Theory for a PCB resonator with special kind of mode

[yefj]

Hello ,I have built a resonator in CST as shown bellow .
I need to make the resonator be in such mode so the H-field will be normal to PCB plane.(W diection in the photo).
Surface shows two peaks.
As you can see I have peak magnetic field at the edges.
Is there some PCB way to excite magnetic field peak in the middle of the ring in the direction of W(normal to PCB plane)?
Thanks.

 

[Baluncore]

Is there a ground plane ?
A ground plane would define the electric, not the magnetic field to be vertical.

To excite and maintain a circulating current in the ring, you would need to remove the ground plane below the ring, so that the magnetic field did not cut the ground plane, which is reflecting and cancelling the ring current.

 

[yefj]

Hello,I remove the ground plane under the ring .
I made the substrate invisible so you could see the remaining ground and the excitation with the ring.
I got a very bad magnetic field .
Its not in the W direction ,its not even closely homogeneous.
Is there something that I did wrong?
Thanks.

 

[Baluncore]

It looks to me like you are exciting the ring as two semicircular dipoles, touching at the ends. Maybe the geometry of your coupling is defining that mode. It may require two points of coupling in quadrature, to define the direction of the circular current in the ring.

The ring circumference may have to be much smaller than a wavelength, for a circular current to form in the ring. With a ring circumference equal to one wavelength, the centre will be surrounded by one full cycle of the wave current, so at the centre of the ring, the magnetic field generated the current in the ring, will cancel to zero.

 

[tech99]

I agree with #4. Maybe make the circumference small compared with the wavelength and obtain resonance by cutting the ring and inserting a capacitor.

 

[berkeman]

I need to make the resonator be in such mode so the H-field will be normal to PCB plane.

@yefj -- I have two questions for you:

1. What is your experience level so far with 13+GHz PCB antennas?
2. What is the application?

 

[yefj]

Hello Balunecore,I succseeded to implement your advices and made a breakthrew .
As you can see I removed the ground plane bellow the ring and I got the H-field prepandicular to the plane.
How do you reccomend to increase the coupling to this mode?
Is there some intuitions I could use?
Thanks.

Hello berkerman, regarding your questions:
1. I am not using it as antenna but purely as resonator.
my goal is to build a resonator in 3GHz from a printed ring with H-field normal to the PCB plane.
the 13GHZ is just a starting point.
2. aplication is in research of NV cells, but I am only trying to build the resonator here.
https://www.researchgate.net/public...le-Cell_NMR_Spectroscopy_with_Quantum_Sensors

 

[Baluncore]

Well done.

With the ring free of the ground plane, and the capacitor suggested by @tech99 you have made an LC resonant tank circuit. You could couple to it capacitively at two points, or with a nearby inductive coupler like the j shaped track. You might think of the LC tank as the secondary of a transformer, or couple to it with a dipole antenna matching network.

That j shaped track was being reflected in the ground plane, then coupling into the ring. Without the ground plane, you must directly couple between the j track and ring, or place a short ground plane track to do the coupling.

Maybe it is time to study directional couplers, and the difference between odd and even modes of coupling.

 

[berkeman]

cutting the ring and inserting a capacitor

What kind of capacitor for 13GHz? A consultant had to recommend very specific (pF value) vendor and type of capacitors for a 2.4GHz radio application that I was helping with...

 

[Baluncore]

What kind of capacitor for 13GHz?

An open-ended transmission line stub, with a length of 3λ/8, will look like a capacitor.

 

[yefj]

Hello,The best method I got to increase the coupling is by moving the ring to a place closer to the excitaion source as shown below.My capacitor at the best coupling phase was 0.5pF and 0.1OHM series resistor.
red plot is 1Ohm series resistor of the capacitor.
green plot is 0.1Ohm series resistor of the capacitor.
All the 0.5pF capacitors I saw are with 1Ohm series resistance.
When I changed the series resistance to 1Ohn I got the coupling dropped drastickly.
So this coupling mething is not working so much.

I know the design of a directionial coupler and the even mode analysis.
How can I implement it here to get a good coupling?
I cant imagine your words.
Is there some drawing I could use to visualise how directional coupler could be used to get a good coupling to the ring?
Thanks.

 

[Baluncore]

Why do you want an axial B field from a ring, if it is not an antenna?

To support resonance, a transmission line ring would need a circumference of nλ. It would also need a ground plane. If the ring was resonant, then there would be no axial B field.

Why does the resonator need to be a ring?
You could use a λ/2 transmission line as a dipole resonator.

 

[yefj]

Hello Baluncore,I started something which is almost works good :-)
It would be great if I could further improve the coupling to the mode made by this ring.
in the last post I showed results ,Can you reccomend a way to improve coupling ?
Thanks.

 

[Baluncore]

To improve coupling, something will need to change.
Please answer my questions in post #12, so I can work out what to change.

 

[yefj]

Hello ,reagrding your questions:
1.A ring is good because we can put a diamong on top of it to use it for spectroscopy NV centers,stuff I dont work with and doesnt know more details.

2.It doesnt have to be a ring , any closed loop shape would be great.
Thanks.