What foamed/pore hard plastic used in some horn / satellite/ etc. antennas?

Sometimes it is machined to form a Frensel lens, sometimes it is very thick, even thicker than a wavelength and allows to make durable horn with very thick metal around it. I want to know what is that material, it looks like there are many small pores in it, very lightweight, different in color, white, gray/yellow, black. I guess with good characteristics, without too much microwave absorption, etc.

Ask the manufacturers! The material you look for is polyurethane. It is hard and durable and low-cost.

Generally spoken when you have a very light-weight foam, the loss can be very low, even when the base material has relative high loss. Low density results in low loss and relative dielectric constant close to 1. From experience with a high Q resonating antenna, prolonged exposure to moisture may increase the dielectric loss.

Some expensive examples you can find at:
http://www.eccosorb.com/products-ecc...ielectrics.htm

Wow, nice =)