  Frequency dissemination and synchronization in free space plays an important
role in global navigation satellite system, radio astronomy and synthetic
aperture radar. In this paper, we demonstrate a portable radio frequency
dissemination scheme via free space using microwave antennas. The setup has a
good environment adaptability and high dissemination stability. The frequency
signal is disseminated at different distances ranging from 10 to 640 m with a
fixed 10 Hz locking bandwidth, and the scaling law of dissemination stability
on distance and averaging time is discussed. The preliminary extrapolation
shows that the dissemination stability may reach $1\times10^{-12}/s$ in
ground-to-satellite synchronization, which far exceeds all present methods, and
is worthy for further study.
