NRf52840 dongle antenna design seems strange

[Jason]

I searched for existing forum posts but couldn't find anyone asking this

Why doesn't the dongle use the inverted F antenna?

The dongle looks like this

The inverted F looks like this. 

I thought the loop to ground added inductance, which helped the antenna gain. 

Also, the GND plane seems to be only 26mm, whereas I expected it to be at least 31.25mm (lambda/4)

Thanks for your help, 

Kind regards,

-Jason

[mathiaso]

Hi, Jason.

Why doesn't the dongle use the inverted F antenna?

You are wondering why it was chosen to use a meandered λ/4 printed monopole antenna instead of a meandered inverted-F antenna?

I thought the loop to ground added inductance, which helped the antenna gain.

If it is desired to have an antenna that radiates more like an isotropic antenna, it isn't desired to have high gain, as this will concentrate the radiation more in a certain direction(s).

Also, the GND plane seems to be only 26mm, whereas I expected it to be at least 31.25mm (lambda/4)

31.25 mm, found by calculating (3e8 m/s / 2.4 GHz) × 1/4, is the wavelength in free space. The relative permittivity of the PCB material also has to be taken into account, which will shorten the guided wavelength. In addition to this, the fact that the dongle will be connected to something will also increase the size of the ground plane.

Best regards,
Mathias

[Jason]

wow, I didn't realise the velocity factor would be so low. I made my PCB slightly over 32mm long because I was told this was the minimum size  *(by a friend)

Thanks for the help, this makes sense. 

Kind regards,

-Jason