nRF52 monopole antenna without PI network?

[Lauri Jämsä]

Hey guys,

I'm designing an antenna for our open source sensor beacon, RuuviTag: http://ruuvi.com

Because we're using 1/4 wavelength monopole antenna, I'm considering to drop the PI network based on what Nordic's nWP-017 antenna tuning white paper states:

There are the following two methods to tune an antenna:

• If the physical dimensions of the antenna can be altered, for example, witha PCB antenna, adjusting the length will be one part of the tuning. Another part is to add a component, inductor, or capacitor, to pull the antenna impedance towards the 50 ohm center point.

• If the antenna cannot be altered physically, more external components must be used to tune the antenna. These external components are called the matching network.

If it is not possible to get the impedance exactly 50 ohm by adjusting the length of the antenna, a component must be used to pull the impedance to the 50 ohm point. It is preferable to use a shunt capacitor since a capacitor is cheaper than an inductor and because a shunt component can be removed without any impact.

Layout (we've also an integrated NFC antenna):

Schematic:

github.com/.../ruuvitag_revb2_schematic.pdf

Antenna tuning will be done by adjusting the length of the antenna and assembling a suitable cap/inductor to pull the impedance towards 50 ohm. What I'm asking is that do you also think that this is an ok way to do it? nRF52 reference design has a 0.8pF capacitor and a 3.9nH inductor:

You can find everything from our GitHub organisation:

http://github.com/ruuvi

github.com/.../

[tesc]

Hi,

Yes and no.

Yes, you can use a shunt capacitor for the matching network at the antenna side, and rely on shortening the length of the antenna as part of the tuning, but:

No, you can not remove L1 and C3 found in the reference schematics. These are required as part of the nRF side matching.

The 50 ohm transmission line is the wire marked RF in the reference schematics, and you must include matching in both ends. The reference schematics show only the nRF end matching. Get the latest reference layout on our infocenter (it also includes reference schematics), and make sure the nRF side matching is identical to the reference both in schematics and layout. Any change in layout for L1, C3 and connected wires may lead to poor matching.

Also do note that shortening the antenna is much much easier than extending it, so the antenna should be longer than needed on the prototype used for tuning. The antenna length for the final design will be found during the antenna tuning process.

Regards, Terje

[Lauri Jämsä]

Thanks, Terje! This was helpful.