How substrate thickness affect the antenna performance?

[mahmoud.tolba]

Hi Nordic Community and Developers,

I'm designing a small Bluetooth module using nRF51822 to use it in my next projects and I need to use thinner substrate material (~0.65mm).

Now I need to know how it will affect the antenna performance and the reference design layout.

Any useful links?

Waiting to hear from you :)

[AmbystomaLabs]

While tools like AppCad will provide a good knowledge base for Mahmoud, I think it is a little unrealistic for a solution to come out of its use.

The initial question doesn't specify which development board they are using as a reference, but assuming it is one of the small beacons these boards were not designed with 50 ohm lines. This was a sensible thing for Nordic to do as one will quickly realize that a 50 ohm line with FR4 on a two layer .062 board will be HUGE. Even if you flood with ground plane and pinch it up to 5 mil from the strip, it will still be pretty big.

So assuming you stay with a two layer design it is unrealistic to put down 50 ohm lines. Further if you do it as 50 ohm the board will need to be built by a vendor that will test and guarantee 50 ohm performance per the design stackup.

It would be better for Mahmoud to just replicate the antenna design and best he can, using transmission line widths that are as big as the components allow and assume some tuning will need to be done.

To facilitate the tuning a pi or t network should be placed near the antenna (essential place holders for components that will be shunt-series-shunt aka pi or series-shunt-series aka t). There is a reason why these style networks work, but it's a rather detailed RF thing and not useful for you.

Then follow some of the well written docs on the nordic site about tuning and monopoles such as: infocenter.nordicsemi.com/.../nwp_008.pdf infocenter.nordicsemi.com/.../nwp_013.pdf

On the tuning side, you will need to figure out a way to measure the field strength from the antenna. Depending on your budget you can get a spectrum analyzer (slightly pricey) and just use any wifi antenna on it. Or, for very little money you can get a usb spectrum analyzer made out of a wifi module. Many of these exist for very little money. Just google it.

I forgot to mention that using a 0.65 mm board will only help you get closer to 50 ohm. Getting close will help simplify your tuning efforts but is not required as I noted. You should however look into how much the board is going to flex. If you use the WLCSP (BGA) version of the 51822 it probably won't survive a lot of flexing before you lose some connections. This will only be exacerbated by the loads of the springs on a button cell holder.

Hope this helped. Cheers!