Ian wrote:
"Can a passive repeater work on my cell phone?"

Whenever you use a passive repeater in a path, you split the path in
two. The attenuation is greatest in the first few feet of the path.
After the first few feet, signal strength declines only about 6 dB every
time distance from an antenna doubles. The much greater loss of two
paths (add about 22 dB) versus the loss in a single path is the reason
passive repeaters are unpopular in most cases.

If very high gain antennas can be used with a passive repeater, its high
loss can be overcome. This is unlikely in a cell phone application.
Microwave systems use passive repeaters in unusual cases such as
periscope antennas and mountain top billboard reflectors that are very
large and close spaced.

If one antenna of a back to back pair can "see" one end of a radio path,
and the other antenna can "see" the other end, the pair may make an
effective repeater, but will have much loss.

Best regards, Richard Harrison, KB5WZI