Bandpass Combiner Principles and Theory of Operation

Starpoint combiners are most often used for simpler combining applications. In theory an infinite number of frequencies can be starpointed, however in application it is generally limited to a maximum of five or six channels with the bulk of the combiners of this type being two and three channel. The luxury of an inexpensive system with fairly conservative size considerations is one of the highlights of the Branch Combiner. This is due to the minimum number of components used in their construction. At this point one might wonder why all combiners are not Starpoint, after all they use bandpass filters, their inexpensive and do not require huge amounts of real estate to set them up. The downside is they do not offer the same level of performance as a Constant Impedance combiner. The isolation of a Starpoint is limited to the rejection of the filters used in the system. If one of the filters shown in Figure 1 is centered at F2 and has a rejection level of 30 dB at frequency F1 then the isolation of F1 to F2 is 30 dB. If the other filter is centered at F1 and has a rejection level of 38 dB at frequency F2 then the isolation of F2 to F1 is 38 dB. The Insertion Loss and Group Delay performance of a Starpoint Combiner will mimic that of the filters. The point here is that the system performance is a direct reflection of the filter performance, high quality, high power filters must be used and even these may not be enough.