1) Ductwork 2) Materials for making ducts 
(from the article `telecommunications media`) ...body of water, with vapour concentration increasing closer to the surface, can cause a bending of the beam toward the Earth that is much sharper ...
guided propagation of radio waves inside a tropospheric radio-duct NOTE - At sufficiently high frequencies, a number of electromagnetic modes of guided propagation can co-exist in the same tropospheric radio-duct.
A system of shafts or tubes designed to carry and protect cables or pipes.
A system of shafts or tubes designed to carry and protect cables or pipes.
ductwork. · materials for making ducts.