Electronic – PCB Antenna Design

pcb-antenna

Before asking the questions, I have read App notes for TI, Cypress and Atmel. Those really helped me clear basics of RF Antenna designs. This is my first time with any RF PCB design, so before embarking on this journey I read as much as I can. Following things are clear to me.

Things I know:

  • Relations bewteen Trace Width, Dielectric Thickness, Trace height, Er => Calculate Impedance
  • No ground plane under Antenna
  • via stitching to GND plane.
  • RF IC -> Balun N/W -> PCB antenna or Chip Antenna
  • Balun's Single Ended Impedance = 50 \$ \Omega \$. So Adjust trace width to match the impedance to 50 \$ \Omega \$.

Things I noticed in any RF IC (BLE) Datasheet:

  • IC's antenna pins are differential in nature and we need to convert it to Single ended to make it work with PCB Antenna.
  • Impedance of these differential pins need not to be 50 \$ \Omega \$. I read somewhere that mostly differential impedance is around 100 \$ \Omega \$. Does that mean that I have to adjust the trace width to match the impedance to 100 \$ \Omega \$? Or I can have any trace width?

Things I Dont Know:

  • Trace Width from Balun's output pin to Feed point of antenna?
  • Should IFA's Trace width needs to be consistence?
  • Trace Width from RF IC antenna pins to balun ? Does it matter? Should I Calculate width of the trace (trace between IC's antenna pins to balun's antenna pins) based on 100 \$ \Omega \$?

I'm just trying to develop a BLE module based on nRF51822. Not much information is available for pcb design guidelines. I did download the reference design and studied that, but I have few questions about that. This is purely for learning purpose. I know that some of the questions are really silly, but for a beginner it matters.

Best Answer

Impedance of these differential pins need not to be 50 Ω. I read somewhere that mostly differential impedance is around 100 Ω. Does that mean that I have to adjust the trace width to match the impedance to 100 Ω? Or I can have any trace width?

The differential 100Ω output can be converted to single ended 50Ω by using a balun.

Trace Width from Balun's output pin to Feed point of antenna?

Just like all the other RF traces, design it for 50Ω characteristic impedance.

I'm just trying to develop a BLE module based on nRF51822

BLE just uses 2.4GHz and there are dozens if not hundreds of examples of 2.4GHz PCB antenna designs. Heck, you could probably just open up a device and measure the antenna with calipers and get a pretty decent result. Just focus on impedance controlled traces, proper antenna feedline, via stitching and other rules you've mentioned and it should be ok. Also consider putting an SMA or some other type of antenna connector in addition to your antenna to test it in case the PCB antenna doesn't work.

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