Abstract: A repeater system in a communication system including a base station and a mobile unit provides a communications link between the base station and the mobile unit when the mobile unit is located in an environment that is substantially closed off to high radio frequency communication between the base station and the mobile unit. The repeater system has a first linear two-way frequency converter including a high frequency port for two-way coupling to the base station and a low frequency port for two-way coupling to a low frequency signal that is capable of distributing radio frequency power through the closed environment. The low frequency port is connected to a cable which propagates and receives low radio frequency power within the closed environment.

Abstract: The invention relates to a dust indicator for detecting the presence of suspended particles, either solid or liquid, in a gas. A concentrated, substantially parallel light beam (1) of suitable wavelength is directed through a measuring chamber (2) containing the gas to be examined. The light beam is surrounded by an internally reflecting elliptical cylindrical reflector (4) in a manner such that the beam (1) coincides with one focal axis (B.sub.1) of the reflector. Arranged in the other focal axis (B.sub.2) of the reflector (4) is at least one light detector (6) with its active surface facing the reflector and the center axis of its field of view directed, preferably perpendicularly, towards the first said focal axis (B.sub.1) along which the beam (1) is directed.