On-board electronics for an active near field multibeam

Some results of a study on active antennas for communications satellites are presented. A comparison between the performances of the antenna system in the two cases of a beam forming network (BFN) at low (IF) or high (RF) frequency is carried out, and the results are presented. The comparison is performed from the point of view of total weight, size, bandwidth, and reliability for a near-field multibeam antenna covering Western Europe. In the case of a low-frequency BFN the system is formed by a diode matrix followed by 37 chains of two cascaded mixers for the two-step up-conversion. The prevailing mass contribution comes from the mixers, even if they are refunded, while the use of the active matrix has the greater impact on reliability and bandwidth limitation. In the case of a high-frequency BFN the single or double (cascaded) bank of 14 mixers is followed by a hybrid BFN. The mixers, even if fully redundant and with a two-step up-conversion, are fewer in number, and therefore lighter, than in the previous case, while the reliability is ensured to a good level