# Wireless energy transfer

#### Man10

Joined Jul 31, 2018
122
If you build a long range wireless energy transfer device, why can't you use a lower frequency than the microwave range to transfer energy? Why does it have to be microwaves? Why not say 10 megahertz?

#### MrSalts

Joined Apr 2, 2020
2,098
Because the wavelength of higher frequencies is short enough to physically shape and direct the "beam" of energy with horn or other shaping antennas of reasonable size. A 10MHz you have 30M wavelength. At 2.4GHz, it is 125mm.

#### nsaspook

Joined Aug 27, 2009
10,122
Most practical wireless energy transfer devices use the reactive magnetic field component of EM energy in a near-field (less than one wavelength) environment for efficiency and electrical safety. Your classic AC utility transformer is an example of near-field (50 or 60Hz) magnetic energy transfer using iron cores for flux concentration of wireless energy between coils inside the transformer.

10MHz 1/4 wave is 7.5 meters
2GHz 1/4 wave is 3.5cm

For long range energy wireless transfer (that's never going to be efficient due to basic physics) you need high gain and highly directional Beam-Forming antennas for far-field energy transfer using RF radiation. It's a lot more practical to build a several wavelength directional antenna at 3.5cm per 1/4 wave element than at 7.5 meters per element.

http://large.stanford.edu/courses/2010/ph240/ma1/

#### Papabravo

Joined Feb 24, 2006
18,991
If you build a long range wireless energy transfer device, why can't you use a lower frequency than the microwave range to transfer energy? Why does it have to be microwaves? Why not say 10 megahertz?
You're welcome to try, but bear in mind anything that you radiate at those frequencies, particularly 10 MHz., will propagate for thousands of miles, and will likely get you a visit from the feds for interfering with a licensed radio service called WWV.

#### Man10

Joined Jul 31, 2018
122
So if I use a lower frequency to transfer energy, the antenna would be too big?

#### MrSalts

Joined Apr 2, 2020
2,098
So if I use a lower frequency to transfer energy, the antenna would be too big?
Essentially, yes. I don't know of any lower frequency transmission antennas tat would be worth the effort vs the benefit of the wireless energy transferred.

#### Papabravo

Joined Feb 24, 2006
18,991
So if I use a lower frequency to transfer energy, the antenna would be too big?
Yes. The wavelength in meters is 300 / frequency in MHz. So:
$\frac{300}{10\;\text {MHz}}\;=\;30\;\text{meters}$
Most practical antennas for this frequency will be 1/4 or 1/2 a wavelength. Other wavelengths would be subjected to substantial power losses from energy reflected back to the source as opposed to radiated. Study physics assiduously you must.

#### nsaspook

Joined Aug 27, 2009
10,122
So if I use a lower frequency to transfer energy, the antenna would be too big?
Look at the original WiTricity hype from smart people who must have known better. Look at the size of a single near-field 9.9 MHz RF resonator designed to minimize far-field RF radiation.
The beam-forming multi-element antenna system needed to optimize far-field RF radiation for long-range wireless energy transfer would be huge.

This was a near-field inductive resonant coupling (RF transformer) demo at 9.9MHz not actual RF radiation.

It's been tried and failed just like the Nikola Tesla system did.

It was not to be, and Intel abandoned its Rezence-based attempts in 2016,[5] and by 2017, Rezence was over in effect, drones never took off, and wireless homes were clearly well out of reach. WiTricity announced layoffs as a result,[6] and now focuses solely on wireless charging systems for electric vehicles (EVs).
https://en.wikipedia.org/wiki/WiTricity
The term WiTricity was used for a project that took place at MIT, led by Marin Soljačić in 2006.[12][13] The MIT researchers successfully demonstrated the ability to power a 60 watt light bulb wirelessly, using two 5-turn copper coils of 60 cm (24 in) diameter, that were 2 m (7 ft) away, at roughly 45% efficiency.[14][15] The coils were designed to resonate together at 9.9 MHz (wavelength ≈ 30 m) and were oriented along the same axis. One was connected inductively to a power source, and the other one to a bulb. The setup powered the bulb on, even when the direct line of sight was blocked using a wooden panel. Researchers were able to power a 60 watt light bulb at roughly 90% efficiency at a distance of 3 feet. The research project was spun off into a private company, also called WiTricity.

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#### Man10

Joined Jul 31, 2018
122
Hypothetically if I built an antenna 0.75 meter long, could I focus the radio waves into a beam? If I had a helicopter hovering 1 kilometer away and hovering 1 kilometer above the ground, could the beam deliver enough energy to power the helicopter?

#### Papabravo

Joined Feb 24, 2006
18,991
Hypothetically if I built an antenna 0.75 meter long, could I focus the radio waves into a beam? If I had a helicopter hovering 1 kilometer away and hovering 1 kilometer above the ground, could the beam deliver enough energy to power the helicopter?
NO. A single conductor fed from one end will have no directional properties to speak of. In order to change the radiation pattern in a useful way you need multiple elements of precise length and spacing. Research Yagi-Uda antennas. They are named after their inventors: Hidetsugu Yagi, and Shintaro Uda in the mid 1920's. these guys were pretty sharp.

#### Man10

Joined Jul 31, 2018
122
Actually I have read the wikipedia article on yagi antenna.

#### Papabravo

Joined Feb 24, 2006
18,991
Actually I have read the wikipedia article on yagi antenna.
That's a start, but there is very little useful design information there. There is a freeware program called EZNEC that can help you with design and simulation of various arrangements and spacing of elements. the lower limit on frequency is the 40 Meter band @ 7.0 to 7.3 Mhz. AFAIK there is no upper limit. I've seen a 35 element job for 1296 MHz. and I've seen a yagi made from paper clips for 3 GHz. Above those frequencies you are probably better off with a dish.