EAGLE library for 2.45GHz planar inverted F antenna (PIFA)

In creating Mark II of a Bluetooth link with my telescope mount, I realized I would need a 2.45GHz antenna for better performance. So I went looking on the Internet for information on Bluetooth antennas. I collected 5 basic options: quarter-wave wire antenna, half-wave wire antenna, chip antenna, external 50Ω antenna, and planar (PCB) antenna:

  1. Quarter wavelength (λ/4) wire antenna. This is a very simple antenna, consisting of a piece of copper wire of exactly 1/4 wavelength. The formula for the wave length λ = c / f where c is the speed of light (299 792 458 m/s ≈ 3E+8 m/s) and f the frequency (2.45 GHz = 2.45E+9 Hz for Bluetooth). Hence the wavelength of Bluetooth is λ = c / f = 3E+8 / 2.45E+9 m = 0.1225 m = 122.5mm. A quarter wave Bluetooth antenna hence has a length of λ/4 = 122.5 / 4 mm = 30.6 mm. A quarter-wave antenna behaves like a dipole antenna if it is perpendicular to a ground plane (which acts as a mirror for the antenna, creating a virtual dipole). The length of the antenna is critical.
  2. Half wavelength (λ/2) wire antenna. Twice as long as the quarter wave antenna. Again, the length of the antenna (61.2mm) is critical. Usually a half-wave antenna is fed from the middle, effectively creating a dipole antenna.
  3. Chip antenna. This antenna type can be very small and can be found for varying gain levels. They're usually expensive and a ground plane underneath may create a shadow zone. However if you're tight on PCB real estate, this is a good option.
  4. External 50Ω antenna. Requires an antenna connector on the PCB and the antenna, hence not cheap either. The external antenna should protrude from your project box.
  5. Planar antenna. The idea is to create an electromagnetic resonator by smartly chosen bits of copper on your PCB. The simplest is the stripline antenna, but more effective antenna designs are known as planar inverted F antenna (PIFA). They take more real estate than a chip antenna, typically 10x20mm, and require a ground plane.

One day while browsing a Dutch electronics forum I found a link to application note SWRA117D from Texas Instruments with a small planar Bluetooth antenna with fairly good efficiency. You'll find the physical dimensions on page 4 of this application note. Eventually I implemented this design into an EAGLE library so I could integrate it in my PCB layout. Here's how it looks like (top left):

BTM-112 Bluetooth module with planar antenna (detail)

You can download this EAGLE library from my website:

Have fun!

Olivier

Vita's picture

BTM-112 Eagle Library

Hi,
Please, where did you find a BTM-112 eagle library. Can you please send link or the library?

Olivier Biot's picture

Re: BTM-112 Eagle Library

Dear Vita,
The BTM-112 library I used, can be found on Google.
You can download it from wisclub.googlegroups.com, more precisely this location.

Oded's picture

PCB antenna

Dear Olivier,
 
Have you already tested the inverted F antenna.? does the Rf link is O.K?
Does your PCB placed in plastic box, Is there much effect of the box on the Rf link?
Thanks
Oded

Olivier Biot's picture

Re: PCB antenna

Hi Oded,
So far I did not have the time yet to make one. I'm still completing another bigger project (14" telescope) and then I'll have some time again for the smaller electronics project requiring this RF antennal (Bluetooth communication with a telescope mount).
Regarding shielding, as long as you use an RF transparent material (I believe plain ABS and PP are quite transparent for RF) there should be no problem. Also, screws and other metal parts in the vicinity will have an impact on the performance of the antenna as they may de-tune the resonator of the fractal antenna.
Hope this helps,
Olivier

RM's picture

Typo

c/f = 3E+8 / 2.45E+9 m = 0.1225 m = 12.25mm
That should be 12.25 cms.
RM

Olivier Biot's picture

Re: Typo

Thanks for catching that typo!
The text has been updated accordingly.

Alfred's picture

Thank you very much for the

Thank you very much for the explanation on different RF antennas, Olivier!
I'm looking into integrating an antenna in one of my projects and it really helped me realise what I need according to my project especifications.
I tried grabbing the eagle library from a link I found on one of your comments (the first on the page) but the link seems dead! Checked the link in the end of the post and it's working fine though.
Also, thanks for recommending using ABS or PP as shielding, it was another issue you helped me solving out!

Great post.
Thanks a lot!

_____
Alfred
AC to DC Converter

Hanson's picture

13.56mhz antenna eagle lib.

Do u have EAGLE library for 13.56Mhz pcb antenna? rather than 2.4GHZ

Tobi's picture

13.56 Mhz <> 2.4 Ghz

The frequency bands you're asking about are *slightly* different:2.4 Ghz means a wavelength of appr 12.5 centimeters13.56 Mhz means a wavelength of appr. 22 meters.
Unless you add various electronic components to shorten your antenna, it ought to be around a quarter wavelength long, or some 5.5 meters. I don't think that a pure PCB antenna would be the most economical approach.

Nemo's picture

Pins are flipped

If anyone is having trouble connecting nets to the schematic symbol, it's because the RF and GND pins are reversed.  Best fix is to edit the library to flip these pins 180 degrees.  Otherwise, just connect the net to the wire INSIDE the package, not to the wire sticking out.
 
Thanks for making this!  Have you have any other Eagle libraries?

Nemo's picture

Pins are flipped

If anyone is having trouble connecting nets to the schematic symbol, it's because the RF and GND pins are reversed.  Best fix is to edit the library to flip these pins 180 degrees.  Otherwise, just connect the net to the wire INSIDE the package, not to the wire sticking out.
 
Thanks for making this!  Have you have any other Eagle libraries?

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