Originally Posted by n0nhp
Most PVC is not RF resistive. The definitive test is to take a small piece and place it in a microwave oven next to (Not in) a cup of water.
Just a comment...
That's a decent general test, but materials can have different properties at different frequencies. Also, one thing that's not detected in this test is the dielectric constant, which will have some effect on the tuning of an antenna, even if it doesn't cause a noticable amount of absorption.
I've made a number of antennas over the years, and PVC makes a pretty decent radome below a few GHz, but in every case, I've had to account for the detuning that takes place when the antenna is inserted into the pipe.