Broadcasting satellite services with a high sense of reality, such as ultra-high definition television (UDTV) and three-dimensional television are expected to be delivered using the frequency band from 21.4 to 22.0 GHz. One of the major technical difficulties in using this frequency band is to overcome large rain attenuation. By using the array antenna technique, in which the radiation pattern can be changed adaptively to transmit high e.i.r.p. only in heavy rain areas while the e.i.r.p. for the rest of the service area remains low, required satellite payload power can be reduced. A prototype mini-TWT array for an array-fed reflector antenna in a 21GHz-band broadcasting satellite was manufactured. Seven mini-TWTs, spaced at a distance corresponds to 1.5 wavelengths at 22.0 GHz, were aligned in a two-dimensional array. The cross-sectional dimensions of each mini-TWT were 15.3 mm x 20.0 mm. Flat heat pipes were mounted between adjacent mini-TWTs to provide sufficient heat transfer for the mini-TWTs located closely. Moreover, thin magnetic shields were applied around each mini-TWT so that leakage of the magnetic fields of neighboring mini-TWTs, which might influence the electron beam focusing, could be reduced. We have confirmed that the performance of the central mini-TWT of this array structure is almost same as that of an isolated mini-TWT.