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?

[Håkon Alseth]

Hi Geir,

The answer would then be different. If you're using discrete match, the components will have a coupling to the mid-layers (if mid-layers = solid), and would then have an effect on out-of-band noise (harmonics and receiver leakage in RX, often called RX_LO) and may degrade output power (not often). You are correct, this is done to get more distance (less parasitic effects) from matching network to the next layer on the PCA10000.

Ideally, you want at least .5 mm distance from the discrete matching network to the next solid layer in the PCB stackup. Integrated matching network (one component BALUN) are built up in a different way, and does not seem to be influenced on how the PCB is stacked up.