Power and ground planes under balun

[Daniel]

Hi

I am developing for the nRF51822 a 4 layer print with a power and a ground plane in the inner layers. I will be using a balun from Johanson Technology (2450BM14E0003) with the Nordic PCB antenna (from the USB dongle).

The product specification of the nRF51822 says that I need to put a keep-out area under the antenna matching circuitry. Is this keep-out also needed when using a balun? Or can I both have a power and a ground layer there?

[Asbjørn]

For the balun it's different and you can have inner layer under the balun, but we would strongly recommend you to have the inner ground plane at least closest to the balun to prevent coupling to the power plane.

The keep out area is for discrete matching network as they are put together by multiple components, they are more susceptible to stray capacitance as you have components from various manufacturers.

[Daniel]

Thank you very much! This explains my question.