The brochure on the site says the beamwidth is 23 degrees.
The antenna gain is listed at 16dBi which would correspond to about 24 degrees of beamwidth so this is probably correct.
@pe1chl: How did you convert dBi to beam width angle? I have been trying to figure it out assuming that the lobe is a round or elliptical cone but I haven't come up with a formula. I know there are other cases like omnidirectional antennas, too.