The Peculiar Behavior of a large electromagnetic coil

[eidolon171]

I have these two very large (ferrous core) coils,

They are basically identical, each 200mm long and have a diameter of 100mm. The core is medium grade steel

One coil is receiving an input of 8V(peak-to-peak) @ 4 MHz from the function generator. The second coil is placed 1m away, and is connected only to an oscilloscope where it showing 20mV(peak-to-peak) while the transmitting coil is operating. My question is this: when I bring the palms of my hands close to the poles of the transmitting coil I get an increase of up to 40mV(peak-to-peak) measured voltage on the secondary coil. What effect is the close proximity of my body having on the poles of the primary coil?

 

[cnh1995]

I think it is happening because of the increase in magnetic permeability of the medium, which is increasing the magnetic coupling between the coils.
Did you have any metal (steel or iron) thing on your hands or body?

Or perhaps the iron in your blood is responsible.

 

[sophiecentaur]

How about Capacitative Coupling, plus the imbalance in the feeds to the coils? Once you look in detail at this sort of arrangement, its equivalent circuit is far from the original circuit that's sketched on the first fag packet during your planning.
To get near to the theoretical situation, you need a Balun at each end to convert the unbalanced source and receiver into balanced feeds. That will take care of a lot of problems.

 

[eidolon171]

I wasn't wearing anything metallic (t-shirt, basketball shorts, running shoes, no jewelry), nor was I wearing a grounding bracelet.

I don't think its my body's iron content, and I'll tell you why. I had a partner help me move two long, high-grade ferrite bars toward the poles of the transmitter coil by pushing them closer with boron rods. There was still an increase in the receiver coil's induced voltage, but it was actually a little bit less than when I used my hands: only 30mV(peak-to-peak).

I feel like what I might be seeing here is related to the same effect you experience when you touch an antenna, which is like a clumsy form of tuning. In which case, my body would be acting like a parallel capacitor, and the coil is starting to behave like an LC tank circuit. I am aware of magnetic resonance as a way to achieve long distance power transfer, but I thought that both the transmitting and receiving coils needed to be tuned to the same frequency. But I don't know, I appreciate your input @cnh1995.

 

[sophiecentaur]

both the transmitting and receiving coils needed to be tuned to the same frequency.

That's for the optimum coupling. A parasitic Capacitance can produce noticeable effects way off resonance. 4MHz is very much 'RF' and needs to be treated with more respect (Impedancewise) than audio or Mains.

 

[jim hardy]

At 4 mhz
a lone picofarad is about 40 kilo-ohms.

so capacitance between your coils is significant at that frequency.
Try holding the 'scope probe at various places along your yardstick, perhaps with a 1" square aluminum foil 'capacitive pickup'.
Then try it again with a few turns of small wire on maybe an inch diameter for a 'magnetic pickup' . https://en.wikipedia.org/wiki/Body_capacitance

Capacitance of a human body in normal surroundings is typically in the tens to low hundreds of picofarads, which is small by typical electronic standards. While humans are much larger than typical electronic components, they are also mostly separated by significant distance from other conductive objects. Although the occasional static shock can be startling and even unpleasant, the amount of stored energy is relatively low, and won't harm a healthy person. The Human Body Model for capacitance, as defined by the Electrostatic Discharge Association (ESDA) is a 100pF capacitor in series with a 1.5kΩ resistor[1]

 

[sophiecentaur]

a lone picofarad

Poor little thing!

 

[sophiecentaur]

I am pleased that you can't have an isolated magnetic pole. Everyone needs somebody.
Best stick on topic though.

 

[Rive]

It's not necessarily electrical or magnetic. Can be some ultrasonic/mechanical coupling too.

Even if 4MHz is a bit off scale.

Ps.: try to replace the 'transmitter' with some piezoelectric sounder.

 

[The Electrician]

Tell us more about the core in your coils. You say the core is medium grade steel. Is it just a solid block of steel, or is it laminated?

The copper wire used for the coils looks quite large to me; what is its gauge?

Have you measured the inductance and Q of the coil at 4 MHz with and without the core in place?

 

[CWatters]

+1 to what Jim said. Is the 20mv signal a real signal transmitted between the coils or just pickup in the scope via the power supply or some other route.

 

[eidolon171]

To dispel some theories: When the primary coil has no input the secondary coil registers no output. Furthermore, as the input voltage is increased in the primary coil voltage output increases in the secondary coil.

Conductivity between coils: There is no contact between the coils, or interfering metals either on my person or the table (which is made of an non-conductive epoxy).

Balancing: I set up chokes to substitute for baluns, this increased the range of the primary coil at either pole, so that the secondary coil could withstand some pretty major misalignments, but it did not increase the maximum magnitude of the voltage in the secondary coil. Which makes sense I guess.

Coil Physical Properties: The copper conductor is 6 AWG (diameter = 4.115 mm) and 7.25 m long (uncoiled). The core is from a company called Everbilt, I was informed by the manufacturer that it was made from a mixture of two low-carbon "mild" steels, typically sold as flat bar for welding. I was told that company specs define the permeability as between 1.25 *10^-5 and 1.25 *10^-4 Henries/meter, and relative permeability estimated between 10 and 100.

The bars were laminated with a spray-on insulator made with vinyl manufactured by Gardner-Bender.

P.S. I formed a length of bar into a horseshoe and put it in the 115V magnetizer and made a pretty strong magnet out of it.

 

[sophiecentaur]

+1 to what Jim said. Is the 20mv signal a real signal transmitted between the coils or just pickup in the scope via the power supply or some other route.

. . . . which is why I said that the measurements must be balanced, to eliminate the effects of any unbalanced induced voltages. (There will be very little balanced voltage coupling). Off the shelf centre tapped transformers (Baluns) could produce a great improvement in balance. You can obtain kits for winding your own TXformer with ratios of your choice. A 4MHz Balun cannot be that hard to wind yourself.
There are a number of approaches to trouble shoot this.
If you observe the supply and output signal waveforms, synced to the input signal, I expect that the phase between the two waveforms can be seen to change noticeably as you wave your hands about.
Alternatively, you can reduce the effect of electric fields by loading the secondary coil with a lowish resistor.
You could do the experiment over a large Earth plane and take the signal leads through holes in the plane.
You cannot afford to be sloppy with RF, as anyone who has tried to breadboard a high frequency analogue circuit will eventually find out.

 

[jim hardy]

Ferro-Magnetization at high frequency does not proceed very far into iron.
Try removing your cores one at a time and repeat your same measurements,,, let us know what difference the core makes ?

 

[sophiecentaur]

When the primary coil has no input the secondary coil registers no output.

That's no surprise. The drive to the primary is unbalanced, I assume (coax or a pair of wires). Any current through the primary coil can be causing an E field in the vicinity as it's the impedance that will count. You could try a number of different methods for picking up this unwanted E field - a wire probe or loop - at various places.

 

[jim hardy]

and relative permeability estimated between 10 and 100.

Iron magnetizes by a couple different mechanisms.
Magnetic domain realignment https://en.wikipedia.org/wiki/Magnetic_domain
and alignment of electron orbit and spin dipoles in the electron shells of the atoms..
http://www.boulder.swri.edu/~dstillman/pdf/Stillman_CSM_06.pdf about page 70 of pdf ( corrections to my understanding are welcome)

The former is somewhat slow and explains ferromagnetism, look up "Barkhausen effect"
the latter goes on up into megahertz range . From what I've read it might give you relative permeability of as much as 10 and curiously that's the lower number you got from Everbilt.

That's why i asked about effect of your iron core. My frequency experiment decades ago with permeability only went up to 400 hz where μ_relative of my steel dropped to around 15.

I expect your core's presence or absence to not change things very much at all, aside from loading the signal generator by absorption of RF energy.

Not challenging you, just I'm curious about magnetism and you have an interesting test bed here.

Have Fun !

old jim

 

[sophiecentaur]

It would be interesting to know what the purpose of the coil + core might be. A core can be there for linking flux to another inductor or for increasing self inductance. In both those cases, it would probably be better to match the core material to the operating frequency. 4Mhz looks like a job for the appropriate ferrite.

Balancing: I set up chokes to substitute for baluns,

What sort of arrangement did you use?

 

[The Electrician]

Why are you doing this? Are you experimenting with wireless power transmission?

Given the thickness of the steel bars in the core and the 6 gauge wire in your primary inductor, you have the makings of an induction heater. If you measure the Q of your inductor at 4 MHz, I think you will find it quite low.

I'm guessing that you may not have the equipment to make such a measurement. In a few days I'll be back in town so I can make a measurement of an inductor with similar core and winding.

I'm curious; why would you use boron rods to push on your ferrite bars? Why not use a wooden dowel or plastic rod? Aren't boron rods a little difficult to obtain?