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:
- 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.
- 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.
- 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.
- 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.
- 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):
You can download this EAGLE library from my website: