<p>I am looking for a plastic which is transparent to radio waves.  I want to place my transmitter in a cylinder. That cylinder would be placed in a big RC plane  ( whose body is made up of cardboard). I want the plane to be both telemetry, and RC controlled.  That cylinder should allow the signals,  should be strong and light.<br>
So which material would you suggest and is that material easy to do 3d print? </p>

For the kind of application you are looking for, transparency to radio signal shouldn't really be an issue, so you are more or less free to choose whatever suits your taste better.
Looking at the 3d printed drone community, the 3 most common materials I see being used there are:
impact-resistent PLA
PETG
ABS
This order also match their "ease of use", with PLA being very easy to print even without heated bed, behaving well with glues in the assembly and being easy to paint on.  The impact resistance of "though PLA" still doesn't match that of - say - ABS, but is typically considered "good enough" for anything but the propellers.
PETG is tougher. Print relatively easy (stringing and oozing being the typical problems) but it is known to be somewhat difficult to glue and paint.  It is also quite dense, so - dimensions being the same - it will weight more than PLA.
Finally, ABS is a classic.  It is strong, durable, easy to finish (with acetone) but it is the most finicky material to print with, requiring an enclosure and proper ventilation (the fumes being toxic).