A Non-reflecting Branching Filter for Microwaves

T ) R E S E N T plans for point-to-point communication by means of microwave radio relay systems call for the operation of several radio channels between each pair of repeaters. A proposed frequency plan for the 40C0 mc common carrier band (3700 to 4200 mc) specifies channels 20 mc wide spaced 40 mc center to center. A possible arrangment for a five-channel radio repeater station is illustrated in Fig. 1. This arrangement is calculated to utilize the available frequency space in an efficient and technically sound manner. If this channel disposition is to be achieved without a costly increase in the number of antennas and the size of the supporting towers, radio frequency branching networks must be provided which connect the individual transmitting or receiving circuits at each repeater point to a common antenna (Fig. 1). If this connection is to be made without excessive loss of power these branching devices must have adequate adjacent channel rejection, low ohmic loss, and good impedance match in the channel bands. An excellent impedance match is especially desirable if circuit disturbances resulting from echoes in the long waveguide lines which lead from the filter assemblies to the antennas are to be minimized (Fig. 1). Since the type of microwave radio repeater now planned 1 obtains most of its gain at intermediate frequencies, the IF amplifiers will reject all spurious 1 See H. T. Friis, "Microwave Repeater Research", to appear in the April 1948 issue of B. S. T. J.