BLE Antenna MIFA on FLEX PCB

Anonymous · ‎Apr 27, 2017

Hello,

We need to design MIFA antenna on 2 layer Flex PCB (polyimide thickness between top and bottom copper layers is circa 80-90um)

Document "CYPRESS AN91445 Antenna Design and RF Layout Guidelines" suggests MIFA examples implemented on PCB thick 0.4mm-1.6mm.

As Flex PCB is much thinner I have few questions related to geometry.

1) Our flex antenna feed would be coplanar waveguide (2-layer flex PCB: W=6mils, GAP=4mils, flex thickness~3.5mils )

2) In AN91445 there is a suggestion to use L tip = 353mils for PCB 0.4mm thick:
Would it be correct to have Ltip=~1500 mils for 80um ? - this is considerably long, how to achieve shorter Ltip at 80um thick flex PCB?
Can I just continue meandering to limit the space required? Is there some kind of simple method to calculated Ltip for flex PCB 80um thickess?

3) Part of the MIFA stub: the trace that is parallel to the meander is grounded. How much will the low thickness of the flex (80um) will affect the performance of the antenna?

May you suggest on make comments on the above matters?

Best Regards, GK.

GuhapriyanT_31 · ‎May 08, 2017

Hello GK,

1) CPW if fine. Please ensure that the trace from ANT pin to the Antenna feed point is 50 ohms.

2) The Ltip values mentioned in AN91445 assumes FR4 PCB. So, far a different substrate, the dimension could be different. Continuing Meandering is fine, but as the optimal length depends on the thickness and substrate material also, it is recommended to get it simulated.

3)The trace that runs parallel to the meander is a short circuited stub to compensate for the parasitic capacitance formed between the ground and the arm of the antenna that runs parallel to the ground. The low thickness does not affect the performance if the length of the Antenna is suitably adjusted.

Thanks,

Guha

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GuhapriyanT_31 · ‎May 08, 2017