# Floating Ground with wireless energy

Building a wireless energy transfer systems with a transmitting and receiving antenna. I have encountered a problem with what appears to be a floating ground. Initial testing is done with a normal LED that will only light when I have my finger touching one of the leads on the LED. Further testing is done by connecting the receiving antenna to an oscilloscope probe. When only the lead of the probe is connected to the antenna, I get a voltage reading in the tens of volts range depending on the distance. When I connect the ground probe of the oscilloscope, I get a voltage reading of only a couple of volts. I am looking for some any kind of explanation to what can be happening here.

Bobbywhy
Gold Member
Tyler1215, Your test setup description is not complete enough for any analysis to be made.

What kind of energy is passing from the transmitting to the receiving antenna?

How is the received energy converted to voltage?

How is the LED connected in the circuit?

When connecting the oscilloscope’s probe to the receiving antenna and you measure “tens of volts” are those volts AC or DC? Voltage is the potential difference measured between two points. With only the probe connected you are not measuring anything reliable because your ‘scope’s ground is “floating”.

When you connect the ‘scope’s ground lead is the positive probe also connected? If yes, where?

If you can provide a schematic diagram of your circuit and specify exactly the points that you are measuring then people here on Physics Forums may be able to provide you with an explanation of what's happening.

In case you use coils resonating at 13.56MHz, be prepared to have huge floating and induced voltages everywhere...

Induction near the receiving coil is like 1V/cm2 so, to make any measurement, you have to think twice... You might try twisted pairs feeding a differential amplifier, or similar things...

And the DC voltmeter that worked best was a stone-old one, with a mobile coil and no electronics... Because induced AC voltage would let the input of electronic voltmeters go mad.

You can forget the oscilloscope probe. Not adequate at 13.56MHz, and far less so with big inductions. And, well... In case you discover HF now, it'll be a hard learning to make sensible measurements in this environment.

Good luck!

Bobbywhy
Gold Member
Enthalpy, How is it that you can know tyler1215 is using 13.65MHZ? He may well be, but he has not yet said what method he is using for his "wireless energy transfer".

i think enthalpy's reply was on the sarcastic side

Enthalpy is actually right. I am using 13.56MHz cause its pretty much a free frequency used for testing purposes. Can you possibly explain more on some induced voltages that I might encounter? The oscilloscope is rated at 50MHz and was working during initial testing but some parameters have changed. The original design frequency was 13.56MHz and I was able to light an LED from 1 foot away. After implementing a power amplifier, the system wont work pass 2MHz and now the new optimal frequency is at 850KHz. Can the shift in frequencies be an issue with the system?

BobyWhy, I am measuring AC voltage, I have not built a rectification circuit yet. The LED is connected across a positive and negative leads of the coils/antennas in parallel with a diode to act as a simple rectification circuit.

Every thing was working fine until the power amplifier was connected to the transmitting side and then I had the frequency drop and the issue with a floating ground and single-wire connections on the receiving side.

I appreciate your speedy response and skepticism about my project, it's what keeps us all in check.

Bobbywhy
Gold Member
Here’s a company that can transfer up to 3.3KW to recharge electric cars using RF:

http://www.witricity.com/pages/ev-charging-system.html [Broken]

Last edited by a moderator:
Thank you. I have already looked at their system and actually been in contact with a couple of the professors that worked on the initial project at MIT.

I am trying to find a different way to do the same thing but hopefully more practical or efficient. If not, then like Edison said "I found a thousands ways not to make a light bulb"

Your help is greatly appreciated with this project.