Frequency Choice

This is the second in a series of blog posts focused on changes needed within the wireless power industry to deliver a better end-user experience.

Tuning into the Right Frequency for Wireless Power

by Ky Sealy, Head of AirFuel Resonant Working Committee & Senior Engineer at WiTricity

We’re at an interesting time in the evolution of wireless power – consumer demand for round-the-clock connectivity is growing (across a vast array of consumer devices), yet the end-user experience still leaves a great deal to be desired. The industry needs to move from first-generation to next-generation technologies to tackle the limitations we’re facing, but a key consideration in that shift is wireless power frequency.

For years, the only real solution for wireless power frequencies was the 100-200 kHz frequency band. This was because using alternative, higher frequencies such as 6.78 MHz was cost prohibitive. But with the advancement in silicon-based power semiconductors, as well as the commercial availability of Gallium Nitride based semiconductors, this is no longer the case. Instead, there are actually significant advantages when using higher frequencies for consumer device electronics. So, what’s holding us back?

The misinformation about frequency

The general perception has been that using a lower frequency will make regulatory approvals easier. This is in fact an industry myth. While regulatory design challenges may differ across frequencies, at 6.78 MHz there is a specially designated industrial scientific medical (ISM) band, meaning that electromagnetic radiated emissions are unlimited at this frequency for designated ISM equipment (such as wireless power systems). Below 6.78 MHz there are no other globally accepted ISM bands to date, indicating further regulatory restrictions may apply to lower frequency consumer wireless power devices.

Another market misconception is that lower frequencies do not have RF exposure concerns or limits as higher frequencies do. Again, this isn’t the case. In an upcoming post we will outline the differences in RF exposure requirements across the various wireless power frequencies – stay tuned.

Time for a new choice

Once we get past the industry misconceptions it’s easier to choose the right frequency for consumer devices moving forward.

If we truly want to have global interoperability for all electronics with wireless power, Qi inductive wireless power transfer simply won’t be enough – 6.78 MHz is necessary to take us to the next level. The reasons for this are relatively simple:

  • Experiential — using magnetic resonance wireless power (MRWP) transfer for consumer devices at 6.78 MHz provides greater spatial freedom. With traditional inductive wireless charging, alignment must be precise in order to facilitate charging – magnetic resonance on the other hand allows a device to move and still keep charging. If you’re charging your phone and you receive a message, for example, the vibration from the message can cause the phone to lose its alignment and to stop charging. This is the same as when you charge your phone while driving and you go over a bump in the road. The bump can cause the phone to lose its alignment and the phone will stop charging. However, with resonant technology (higher frequency) users can move their device or have multiple devices, for example, charging at the same time and the devices will still charge even if they’re displaced. Furthermore, magnetic resonance allows wireless transfer through thick materials such as tables and desktops thus eliminating the need to drill into the surface for a truly wireless experience. This is a huge shift in terms of user experience.
  • Industrial design and form factor — at 6.78 MHz wireless frequency electronics and coils become smaller and thinner in size, providing designers with increased design In addition, high quality coils can be built using ubiquitous flexible printed circuit board materials, and you can bend the material and have different product shapes, e.g. curves. The increased quality factor at the higher frequency also means lower loss in the resonant coils and hence higher coupling efficiencies.
  • Regulatory — 6.78 MHz is the lowest common global frequency, deemed as an ISM frequency by most, if not all, regional regulatory agencies. Global and local regulatory recommendation groups such as the ITU-R1 and Broadband Wireless Forum2, also specifically recommend that wireless power systems for consumer devices use the 6.78 MHz band. Beside the functional benefits afforded at 6.78 MHz, wireless power systems operating at this frequency are much less likely to interfere with licensed radio bands.
  • Safety — the metal object heating caused by magnetic fields at 6.78 MHz is negligible compared to lower frequency wireless power. This means that devices are safer as the frequency doesn’t heat up everyday objects such as keys, coins, etc. that could come in contact with them.

The 6.78 MHz frequency with MRWP therefore provides greater freedom both in terms of design as well as user experience and should no longer be ignored as the number one choice for frequency when developing consumer wireless devices.

Interested in finding out more?

If you’d like to learn more about the frequency choices for wireless power, please email me at: ky.sealy@witricity.com. We’d love to hear from you and to share the work we’re doing to deliver next-generation wireless power.


1 https://www.itu.int/pub/R-REP-SM.2303-2-2017

2 http://www.yrp.co.jp/en/yrprdc/bwf/contents_bwf/files_bwf.html