RE: Designing trace antenna nRF51822-CFAC + BAL-NRF02D3, surrounding ground?

Bhaskar — Tue, 08 Nov 2016 11:49:24 GMT

Hi,

As per antenna design guidelines the antenna should not have ground around. Even reference design nRF51-DK has no ground around the antenna trace in all 4 layers. In such case how PCB fabricator can calculate antenna trace width for antenna to achieve 50ohm without reference plane. Please help me on this..

RE: Designing trace antenna nRF51822-CFAC + BAL-NRF02D3, surrounding ground?

Brian — Wed, 03 Feb 2016 17:37:50 GMT

Terje,

Thank you very much for the detailed answer. As always, this forum is unbelievably fast and helpful.

Brian

RE: Designing trace antenna nRF51822-CFAC + BAL-NRF02D3, surrounding ground?

tesc — Wed, 03 Feb 2016 10:13:21 GMT

Hi,

The RF part consists of the nRF side, the 50 Ohm transmission line and the antenna side. Only the nRF side is included in the reference layouts, as well as the beginning of the transmission line.

For the complete layout the first image is correct. The antenna should not have ground around. The lack of ground is what makes the antenna an antenna (as opposed to a transmission line.) Antenna length is measured following the antenna from the edge of the ground plane. The ground plane edge should be the same on both (all) layers.

You should make sure that the transmission line between the balun and the start of the antenna is 50 Ohm. For the width of the antenna anything around 2-3 mm is good.

Tuning is required in both ends of the 50 Ohm transmission line. On the nRF end you should follow the reference layout closely (as is done on your images). On the antenna side, you can choose one of the following:

Use one shunt capacitor and make the antenna longer than it should be. During the tuning process the antenna is physically shortened (this is done on a prototype board), and a good value is found for the shunt capacitor. Then the antenna length found during tuning is used for production boards.
Use a pi network. Tuning is done by finding suitable components for the pi network. This is the only option if using an antenna that can not be physically altered for tuning purposes.

For more on antenna tuning, se the nWP017 - Antenna tuning whitepaper.

Regards, Terje