Ben Flewett wrote:
A number of times I have switched on my transponder
and called for a clearance but ATC have not been able
to see me. I can see my transponder being interogated,
I have good battery power, everything seems to be working
but ATC still can't see me. This has happened to a
number of pilots I know.

If you don't call for a clearance you have no idea
whether you are transmitting or not.

Man, if I had a nickel for every time ATC said they didn't have
me but the airliner 10 miles out had me plain as
day, I'd have some nickels!

Too low, in line with another transponder, receiver too
far away, etc. If I let an ATC guy convince me to swap out
a $1000+ part, I'd get his avionics repair certificate number
first... ;(