Bluetooth only uses one frequency band – the 2.4GHz ISM Band (2.4-2.4835 GHz).
Bluetooth devices hop between frequency channels in order to coexist with the entire menagerie of devices that share the 2.4 GHz ISM band, including other WLAN technologies and microwave ovens.
Bluetooth Low Energy (BLE) has fewer channels (40) than standard Bluetooth (79), but from your antenna’s perspective there’s no difference – it’s all the same frequency band.
So if you’re designing a Bluetooth device, the 2.4 GHz ISM band defines your fundamental antenna parameters for any Bluetooth RF design.
The freespace (in air) wavelength of an electromagnetic wave is calculated by taking the speed of light in freespace divided by the frequency:
In freespace,the speed of light ‘c’ is 299,792,458 meters / second.
Therefore, the range of freespace wavelengths of Bluetooth signals goes from =c=2.4835GHz to c=2.4GHz
Put simply, a bluetooth signal’s wavelength in free space is between 120mm and 125mm.
With no special RF voodoo, we can make a very efficient antenna, with length Lo, (the “unloaded” length an antenna) from a quarter-wavelength conductor, as long as we have a reasonable ground plane to drive the antenna against so the combination of antenna and its ground plane will effectively be a ½ wavelength long.
Lo = Lambda/4= 31mm for the “long” wavelength 2.4GHz end of the band
So 31mm is the desired length of a Bluetooth antenna without “dielectric loading” (see next section).
Here’s a rule of thumb we suggest: if you have a 40mm x40mm square available on your PCB for your ground plane and antenna, you can consider a relatively large off-the-shelf chip antenna or PCB trace antenna for your bluetooth application.