Antenna width for nRF52840

[Sylvain]

Hello DevZone,

I have few questions relative to the design of the PCB antenna for a nRF52840 chip.

I'm going to use the Eagle library find on the github. But...

1) Why the antenna width of the Eagle library (1.5mm) and the antenna width used in the Altium reference design (1.27mm) are different?

2) Furthermore, 1.5mm or 1.27mm doesn't correspond to a Z0 of 50 ohms. For an FR4 PCB (h=1,6mm, t=0,035 mm, er (relative dielectric constant) = 4.7) I found Z0 = 77.7 Ohms for the 1.27 width and Z0 = 72 Ohms for the 1.5 width. I suppose that the Z0 impedance doesn't need to be 50 Ohms, but it only needs to adapt the PI-NETWORK to the Z0 value of the antenna (77.7 Ohms or 72 Ohms)?

I have already read the differents white paper ...

Best regards,
Sylvain.

[ketiljo]

The width of the antenna isn't very important. The length of it is far more important. A wide antenna track will have more bandwidth than a very thin one.

The calculations you do is for the impedance of a transmission line and doesn't apply for an antenna as you don't have a ground plane under the antenna. 

Just use a a width in the area of ~1.5mm and you will do just fine. 

[Sylvain]

Thank for your answer. I'm understanding better now.