Function of solid plane in the NFC antenna

[michael_d_1983]

Hello,

We have integrated our NFC antenna onto a controller board.
Our antenna measures approximately 90 mm x 40 mm and has 3 turns.
It is already surrounded by ground planes on two sides. The distance between the ground planes and the antenna is approximately 5 mm.

Our PCB manufacturer (of a multi-layer PCB) wants us to fill the antenna area of the inner layers with copper.

Since I found NFC demo boards from NXP where this is implemented in a similar way, I think it should work and would like to implement it similarly.

Are there any reasons why it was implemented this way on these evaluation boards?

To what extent does the copper area affect the range?
Does the copper area affect EMC? Is it better or worse?
What should be the minimum distance between the copper area and the antenna?
Is it better to use a single large area or several small areas?

Thank you very much, and best regards,
Michael

[Tomas_Parizek]

Hello @michael_d_1983 ,

As mentioned, we use the copper tiles to demonstrates a a bit more realistic environment.

This means that you may observe a slightly lower reading range, which is more reflecting the reality than an ideal antenna in free space. This because of some energy is absorbed the the metal tiles. 

But there is no change in the TX current and EMI, if the tuning stays the same. 

Therefore there is no need to implement such design into the "real" product. 

BR

Tomas 

[michael_d_1983]

Hi Tomas,

Thanks for your feedback!

We actually don’t want to include the copper areas at all. The PCB manufacturer wants us to do that.

(The reason is that if the copper distribution on the PCB is uneven, problems arise during the lamination process in PCB production—such as air pockets and delamination—because the resin in the prepregs isn’t sufficient to compensate for the missing copper.)

We just want to clarify whether there’s any fundamental reason against this that we haven’t considered yet.

We’ve tested it (reader functionality). Everything seems to be working as usual.

Best regards,
Michael

[Tomas_Parizek]

Hello Michael, 

The main reason why we use this inner metallization is mainly for a demonstration purpose to simulate a realistic environment around the NFC antenna e.g. in the POS terminal. 

Also this metal filling may help to reduce the detuning effect by a different object because the antenna is already tuned with the "metal loading" inside.  

However for the real product, I would go for a conventional NFC antenna without any filling inside.

BR

Tomas 

[michael_d_1983]

Hi Tomas,
Thank you very much for the information.

We have since conducted tests by gluing copper tiles (8 mm x 8 mm, spaced 2 mm apart, 2 layers each) to the antenna surface on both the top and bottom.
We then measured the antenna and rematched it.
We can’t detect any differences in terms of functionality (range, current measurement, and EMI).

You wrote that you would recommend a conventional design.
Are there reasons for this, and if so, what are they?

Maybe several smaller areas with more spacing between them would be better than the 8x8 mm with 2 mm spacing?

Thank you very much,
Michael