A window has two plates of radiation-propagative material, such as two plates of glass or two films of transparent dimensionally-stable plastic which serve as window panes through which electromagnetic radiation can propagate. The panes are spaced apart by one quarter wavelength of the radiation by a spacer block to provide for cancellation of any reflected wave. The window is readily disassembled to permit replacement of the spacer block to increase or to decrease the spacing as may be required to adjust the window for a different frequency band of radiation. Also, the construction of the window permits its mounting within the aperture of a wall, and to serve for joining two waveguides, thereby, to facilitate use of the window in the conduct of tests in a laboratory. The spacer block with the transparent plates thereon are positioned between a base and a cap, both of which are configured for attachment to waveguides. The base and the spacer block and the cap are provided with openings arranged coaxially and through which radiant energy propagates. A set of bolts and alignment pins extend through the window components for facile assembly and disassembly.

A high average-power microwave window is provided whose thermal conductivity has been enhanced to enable it to transmit higher average RF power levels than conventional windows of the same size. Such a window is suitable for use with high-average power RF sources such as klystrons and magnetrons. The window comprises a ceramic substrate, typically a low-loss ceramic such as alumina or quartz, to which narrow strips of a high thermal conductivity material have been bonded. One such high thermal conductivity material is synthetic polycrystalline diamond, which can be bonded to the surface of a dielectric substrate using a high-temperature cement or can be directly deposited on the surface by a process such as chemical vapor deposition (CVD). High-purity alumina, a commonly-used material for high-power RF windows, has a thermal conductivity of 26.4 W/m.multidot..degree. C., while synthetic diamond has a thermal conductivity of 1000 W/m.multidot..degree. C., 2.6 times that of copper and 38 times that of alumina. The novel feature is the use of high thermal conductivity strips to increase the effective thermal conductivity of a microwave window by providing low-resistance paths by which heat can be extracted from the window, resulting in a significant increase in the window's power-handling capacity.