nRF51422 External Antenna Design

Question

Issue: With my external antenna design, I get lower dbm than PCB antenna (nRF51DK) with same distance. 
 My external antenna design: -70dbm 
 nRF51DK: -40dbm 
 In this answer , you mentioned it's possible to use an external antenna for nRF51422. 
 I use your hardware design document as reference and an U.FL. socket for external antenna connection. 
 Here are the links for the cable and antenna I'm currently using:
 Cable 
, External Antenna 
 Here is my design:
 
 
 I'm a software developer instead of a professional electrical engineer. 
 Could you provide some guidance? I really appreciate that.

Jørn · Accepted Answer

Hello Kangmin 
 I see you have copied the reference circuit from the product specification, which should work well considering the cable, its connectors, and the antenna you have are specified for 50 Ohm. If you haven't already, it could be a good idea to measure your cable and antenna, while connected together, with a vector network analyzer to ensure it truly is 50 ohm at center frequency. If you do measure it and find it is not 50 Ohm, a matching network can be added to compensate for this. 
 Your layout differs somewhat from what is described in the PCB guidelines of the nRF51422 product specification. At high frequency, an electronic system becomes very sensitive to changes in the physical layout. All PCB traces in the RF signal path should be treated as microstrip transmission lines, be designed to have a specific characteristic impedance, and be kept as short as possible. A ground-plane (preferably as large as possible and unbroken) should be present on the bottom layer. The ground-plane should at least cover a large area underneath the components which takes part in the high frequency signal path. 
 I recommend you read through section 11.1 PCB guidelines of the nRF51422 product specification. I also recommend you look at the layout found in the nRF51 development kit (pca10028) hardware files, and copy it, as it has been carefully designed to fulfill all the above criteria. As you don't need the monopole PCB trace antenna you can omit it from the design. 
 nRF51 DK hardware files: 
 www.nordicsemi.com/.../64338 
 If you are interested, here is an introduction to microstrip transmission lines. 
 www.microwaves101.com/.../microstrip 
 Best regards 
 Jørn Frøysa

nRF51422 External Antenna Design

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