore
    Commented Jan 4, 2019 at 19:42
  • $\begingroup$ @Pelinore There are some references to much smaller antennas but they're also way less powerful and really limited in range. I can find some proof-of-concept research projects that can "trickle charge" batteries within 20 feet with short (<1 foot) antenas. If you're willing to be flexible on "hard science"... well you could probably include it. $\endgroup$
    – Ranger
    Commented Jan 4, 2019 at 19:48
  • $\begingroup$ I already switched the hard science tag for the science based tag, the site doesn't seem to have recognized that yet & still has the hard science disclaimer up. $\endgroup$
    – Pelinore
    Commented Jan 4, 2019 at 19:52
  • $\begingroup$ "I can find some proof-of-concept research projects that can "trickle charge" batteries within 20 feet with short (<1 foot) antenas"! : anything up to a 40 foot transmitting antenna would be suitable for what I wanted, the receiver shouldn't be more than a 1 foot though. $\endgroup$
    – Pelinore
    Commented Jan 4, 2019 at 19:56
  • $\begingroup$ a beam antenna is not broadcast. $\endgroup$
    – Jasen
    Commented Jan 5, 2019 at 3:12

What you need is induction coils

This is a bit of a hybrid technology but should suit your purposes. It's considered on and off as one of the options for the future of electric vehicles. Induction loop in the road charging via loop in the car. It's been rattling around for years as a viable technology, including now for charging phones and powering rfid systems.

It's that use for charging cars that you're more interested in though. Your robots have independent power from their batteries, but most of the time they're likely to be performing routine tasks in predictable areas. You can lay induction coils in those areas, allowing your robots unlimited uptime on regular tasks while their batteries allow limited free roaming for exceptional requirements.

  • $\begingroup$ That's a good idea but I want something that will charge them back up if they stray outside their normal area of activity then run out of juice. $\endgroup$
    – Pelinore
    Commented Jan 4, 2019 at 20:17
  • $\begingroup$ near field, not broadcast. $\endgroup$
    – Jasen
    Commented Jan 5, 2019 at 3:12

Crystal radios are powered solely by broadcast power.

In the early days of radio, people made homemade radios. These used a radiosensitive detector and could convert the energy fo the signal into audible noise - no battery required. The article is super interesting; I did not know that WWII GIs made their own crystal radios in the field using things like lead from pencils and razor blades.


As a crystal radio has no power supply, the sound power produced by the earphone comes solely from the transmitter of the radio station being received, via the radio waves captured by the antenna. The power available to a receiving antenna decreases with the square of its distance from the radio transmitter. Even for a powerful commercial broadcasting station, if it is more than a few miles from the receiver the power received by the antenna is very small, typically measured in microwatts or nanowatts.

So here is a (century old!) precedent for powering a device with broadcast power. I wondered: could someone use that same power gathering technique to, say, light an LED. Yes!

Crystal Radio powered Lamp

Re: Crystal Radio powered Lamp Quote Post by cheungbx » Thu Jun 08, 2017 4:18 pm

crystal radio lamp

My home is on the 56/F of an apartment. I have a 30 meter antenna hanging out of my window at night when I need to test, I push it out using a fishing rod so it is 1.5 to 2 meter away from the wall of the building. My building is located on on the coast facing a transmission status 10KM away on the island called Peng Chau. There is no obstruction from my building to the transmission station unless there are huge cargo ships on the waters. There is another transmission station on the shore 10kM over a hill called "Golden Hill" with lots of obstruction.

I think the LED guy is so awesome!! So - it is possible to power small devices with radio waves. It is not new technology. People in the area do not explode into flames.

How much radio wave energy can you pump into an area before people explode into flames? I have looked for reports of accidents with radio waves but no luck - it would seem radio waves are pretty low toxicity for life. You can see loads of healthy vegetation growing right next to powerful radio towers.

For your story you could pump loads of energy into the area and let the robots harvest wherever they are. Maybe it could actually be radio waves carrying popular music which other other characters listen to on their crystal radios. Another alternative - the robots might be far away but if you can hit them with broadcast power the reverse is true. They can tell you where they are. If you know where they are, you can train a dish on them from your tower and broadcast power right at them. That would be way more economical.

  • 1
    $\begingroup$ You're making a bit of a scale error here. Crystal radios draw trivial amounts of power -- in the microwatt range -- and require antennas tens of meters long to get that much. The LED lamp you link to draws 5 milliwatts or less, and uses a 30-meter antenna to get that much from a nearby transmitter. The question calls for charging a car battery in a matter of hours, which requires hundreds of watts -- about five orders of magnitude more than you can get from broadcasting, and not far from that involved in a microwave oven. $\endgroup$
    – Mark
    Commented Jan 6, 2019 at 2:22
  • $\begingroup$ @Mark : as long as it can provide enough power to trickle charge a battery it works for what I want, days (or longer) rather than hours is disappointing (for me) but can can still work for what I wanted. $\endgroup$
    – Pelinore
    Commented Jan 6, 2019 at 16:05
  • $\begingroup$ @Mark : "30-meter antenna" does that have to be a straight antenna or can it be coiled into a smaller space? $\endgroup$
    – Pelinore
    Commented Jan 6, 2019 at 16:08
  • $\begingroup$ @Mark - I did not mean to suggest that the LED crystal radio guy would charge his car with the rig he describes. It is proof of principle: power sent via radio waves and collected at a distance to do useful work. Pelinore can fiction up a better resonant crystal to collect more, and a radio tower which kicks out more and that will serve his story. The apocryphal reason Tesla's broadcast power station lost funding was not impracticability, but an inability to identify and charge users. In a densely populated city, a tax funded broadcast power source might work. Maybe for streetlights? $\endgroup$
    – Willk
    Commented Jan 6, 2019 at 16:56
  • $\begingroup$ @Pelinore, straight, at least at the wavelengths involved in the AM radio bands. If you're working with centimetric waves or shorter, you can probably use a dish or fold the antenna up into a grid or something. $\endgroup$
    – Mark
    Commented Jan 6, 2019 at 19:58

Current wireless charging technology, is all near field, not broadcast.

The only type of practical broadcast power is that used by bumper cars - a metal floor and electrified ceiling. (but this is not a wireless broadcast)

proposed satellite power down-links are beam (directional) antennas not broadcast.

everything else, is mis-representation or a toy


The most powerful is the sun
You already have the most powerful broadcast power device in the solar system available during the daytime. It's called the sun.

The sun is a big ball of flaming hot stuff that radiates power in all directions. By the time it gets to earth and filters through our atmosphere, you're left with around 300 watts per meter squared of area. So while you can't have an antenna due to the dominant wavelengths being much higher, you can have a receiving panel. If you have a 10% efficient panel of 50cm each side (0.25m2), you can get a theoretical 7.5 watts. Not really enough for a robot mobile, but if you pretend to have better solar panels you could get up to 75 watts, which is enough for some moderately energy efficient systems (a laptop charger is about 75 watts, the curiosity rover's RTG outputs about 100W).

Going through your requirements:

  • omnidirectional.
  • No transmitting antenna
  • 1 (square) foot or thereabouts.
  • Safe for people to live as per normal
  • A car battery is about 600Wh (45Ah, 12V), so a 100% efficient 50cm-per-side solar panel would take about 2 hours.

No, transmitting towers aren't good enough
Let's say that the sun isn't good enough for you and you have to have a transmitting tower. It's going to have to be a powerful transmitting tower. Lets say you want to get that 300W/m2 at 1km radius. The inverse square law is the killer. If you're 1 meter away and receive 1W/m2 of energy, then being 2m away gives you 0.25m2 of energy. By the time you're 1km away, you're receiving 0.000001 W/m2 of energy. Reversing this tells us that to get 300W/m2 of energy at 1km requires a power output of something like 300,000,000 W/m2 at 1m radius distance. For reference, if you switched on all the backup transmitters of the most powerful transmitter in the world while it was operating, it would only output about 3MW, still 100 times too low.

Because we've gone with the same power output as the sun at 1km range, we meed all the power requirements from your question. However we now have:

  • Huge transmitting antenna (to avoid it melting)
  • Enormous operation costs
  • Instant burns to anyone near the transmitting tower.

So: Use solar power. Seriously. Everything else only operates at close distances or requires cancer-inducing levels of power. (Yes, the sun does output cancer-inducing levels of power, but at least its a very long way away).

The Asimo Robot draws a huge amount of power
The asimo robot has a lithium battery that weighs about 13 pounds (5.8kg). Most lithium chemistries have a power density of about 100-200 Wh/kg. This means the robot has about 600-1200 Wh of energy, which it drains in half-an-hour which results in an average power draw of 1.2-2.4kw.

So it turns out that a 100% efficient 0.25m2 square solar panel outputting 75 watts is about 20 times too low. You'd need a 100% efficient solar panel of about 5m2 surface area to power it continuously.

Powering the Asimo robot via broadcast power is not feasible. The Sun is the most powerful thing you will find that is "safe" and omnidirectional, and it's around 20 times too weak. As it turns out, there are reasons why we don't power robots via broadcast power anywhere other than micro robots on laboratory tables.

To get this to work, you'd have to handwave more efficient robots. Even 100% efficient solar panels, or 100% efficient transmitter/receiver antennas isn't enough.


"The transmission must be omnidirectional."

This is the primary impediment to using this as an efficient means of transmitting power. By its very nature, it will limit the power received, by the SQUARE of the distance from the transmitter. (a factor of 1/r^2)

Consider a spherical shell of photons/fields/particles moving outward. As the radius increases, the same amount of original stuff, must now be spread out over a greater and greater area. The result is that a given square meter of the shells surface area will contain less and less stuff and the shell expands. This is called the "inverse square law" and wikipedia has a great image that displays this well.

The end result is that, unless your power recievers are very large, or very close to the power source, most of your output power will be lost to space.

One possible workaround for this would be to change the requirement of "omnidirectional" to "multi-directional".

[Another, more complete, answer mentions this, but I wanted to provide some more details than a comment affords.]

Inverse square law.svg
By Borb, CC BY-SA 3.0, Link


As an example: consider a robot 1 cubic meter in size, that is 2 km from your power source. The robot has a cross section of 1 square meter, the sphere at 2km has a surface area of about 50 MILLION square meters. So, if the robot requires 1 watt of power to get out from under a tree. Your omnidirectional transmission source will need to put out: 50 Megawatts of power.

  • $\begingroup$ "This is the primary impediment to using this as an efficient means of transmitting power/" efficiency isn't particularly important, that your nearby autonomous mobile units have a backup other than solar (in the event they happen to be under a tree or in a house) so they don't (or are less likely to) run down somewhere & gently rust for eternity is the primary (if not sole) goal. $\endgroup$
    – Pelinore
    Commented Jan 6, 2019 at 16:28
  • $\begingroup$ I think you may be underestimating the efficiency loss. consider a robot 1 cubic meter in size, that is say.. 2 km from your power source. The robot has a cross section of 1 square meter, the sphere at 2km has a surface area of about 50 MILLION square meters. So, if the robot requires say.. 10 watts of power to get out from under a tree. Your omnidirectional transmission source, will need to put out: 500 Mega watts of power. $\endgroup$
    – Glurth
    Commented Jan 7, 2019 at 17:50
  • $\begingroup$ I think your ignoring the point that it only needs to trickle charge the battery rather than power the unit? $\endgroup$
    – Pelinore
    Commented Jan 7, 2019 at 18:10
  • $\begingroup$ I'm not, the same ratio (50 million to 1, in that example) would apply to any amount of power. Weather the power lost is "acceptable" or not - yes, I'm ignoring that. $\endgroup$
    – Glurth
    Commented Jan 7, 2019 at 22:15

Nikolai Tesla invented something he claimed delivered power through airwaves. Too bad nobody understood how it worked. But yes it should be possible.

  • 1
    $\begingroup$ You mean this? sites.suffolk.edu/xenia/2016/02/17/… first hit I get with the search-string "Tesla transmitting power without wires" he was using radio frequencies. $\endgroup$
    – Pelinore
    Commented Jan 4, 2019 at 23:57
  • $\begingroup$ @Pelinore yes that's the thing. Even though the mainstream has always poo-poo'd it, Tesla was a genius and he was quite convinced it would work for long distances. He never got to test it at scale because he was terrible with marketing and lost his funding halfway through the project. $\endgroup$
    – ken
    Commented Jan 5, 2019 at 0:05
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    $\begingroup$ I don't think they ever "poo-poo'd" that it works, just that the range is sufficient for purpose for wide scale use & the efficiency for power input & broadcast to power received at the other end was sufficient for commercial operation of the system in most markets. $\endgroup$
    – Pelinore
    Commented Jan 5, 2019 at 0:11
  • 3
    $\begingroup$ If you're referring to Wardenclyffe Tower, it's well-understood what Tesla both thought he was doing (ground-resonance power transmission) and what he was actually doing (far-field broadcast power transmission). It's also well-understood that neither is particularly workable because of inherent inefficiencies. $\endgroup$
    – Mark
    Commented Jan 5, 2019 at 3:24

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