The internal antenna was designed with the small gap above WLAN in the 2.4 GHz ISM band as target.
The actual antenna seems to be closer to the lower part of the 2.4 GHz band, though.
These are some measurements made with a ZVH8 network analyzer:
These show that the antenna of this specific rad1o is most sensitive around 2.36 GHz and is still sensitive up to 2.45 GHz. Above that, the sensitivity starts to drop.
This measurement was made with an SMA connector attached to the rad1o and a 0 Ohm resistor bridging the SMA connector to the internal antenna:
Please keep in mind that different rad1os might have slightly different characteristics.