What's the precision needed to make a Magnetic Resonant Loop?

Thread Starter

Mariemation

Joined Sep 26, 2016
18
Hello

I want to make a resonant magnetic loop of 30 cm in circumference, I have a few questions:

1* do i need to cut more than 30 cm in wire in order to compensate for the edges of the loop (which need to be implanted in a circuit board)?

2* does the loop's shape have to be perfectly circular? because i don't have the right tools to do that, is that going to affect the theoretical resonance frequency of the loop?

3* I want to use a gimmick capacitor as a tuning one because i don't have a variable capacitor, so i am planning to attach two enamel wires to the loop and then twist them together searching for the resonance frequency, the problem is i might be getting away from the RF instead of getting closer, any suggestions on how to both increase and decrease capacitance to tune the loop?

4* how do i know the range of the frequency that the tuning capacitor need to scan through ito reach the desired resonance frequency?

I already tried to make the loop but it wasn't resonant at the calculated theoretical frequency, here it is:
loop 2019-02-25 at 5.35.24 PM.jpeg

The characteristics:
loop diameter: 10 cm
wire diameter: 1 mm
inductance: 2.94E-7 H
it is in parallel with a capacitor of C = 50 micro Farads
which makes a resonance frequency of 41093.62 Hz

This project is aimed at increasing the distance of wireless power transfer using passive resonant circuits.

Thank you in advance.
 

MrChips

Joined Oct 2, 2009
30,705
You are not going to create 50μF by twisting two wires. Maybe 50pF = 0.000050μF, six orders of magnitude too small.
 

Picbuster

Joined Dec 2, 2013
1,047
Hello

I want to make a resonant magnetic loop of 30 cm in circumference, I have a few questions:

1* do i need to cut more than 30 cm in wire in order to compensate for the edges of the loop (which need to be implanted in a circuit board)?

2* does the loop's shape have to be perfectly circular? because i don't have the right tools to do that, is that going to affect the theoretical resonance frequency of the loop?

3* I want to use a gimmick capacitor as a tuning one because i don't have a variable capacitor, so i am planning to attach two enamel wires to the loop and then twist them together searching for the resonance frequency, the problem is i might be getting away from the RF instead of getting closer, any suggestions on how to both increase and decrease capacitance to tune the loop?

4* how do i know the range of the frequency that the tuning capacitor need to scan through ito reach the desired resonance frequency?

I already tried to make the loop but it wasn't resonant at the calculated theoretical frequency, here it is:
View attachment 172463

The characteristics:
loop diameter: 10 cm
wire diameter: 1 mm
inductance: 2.94E-7 H
it is in parallel with a capacitor of C = 50 micro Farads
which makes a resonance frequency of 41093.62 Hz

This project is aimed at increasing the distance of wireless power transfer using passive resonant circuits.

Thank you in advance.
Please read http://www.antenna-theory.com/antennas/smallLoop.php
to get information on single loops.

picbuster
 

DickCappels

Joined Aug 21, 2008
10,153
1* do i need to cut more than 30 cm in wire in order to compensate for the edges of the loop (which need to be implanted in a circuit board)?

Only if you want to connect it to something by using the wire leads. In short, yes, leave some at the ends and trim the unneeded parts later.

2* does the loop's shape have to be perfectly circular? because i don't have the right tools to do that, is that going to affect the theoretical resonance frequency of the loop?

No, doesn't have to be perfect and yes the shape will affect the frequency some, but probably not much. To get a nicely formed circuit look for cylindrical objects that upon which you can wind the coil. After it is wound you can slip the coil off of the form and wrap with tape to hold its shape.

3* I want to use a gimmick capacitor as a tuning one because i don't have a variable capacitor, so i am planning to attach two enamel wires to the loop and then twist them together searching for the resonance frequency, the problem is i might be getting away from the RF instead of getting closer, any suggestions on how to both increase and decrease capacitance to tune the loop?

That is probably only going to work at frequencies above 20 MHz or so because you can't get much capacitance in a gimmick as MrChips has already pointed out.

4* how do i know the range of the frequency that the tuning capacitor need to scan through ito reach the desired resonance frequency?
Start with this formula, upload_2019-3-16_16-52-2.pngthen estimate the tolerance of your inductance so you can figure the range of capacitances that would be needed.

inductance: 2.94E-7 H
it is in parallel with a capacitor of C = 50 micro Farads
which makes a resonance frequency of 41093.62 Hz


The problem with not hitting the correct frequency might be down to your capacitor having a large tolerance. The reactances of your capacitor and inductor are about 78 milliohms each. If you increase the reactance a lot you can use a much smaller capacitor which would allow you to use lower tolerance capacitors and probably allow you to find higher efficiency (higher Q) capacitors. The down side is that you have to wind more turns
 

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Thread Starter

Mariemation

Joined Sep 26, 2016
18
1* do i need to cut more than 30 cm in wire in order to compensate for the edges of the loop (which need to be implanted in a circuit board)?

Only if you want to connect it to something by using the wire leads. In short, yes, leave some at the ends and trim the unneeded parts later.

2* does the loop's shape have to be perfectly circular? because i don't have the right tools to do that, is that going to affect the theoretical resonance frequency of the loop?

No, doesn't have to be perfect and yes the shape will affect the frequency some, but probably not much. To get a nicely formed circuit look for cylindrical objects that upon which you can wind the coil. After it is wound you can slip the coil off of the form and wrap with tape to hold its shape.

3* I want to use a gimmick capacitor as a tuning one because i don't have a variable capacitor, so i am planning to attach two enamel wires to the loop and then twist them together searching for the resonance frequency, the problem is i might be getting away from the RF instead of getting closer, any suggestions on how to both increase and decrease capacitance to tune the loop?

That is probably only going to work at frequencies above 20 MHz or so because you can't get much capacitance in a gimmick as MrChips has already pointed out.

4* how do i know the range of the frequency that the tuning capacitor need to scan through ito reach the desired resonance frequency?
Start with this formula, View attachment 172487then estimate the tolerance of your inductance so you can figure the range of capacitances that would be needed.

inductance: 2.94E-7 H
it is in parallel with a capacitor of C = 50 micro Farads
which makes a resonance frequency of 41093.62 Hz


The problem with not hitting the correct frequency might be down to your capacitor having a large tolerance. The reactances of your capacitor and inductor are about 78 milliohms each. If you increase the reactance a lot you can use a much smaller capacitor which would allow you to use lower tolerance capacitors and probably allow you to find higher efficiency (higher Q) capacitors. The down side is that you have to wind more turns

Thank you so much for your answer

about the capacitor: i was not planning to reach 50μF using only the gimmick capacitor, i already have a capacitor in parallel with the loop, i wanted to add the gimmick capacitor in parallel to search for resonance frequency near the theoretical value.
 

Thread Starter

Mariemation

Joined Sep 26, 2016
18

The problem with not hitting the correct frequency might be down to your capacitor having a large tolerance. The reactances of your capacitor and inductor are about 78 milliohms each. If you increase the reactance a lot you can use a much smaller capacitor which would allow you to use lower tolerance capacitors and probably allow you to find higher efficiency (higher Q) capacitors. The down side is that you have to wind more turns
but isn't adding more turns going to increase the tolerance on L?
Is 10% considered high tolerance in the capacitor?
 

MrChips

Joined Oct 2, 2009
30,705
1) Twisting two wires together will add about 10pF. This will have zero effect on a 50μF capacitor in parallel.

2) It will be difficult to source a 50μF capacitor that will give reliable and consistent results. The tolerance of a typical 47μF electrolytic capacitor is about ±20%.

3) You would be better to choose a non-polarized capacitor under 1μF and adjust all other variables.
 

AnalogKid

Joined Aug 1, 2013
10,986
If you look at the wireless charging products and kits already on the market, you will see that they are based on coils with dozens or hundreds or turns.

ak
 

BR-549

Joined Sep 22, 2013
4,928
The problem is the use of the term "magnetic resonance loop and resonance magnetic loop".

A resonant loop is a special configuration of a coil. The length of the one turn coil is the resonant wavelength.

A loop can be resonated at many frequencies....but only one frequency is natural with the perimeter of the loop.

What is the wavelength of 40KHz? That's a big perimeter.

So, at 40KHz, you want a resonant induction coil, not a resonant loop.

What is the resonance of a loop, 1 meter long?
 
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