  We demonstrate a remote microwave/radio-frequency (RF) transfer technique
based on the stabilization of a fiber link using a fiber-loop optical-microwave
phase detector (FLOM-PD). This method compensates for the excess phase
fluctuations introduced in fiber transfer by direct phase comparison between
the optical pulse train reflected from the remote site and the local
microwave/RF signal using the FLOM-PD. This enables sub-fs resolution and
long-term stable link stabilization while having wide timing detection range
and less demand in fiber dispersion compensation. The demonstrated relative
frequency instability between 2.856-GHz RF oscillators separated by a 2.3-km
fiber link is $7.6 \times 10^{-18}$ and $6.5 \times 10^{-19}$ at 1000 s and
82500 s averaging time, respectively.
