ground plane under balun (and antenna net)

[Geir Strand]

Hi,

I am developing a test board with a nRF51822 and ST balun (NRF01D3) and a chip antenna (probably Johanson 2450AT18A100 or B100). The board will have 4-layers. The inner layers are ground and power planes. Should they be removed under the balun and antenna net?

[Håkon Alseth]

The ST BALUN is designed to work with 2 or more layer boards without any modifications or keep-out areas. No need to make any alterations wrt. to the BALUN.

However, the antenna must have a keep-out on all layers, with a given clearance length, as described in the manufacturers datasheet. If the antenna is not placed to radiate freely, it will not function properly.

We recommend using a PI-network as an antenna tuner (shunt-series-shunt network) between the BALUN and the antenna. All antennas are not 50 ohm by default, and their impedance at 2.45 GHz will always differ from design-to-design.

[Geir Strand]

Hi Håkon.

Thank you for your quick answer. I have included the pi-network. Would your answer have been different if we used discrete components instead of a balun? I just found the reference design for PCA10000, which has 4 layers. This have polygon pour cutouts on all layers except bottom layer. Why is this layer treated differently? Is it because of the distance from top layer?