dan,
if i understand you correctly, u would like to replace the antenna on the hand held scanner to improve reception while still being able to be mobile...
if the HH scanner has a short rubber duckie type antenna, a small improvement may be had by going to a longer one if available for it i am not familiar with the unit. how much improvement remains to be seen by trying it.
i am not sure that it would be worth it really.
when you are on the fringe of a signal pattern,,, gaining elevation, improving line of sight, or getting closer may be the best options.
best of luck hope this helps a bit