They show up on most ATC radars only because they have a transponder.

Nonsense they show up on primary radar very well

As posted, I seemed to be invisible to the PSM tower when I asked
permission to descend through the Delta airspace earlier this summer.
Flying a rag & tube Cub with no transponder. Can't speak for spam
cans--all the local ones have transponders anyhow.

all the best -- Dan Ford
email: www.danford.net/letters.htm#9

see the Warbird's Forum at www.warbirdforum.com
and the Piper Cub Forum at www.pipercubforum.com