[SOLVED] Wireless power transmission antenna design(circular/rectangular)

I am at present designing antenna for wireless power transmission for receiver side using magnetic resonance, Actually the thing is that i have used circular 2 turns helix coil as resonating coil and rectangular spiral coil as coupling coil, i am getting some what good results, but when i am trying to replace circular 2 turns spiral coil with rectangular spiral, the result is degrading.The thing is that i have a size limit of 48 mm of diameter on antenaa design at receiver end and i am working on 18.26 MHZ.

my results with previous configuration were as follows:

S 2,1 -1.7967 db
S 2,2 -5.3659 db
S 1,1 -7.2194 db

I just want some idea, that using which structure the efficiency would improve?, any help is highly appreciated.

Thanks,

Jeetkumar said:

I am at present designing antenna for wireless power transmission for receiver side using magnetic resonance, Actually the thing is that i have used circular 2 turns helix coil as resonating coil and rectangular spiral coil as coupling coil, i am getting some what good results, but when i am trying to replace circular 2 turns spiral coil with rectangular spiral, the result is degrading.The thing is that i have a size limit of 48 mm of diameter on antenaa design at receiver end and i am working on 18.26 MHZ.

my results with previous configuration were as follows:

S 2,1 -1.7967 db
S 2,2 -5.3659 db
S 1,1 -7.2194 db

I just want some idea, that using which structure the efficiency would improve?, any help is highly appreciated.

Thanks,

Click to expand...

From your explanation it is not clear why you do not operate both coils in resonance? If you use the single frequency, 18.26 MHz, I think when you tune the transmitting and receiving coils both to resonance, the power transfer is optimized. Both circuits form essentially a band-pass filter.
Recently, this simple fact known since ~1906 was "discovered" by MIT researchers and utilized to transmit some power like ~40 W over ~50 cm. Their coils, however, at those dimensions and 6.5 MHz, were huge ~0.5 meter square coils.

Thanks for your reply, Actually i have a distance of 20 cm between Tx and Rx, and extending more than 20 cm is degrading my results, my Tx coupling coil is 150mm in radius and Tx resonating coil is of 200mm and distance between those coil is 300mm, so i am not having clear idea of whether i should change distance between two coils of TX or increase the radius of those coils, i have no limit for transmitting side, but i have size constraint on receiving side, so was wondering that what should be the best way to approach, i am at present not getting desired results with Rx circular coupling coil and replacing it with rectangular coil further degrades the results, the reason i am converting to rectangular spiral is that i have to place both Rx coils in 8mm distance, so i thought of converting circular 2 turns helix loop to rectangular may help me to reduce dimensions, my circular helix loop is 3mm in length and rectangular gets reduced to 1 mm in thickness.
So what would be best possible way to resonate both coils?

Thanks for your reply, appreciated

Jeetkumar said:

Thanks for your reply, Actually i have a distance of 20 cm between Tx and Rx, and extending more than 20 cm is degrading my results, my Tx coupling coil is 150mm in radius and Tx resonating coil is of 200mm and distance between those coil is 300mm, so i am not having clear idea of whether i should change distance between two coils of TX or increase the radius of those coils, i have no limit for transmitting side, but i have size constraint on receiving side, so was wondering that what should be the best way to approach, i am at present not getting desired results with Rx circular coupling coil and replacing it with rectangular coil further degrades the results, the reason i am converting to rectangular spiral is that i have to place both Rx coils in 8mm distance, so i thought of converting circular 2 turns helix loop to rectangular may help me to reduce dimensions, my circular helix loop is 3mm in length and rectangular gets reduced to 1 mm in thickness.
So what would be best possible way to resonate both coils?

Thanks for your reply, appreciated

Click to expand...

To create a good band-pass filter from your coils, there are two necessary things to do: First, use a variable capacitor (from an old transistor radio, 50-300 pF approx._ to tune each coil to resonance. When you reach the resonance, you should see a peak in transmitted power for each of the coils. Second, each coil should be optimally coupled to the input and output transmission lines. I would suggest that you experiment: you can use one or two coupling turns, a coil located close to each "main" coil; you can find a tap on each of the coils, the other end to be connected to ground. Or third, when you reach the resonance by the variable capacitor, you can design a two-capacitor combination for an optimum coupling.

For a correct band-pass filter design there are many books and manuals. I still think that you can achieve a good result by simple experiments like the above. If there are metal objects around or close to your coils, try to put them far away possible, or, make sure those objects are made of aluminum or copper, so they would not introduce loss into your propagation path. If your coils are located e.g. in a steel pipe, I would add a thin aluminum pipe in it, to reduce loss.

The distance between your coils determines the coupling coefficient in the tuned band-pass filter, so try to fix it. If you need to vary the distance, the optimum tuning to a resonance in both coupled LC-circuits would be needed.
For a "long" distance between the coils, you possibly can also use a coupling link made of two 2-3 turn coils connected to a twin lead; the coupling coils should be coupled to "main" coils as close as possible. You can also try to use ferrite cores in your resonant coils (like a ferrite rod antenna), maybe the power transmission can increase.

Thanks will try to implement using your guide lines, and i am at present designing on HFSS software.

Thanks

You'll have to explain things a little more clearly. You seem to at least three coils: transmit, receive, and coupling. What is the function of the coupling coil? Is it just to sit in between the TX and RX and help the transfer of energy? That can work in theory, but also keep in mind that the greater the coupling between coils, the more they will detune each other.

Are you trying to match your TX and RX coils to 50ohms to just transmit RF power, or are you going to need to rectify the power into DC on the receive side as well (and generate it on the transmit side)?

I have to transfer power wirelessly and than will add ac to dc converter, but as of now i am working to transfer power wirelessly.
I have four coils in total, at Tx side first there is coupling coil which is connected to supply, next there is resonating coil with added capacitance at Tx side, than there is resonating coil at rx side with added capacitance and than again a coupling coil to deliver power to load. And yes i have matched it to 50 ohm.

So what is the spacing of the coils like? The numbers you gave don't seem to make sense (you said there is 20cm between TX and RX, but the distance between the TX resonating and coupling coils is 300mm? I don't understand). Depending on the geometry, having separate coupling and resonating coils may decrease efficiency. A few years back I did a summer of R&D with wireless power, and found that extra resonating coils were useful if they were spaced evenly between the TX and RX coils, but if not they would degrade efficiency as opposed to simply having resonant TX and RX coils and nothing else. Since in wireless transfer it's often not allowed to have extra coils floating in between, I ditched them and just went for a two coil system.

Following are complete details,

Thanks for your reply, Actually i have a distance of 20 cm between Tx and Rx, and extending more than 20 cm is degrading my results, my Tx coupling coil is 150mm in radius and Tx resonating coil is of 200mm in radius and distance between those coil is 300mm, so i am not having clear idea of whether i should change distance between two coils of TX or increase the radius of those coils, i have no limit for transmitting side, but i have size constraint on receiving side, so was wondering that what should be the best way to approach, i am at present not getting desired results with Rx circular coupling coil and replacing it with rectangular coil further degrades the results, the reason i am converting to rectangular spiral is that i have to place both Rx coils in 8mm distance, so i thought of converting circular 2 turns helix loop to rectangular may help me to reduce dimensions, my circular helix loop is 3mm in length and rectangular gets reduced to 1 mm in thickness.
So what would be best possible way to resonate both coils?

Thanks for your reply, appreciated

I asked you to clarify, but you literally just repeated the same thing over again... if you can't explain with words, please provide a picture, giving the spacing between all four coils, not just one or two of them. Also a schematic of what kind or resonating/matching networks you're using would help.

I can say in general that if you increase the size of one coil and keep the other small, it will only help efficiency up to a point, and you might have already crossed that point. When that happens the only way to improve is to increase the size of the other coils, bring them closer together, or increase the Q of either coil. It's not too surprising that a rectangular coils would have a somewhat lower Q, due to increased proximity effect near the corners. But it may improve overall performance if the area of the coil is greater than a spiral coil.

Sorry for inconvenience..here are the images with distances specified.

Thanks.

Okay that does clear things up, thanks.

So your RX coupling and resonating coils are very close together, and thus probably have poorer performance than just a matched resonating coil would alone. The TX resonating coil, on the other hand, probably will help because it acts to increase the coupling between the TX and RX sections. So keep that around.

After looking at the shape of the RX coils it's certainly no surprise that the square spiral performs worse than a helical coil would. Consider that all the magnetic flux which passes through the conductors in the coil will induce eddy currents and cause losses. So in general you want to minimize the area of the conductor which is orthogonal to the B field. The square spiral has much more area facing into B, so it will have more losses. If you can, you should try just using a helical RX coil (just one, which is tuned to resonance). That should give much better results.

If you do have to fabricate the RX coil on a PCB or something, try doing a double layer spiral coil. That should give a higher Q.

Thanks

But the thing is that i cant exceed 8mm horizontal distance(i.e i have to work under 8mm) between two coils at Rx side, and circular helical coil itself covers 3mm horizontal distance so needed to keep 2.5mm distance between two coils at Rx side, and yes i have used circular coupling coil with 1 turn but gave worst result than rectangular coupling coil, and i read in 1 paper that rectangular gives better efficiency, you can have just look at abstract of ieee paper "Analysis on Transmission Efficiency of Wireless Energy Transmission Resonator Based on Magnetic Resonance"

thanks for your continuous replies, appreciated.

Jeetkumar said:

Thanks

But the thing is that i cant exceed 8mm horizontal distance(i.e i have to work under 8mm) between two coils at Rx side, and circular helical coil itself covers 3mm horizontal distance so needed to keep 2.5mm distance between two coils at Rx side,

Click to expand...

I'm saying you shouldn't have a coupling coil and resonating coil on the receive side. Use a helical coil as both your coupling and resonating RX coil.

As far as I can tell, that paper you posted gives no justification for having two coils on the RX side. When they're that close together, their coupling is so high that they effectively act as one coil, except that the coupling isn't perfect and therefore there are some losses. I remember that the original Witricity experiment also used coupling coils, but I don't believe they did that for the sake of improving efficiency. It was just convenient because it made it more immune to detuning, or something.

and i read in 1 paper that rectangular gives better efficiency, you can have just look at abstract of ieee paper "Analysis on Transmission Efficiency of Wireless Energy Transmission Resonator Based on Magnetic Resonance"

Click to expand...

Rectangular can give better efficiency in some cases. Notice exactly how they do their comparison. They just compare single turn, flat coils. No spirals or helixes involved. In their case, the conductor area facing the B field is roughly the same, and thus the square coil has better performance due to its larger coupling area. However in your case you're comparing a multiturn helical coil vs a multiturn square spiral coil, and of course the square coil will perform worse, for reasons I explained before (more conductor area facing the B field).

Thanks a lot, will work the way you suggested me before and will try to improve.

It was really helpful and knowledgeable from your part.

Can anybody plz guide me in making of square helix in HFSS ??? i cant figure out how to make spiral square ??
Waiting for your reply

Hi Anwar,

for rectangular spiral or square helix, here are the steps to follow first go to draw section-> user defined primitive->sys lib->examples-> rectangular spiral, and for helix structure follow same steps till sys lib ->segmented helix-> rectangular helix.

Once you have selected spiral or helix you can change the size and number of turns as per your requirement.

Thanks kumar
i followed your instructions... but i want to made square helix along z axis...through your way it remains in circular form...i want to make the helical structure in square form..

How can it be circular?? when you select rectangular spiral, its just the same instruction you need to follow and regarding axis once you have designed the structure you can change axis to z axis using rotate in main tab.

again segmented square helix is formed in a circular structure

---------- Post added at 20:54 ---------- Previous post was at 20:24 ----------

thank kumar thanx a lot u helped a lot i made it thanx again