RE: NORDIC WP008 , is the quarter wave length correct

Oystein — Thu, 29 Jan 2026 15:20:33 GMT

Of course, this makes sense, I never given DK_eff to much consideration but if the ground plane is removed the it makes perfectly sense.

Thansk for clarifying :)

Øystein

RE: NORDIC WP008 , is the quarter wave length correct

Szabolcs — Mon, 26 Jan 2026 14:49:28 GMT

Hei Øystein,

The 3.1 value you are referencing is the effective relative permittivity (epsilon) for a microstrip line. For a microstrip line, you have a ground plane on the other side of the PCB, and the majority of the EM field is inside the substrate (between the microstrip and the reference plane). That is why the effective epsilon is closer to the PCB epsilon.

However, in the antenna design, there is no ground plane below the antenna on any other layer, so most of the EM field is in the air, and the effective epsilon will be closer to that of the air than the substrate. It is difficult to estimate it properly (I haven't found any articles on this topic), and the problem gets more complicated if you account for the size of the ground plane. I think there is no closed formula to calculate this, it can only be determined by simulation.

Overall, the 0.75 multiplier for the length is more like an empirical number, our experience shows that it is a good estimation for the initial value, but depending on the ground plane size, shape, antenna placement, and other factors, the antenna length usually needs to be customized to the device. This can be best done by tuning and measurement.