Impedance Calculation for Bluetooth Antenna

[A_neves]

Hello all,

I am designing a PCB with Bluetooth comunication. So I am using a ceramic chip antenna with 50 ohm impedance.
For the antenna conection I should calculate the width to fit 50 ohm right?
I tried to calculate the impedance, but I am getting a large width for antenna track (1.5mm). I am designing a 4 layers PCB and under the antenna trace I have a keepout, so there is no cooper on the 2 internal layers ( L2 and L3).
Should the Isolation height parameter for a 1.6mm PCB the distance between top and botton layer (1.6mm - (35um +35um)) ?
Another question the online calculaters does not have trace clerance to calculate the impedance. What value should I use?

Thank You!

[robertferanec]

- What is the module?
- Is there something about the trace and layout in the datasheet?
- How they did it on the reference board?

Usually the module manufacturer writes in datasheet how to route the antenna.

[A_neves]

robertferanec Thank you for your reply.

The module is a Nordic Nrt52840. On the reference board they used a trace width 0.765mm.
But if I use a different Pcb manufacture could the impedance change right?
I tried to simulate their calculations, but I am not getting their width values.

[robertferanec]

yes, for 50OHM on two layer PCB, the tracks will be wide (you would need to know the exact stackup they used to calculate it precisely):
(from https://infocenter.nordicsemi.com/in...f5gs.html&cp=1 )

[A_neves]

Thanks for your answer robertferanec .

My doubt is what is the correct way to calculate the impedance for this layout?
I mean, what should be de distance "h" between antenna microstrip and ground plane in this case, since there is a keepout polygon under antenna track?

Thank You!

[robertferanec]

what should be de distance "h" between antenna microstrip and ground plane

- you answered your question. The distance is between GND plane and the antenna track.

[A_neves]

Hello robertferanec ,

But what should I consider the ground plane, since under antenna track there is a keepout polygon?
Should be "h" the dielectric material between top layer and mid layer1 or should be the distance between top and bottom layer (because it is the ground plane under antenna track, because of keepout polygon on mid layer 1 and mid layer 2)?

Thank You!

[robertferanec]

It should be distance between L1 to L4

Track is on L1, GND is on L4. Distance between track and GND is the distance between L1 and L4:

[A_neves]

Thanks for your answer robertferanec .

This is the stackup that Nordic used on nRF52840-DK.

I tried to compute their width for track, using 0.0175mm for Trace Thickness, 0.762mm Trace Width and 1.47mm Substrate Height but I am getting 97 Ohms impedance using this website:

https://www.eeweb.com/tools/microstrip-impedance

What am I doing wrong?

[robertferanec]

Also I do not know how they calculated it. When I try to put the numbers into Saturn PCB, I am not getting 50OHMs.

Sometimes, the real stackup may be different, sometimes they will not make the tracks exactly to meet the impedance.

If you are not sure, I would try to contact their support. Or maybe, try to have a look at similar modules and check their stackups and layout (how others routed 50OHMs antenna track). That is what I would do.

[A_neves]

Hello robertferanec

Ok. Thanks for your help.