The work of double disk window design for JET EP ECRH project has been done at FZK Karlsruhe. The window unit has to provide a reliable barrier between the torus and the outer system for tritium containment. An additional requirement is that the microwave transmission of the double disk arrangement has to be suitable for two frequencies: 113.3 GHz and 170.0 GHz. Based on a new development of the brazing technique the design of the window unit has been modified. Detailed calculations of the microwave transmission characteristic of a double disk window have been performed. In particular the influence of mechanical tolerances on the transmission characteristic have been investigated in order to be able to specify the mechanical dimensions of the CVD-disks and the window unit. The calculations have been performed for several distances dgap between the disks. The first result was that an uncertainty of ± 10 micron in the thickness of each disk of the pair used in a window unit cannot be tolerated since it may cause an unacceptable reduction of the transmission characteristic (power reflection > 1%). In order to obtain a sufficient tr ... mehransmission characteristic the pair of the CVD diamond disks have to be within the following geometrical specifications: mean thickness of both disk used in one window unit: ddisk0 = (1.111 ± 0.010) mm; thickness of the each disk: ddisk = ddisk0 ± 0.005 mm. The optimum gap distance dgap between the disks depends slightly on the mean thickness ddisk0 of both disks and has to be determined for each window unit individually taking the mean value ddisk0 of the selected disk pair. Depending on ddisk0, the optimum gap distance is between dgap @ 3.4 mm and 3.2 mm for 113.3 GHz. Unfortunately, the optimum gap distance dgap is slightly different for 170.0 GHz, namely between dgap @ 3.2 and 3.0 mm. Another possible gap distance suitable for 170 GHz is e.g. between dgap @ 4.1 and 3.9 mm or between dgap @ 5.8 and 5.6 mm. The mechanical design of the window unit has to allow the modification of the distance between the disks in order to fulfill the requirements of operation at both frequencies. With the assumed thickness tolerance of ± 5 micron the reflectivity of the double disk window unit is in the worst case -25 dB at 113.3 GHz and about -22 dB at 170 GHz. In both cases the bandwidth is as large as about 2 GHz.