Hi Klaus,
i was thinking of something about maybe ten inches in dia,
very light-weight, sort of a clip-on thing that would
be maybe a fold-up assembly, like those camera flash units.
I don't know the frequencies involved, but i guess that a
ten inch dish would give a reasonable capture.
Also i'm not sure that the Yagi arrangement would act as
a transmitter, it's primarily for reception.
I'm not that well up on RF propagation, but i don't think
the Yagi reflectors would operate in reverse, ie: sending.
But i could be wrong there.