Question on Antennas (Basic Stuff)

[zero_coke]

So, I'm doing an experiment on wireless power transfer and I have some questions.

Our transmitter involves:

Power Supply --> Oscillator --> 140W Power Amplifier --> Driving Coil (at resonance)

And then the receiver:

Receiving coil (at resonance, with load attached to it).

What we're worried about is the safety of this setup. Is our driving coil an antenna or not? Because I don't want this 140W to radiate in all directions like how radio towers do. I'm having a bit of a hard time understanding if I'm radiating power or not. My understanding is that if I'm feeding my LC coil (the driving coil) 140W of power it would simply setup a large magnetic field no? Would it interfere with cellular services or cause any harm to humans? The only thing I think we should be worried about is the magnetic field strength of the LC driving coil to be within safety limits (< 4.89 A/m for up to 10 MHz here in Canada). Mind you, I should also mention that we're working with frequencies in the range of 1-10 MHz and we haven't picked one yet but most likely around 3 MHz. I just wanted to know the implications of using a 140W amplifier in this experiment because I was able to replicate the experiment successfully by using a standalone signal generator and the coils and able to light up an LED. What we're trying to achieve in this more advanced setup is to get at least 1-2 meters of distance and be able to power up a decent load, say a 20W light bulb.

 

[goldsmith]

Hi zero_coke

What we're worried about is the safety of this setup

what kind of safety you're referring to ?

Is our driving coil an antenna or not?

What should an antenna do ? transmit the signal . it depends on how much is frequency of operation of your transmitter .

Because I don't want this 140W to radiate in all directions like how radio towers do

you can easily use a limited directional antenna like yagi with special directors or something else like that .

My understanding is that if I'm feeding my LC coil (the driving coil) 140W of power it would simply setup a large magnetic field no

I hope your mean isn't that you didn't put any load ! if so how you've measured the power around 140 watts ?? is your load an inductor ??!! do you know about theorem behind signal transmission and antenna ?

What we're trying to achieve in this more advanced setup is to get at least 1-2 meters of distance and be able to power up a decent load, say a 20W light bulb.

I'm a bit confused . is your mean something like this : your transmitter be able to transmit it's signal through 1-2 meters ? did you know if a 140 watt transmitter being connected into a proper antenna what will happen ? how much would be the distance ?!!!! or perhaps i couldn't get your exact meaning ?
can you tell me a bit more explanations please ? ( clear information )
Best Wishes
Goldsmith

 

[tony_lth]

I think the driving coil is an antenna. But because of the way wound of the coil, It has a fixed direction. Many layers and many turns.

 

[zero_coke]

Hi zero_coke

what kind of safety you're referring to ?

I am referring to the magnetic and electric field strength. In my country (Canada) the maximum allowable strength limit is about 4 A/m for 0.03 - 1 MHz and 4/f A/m for f = 1-10 MHz.
I was wondering how I can measure the electric field and magnetic field strength of my coil.

What should an antenna do ? transmit the signal . it depends on how much is frequency of operation of your transmitter .

The frequency I am using will be between 0-10 MHz. I need to find optimal frequency for most efficiency and safety.

I hope your mean isn't that you didn't put any load ! if so how you've measured the power around 140 watts ?? is your load an inductor ??!! do you know about theorem behind signal transmission and antenna ?

I didn't measure the power. The amplifier I purchased says it will output 140W into a 50 Ohm load. The load is not attached directly to the transmitter. The load is attached to my receiver.

I'm a bit confused . is your mean something like this : your transmitter be able to transmit it's signal through 1-2 meters ? did you know if a 140 watt transmitter being connected into a proper antenna what will happen ? how much would be the distance ?!!!! or perhaps i couldn't get your exact meaning ?
can you tell me a bit more explanations please ? ( clear information )
Best Wishes
Goldsmith

Goldsmith, my goal is to transmit power for 0.01-2 meters wirelessly. For this, I am using magnetic resonance using primary coil and receiving coil. My problem is, I don't know what will happen when I connect my 140w amplifier to the driving coil. Will it act like an antenna or just be a regular coil and setup a large magnetic field? I don't want to radiate the signal like antenna, I want to keep the signal for 0.01-2 meters. I want to stay in near-field. How I can do this?

 

[biff44]

yes, at 140 watts I would be worried about safety too! The good news is that you are at a low frequency where dielectric heating of your tissue will be less. The bad news is that the wavelength will be longer, so you will be in the near field of the radiator--meaning it will be hard to avoid a dose of RF.

I would get an RF safety monitor, like the type Narda makes, and use it before approaching the system with the power on. You might be ok.

You could do the testing inside of a faraday cage room, and only go in when the power is off.

 

[flanello]

You should also take care of metallic or magnetic parts in the near environment. They could influenced by some magnetic forces or could heat up because of eddy currents:https://en.wikipedia.org/wiki/Eddy_current

 

[zero_coke]

Thanks guys!

I have another question: If my frequency is low (2 MHz), then technically I am in the near field when I'm around the transmitter. However, I want the majority of the power to stay in the near field and not radiate. How can I make this happen? I know you cannot stop an antenna from radiating if the amplifier is matched to the antenna, but I want the radiated power to be much less than reactive power in the near field. If I make a poor antenna, technically, it's a poor radiator, but does this mean it will be good for near field??

 

[zorro]

Hi,

No antenna can be really efficient as a radiator if its size is significantly smaller than a quarter wavelength.
At 2 MHz lambda is 150 meters, so I think that radiated power will be only a small part of total power in your case; reactive power in near field should be much more that radiated (active) power. You tell that the amplifier is matched to the coil, but the most part of active power delivered to it must be dissipated in losses.
Obviously, you can harvest power from near field. But it would not be radiated power (i.e. power that otherwise woul continue to propagate in free space. Rather, it would increase the losses seen by the amplifier.
Regards

Z

 

[zero_coke]

How can I measure the radiated power in the near field and far field? Is it possible to calculate or must I use equipment like EMF meter?