nrf52840 reference antennas, transmission lines and matching networks

[JEIZIZIAH]

Hi, I am working on a pcb design utilising the nrf52840. I want to use a pcb antenna. I am a hobbyist so It’s very likely I’m misunderstanding something. I am interested in using the ¼ wave monopole like the nrf52840DK antenna design instead of the meander style antenna on the nrf52840 dongle for the normal reasons, easy to tune, the board will have enough space, sounds like it might be more forgiving with manufacturer tolerances etc.

I have read the frequently referenced white paper (l/4 printed monopole antenna for 2.45GHz), it assumes a dielectric constant of 4.4 for 1.6mm FR4 but doesn’t explain what to do if the dielectric constant is different. I want to go through JLCPCB which suggest a dielectric constant of 4.5 for their 2-layer boards. Will this make a big difference? Also, the white paper suggests the antenna has a width of 1.5mm, however, the nrf52840DK antenna is 1.25mm, is the width important?

The matching network on the chip side of both the DK and the dongle are the same. However, the matching network at the antenna end of the DK and the dongle are different, 1.2pF & 1.5pf capacitors respectively. Which should I use for a 2-layer board with a printed ¼ wave monopole antenna? is this value based on the antenna type, or build up of the board? or is it a combination of all things and requires tuning to get “perfect”?

Last question for this post. The dongle has a grounded coplanar wave transmission line dimensions entered below which basically gives the target 50 ohms.

 

However, the DK grounded coplanar wave transmission line dimensions entered below are way off 50 ohms.

 

Is there a reason for this or have I got something very wrong/misunderstood?

 Any help on the above matters is appreciated, thank you.

 

J

[Kaja]

Hi,

I will answer you case tomorrow.

Best regards,
Kaja

[Kaja]

Hi,

Antenna selection in general: 

Antenna choice is complex and depends on many factors including technical and commercial considerations. Here are some alternatives:

Monopole, printed PCB antenna: This is easy to make and easy to tune, you also only need one impedance matching component, so it’s cost effective. Here the spacing is the issues, you need to make it about 23 mm long needs a minimum of 5 mm clearance to the ground plane. High bandwidth, making it fairly resistant to detuning. Link to our whitepaper: https://infocenter.nordicsemi.com/pdf/nwp_008.pdf?cp=12_18

Meander antenna, printed PCB antenna, ex. our dongle antenna design: Requires a smaller area than the monopole antenna, but usually requires a pi-network for tuning in addition to length. Lower bandwidth than a quarter wave monopole antenna. Here is a link to our nRF52840 Dongal design files as an example of this: https://www.nordicsemi.com/Software-and-Tools/Development-Kits/nRF52840-Dongle/Download#infotabs

 

Chip antenna: Higher BOM, but the antenna is small. The downside is that it usually has less gain. It requires a matching network, based on the vendors recommendations. It has a lower bandwidth than a quarter wave monopole antenna so it can be sensitive to detuning.

The type of matching network needed for the antenna depends on the antenna type, and you need two matching network in total, one for the nRF radio and one for the antenna. This is why this is the same for the dongle and DK on the radio side, but different on the antenna side:
 - example with nRF52833 and antenna. 

I have read the frequently referenced white paper (l/4 printed monopole antenna for 2.45GHz), it assumes a dielectric constant of 4.4 for 1.6mm FR4 but doesn’t explain what to do if the dielectric constant is different. I want to go through JLCPCB which suggest a dielectric constant of 4.5 for their 2-layer boards. Will this make a big difference? Also, the white paper suggests the antenna has a width of 1.5mm, however, the nrf52840DK antenna is 1.25mm, is the width important?

- Changing the substrate does not make a big difference.. and you can use both 1.25 and 1.5 mm as width.  If you copy the one form the DK, add some extra mm for editing the length when doing antenna tuning. 

If you follow the white paper, also add some extra length ~ 25mm in total. 

However, the DK grounded coplanar wave transmission line dimensions entered below are way off 50 ohms.

 

Is there a reason for this or have I got something very wrong/misunderstood?

The important thing is that you do the calculations for you design, and that the transmission line is ~50 Ohm. 

Best regards,
Kaja