Microstrip impedance of 50 ohm PCB antenna

[Brett]

I'm looking at reference schematic/layouts for the NRF52840 and fail to see how the PCB antennas are 50 ohms. (I understand that it's frequency dependent, and that resonant frequency is a function of length. I'm referring to the characteristic impedance of the trace itself)

Using a few online calculators, with a thickness of 1 oz/ft^2, trace width of 50 mil, height of 63 mil, and dielectric of 4.3, (my understanding is that those are the characteristics of the reference designs), I'm getting an impedance of ~80 ohms for the antenna. I then thought that my height was wrong, since there is no ground plane under the antenna, and I used the distance between the antenna and the ground plane (197mil), and got 120 ohms impedance. How does that distance (197 mil between the antenna and ground plane come into play?

I must have one of my parameters wrong, or have a misunderstanding about the PCB antenna. Can someone clarify?

[Jared]

Hi,

Calculators doesn't always give an accurate result of how it really is. You would have actually measure on the circuit with a Network analyzer to get an accurate result. Which is what we have done with the reference design. The matching network in the reference design has been measured and tuned to the antenna so that it's 50 ohm. Without the capacitor it would be around 30 ohm.

regards

Jared