nRF52840 Dongle Antenna Simulation in CST

[Radio]

Hi All,

I am trying to simulate the nRF52840 Dongle Antenna given in the hardware files using CST studio. I am using the following substrate and trace width which was in the hardware PCB files.

FR-4 - Dielectric 4.8
h = 78.74 mil
Antenna microstrip trace width, w = 10 mil

I first wanted to simulate the antenna section starting from the RF feeder (left of the green arrow) as shown below. 

My CST design is as follows. There is no ground plane because it is a monopole antenna.

 

As you can see from the above images, my simulation results are really bad.

1. I am thinking it could be because of my port settings in CST. If so how can I assign ports to such an antenna CST?

2. Are there anything else I am doing wrong here? I am thinking that I should see little bit of matching at 2.4 GHz without the matching network. But it looks like there is no resonance at all at this frequency. 

Thanks 

[Radio]

Hi,

In my above simulation, it does not have a ground plane. This is the reason why the results were wrong. In the nRF dongle, RF feeder line width is 35 mil which I think had calculated as a CPW. So I added a feeding CPW Microstrip line to the antenna with width 35 mil. Now I am getting some what sensible result, but the return loss is still not very good yet.

Below please see the images of the new port configuration and the results. Any idea what I am doing wrong. I am also getting some warnings while running the simulation.

Thanks!

[Kaja]

Hi, 

what do you want to simulate? 

Are you trying to simulate the antenna, so that you can find out the components values for the matching network? 

Best regards,

Kaja

[Radio]

Hi Kaja,

Yes I first want to simulate the existing nRF52840 dongle antenna with the substrate definitions Nordic has used. Then use this simulated S-parameters to match to the chip impedance and compare with the matching network in the given nRF52840 dongle schematic.

Reason to do this is because, based on the manufacture, my PCB specifications might change. So then I can do a re simulation and have a feeling on the results, based on the substrate I am going to use to manufacture.

Further note, I did a simulation including the entire ground plane according to the chip size. I also had to include the CPW feeder that connects to the ground. I was able to get a better result now. 

However, If I don't include the CPW feeder I get bad results. I am not sure why this is so. Also Waveguide port dimensions have significant effect on the response as well. Please see the attached images.

Thanks!

[Kaja]

Hi, 

I would recommend you to copy the design form the dongle, and then leave the components in the matching network for the antenna NC and with a 0 Ohm resistor. 
It's must easier to tune the components values on a physical board, then it is to simulate it. We offer to do this service for all our costumers. 

Same with the radio, copy the reference design, then we fine tune the components values for the specific design. 



Best regards,
Kaja

[Radio]

Hi Kaja,

Thanks for the reply. May I know what is the procedure to do the matching service?

Thanks!

[Kaja]

Hi, 

so we first do a design review, when the design look okay you can send us two working boards and one enclosure. 
At the lab we do RF measurements and tune the radio matching components and crystal (32 MHz) load capacitors. 
We also tune the antenna, components (and length, depending on the antenna type). 
Then we send you an report with the results, and suggest components values. 

Best regards,
Kaja

[Radio]

Hi, 

This should be done by opening a private ticket?

Thanks!

[Kaja]

If you would like you design files to be confidential, then yes; you should do this in a private ticket.

Best regards,
Kaja