Elimination of fine tuning in high power, low-PIM diplexers for combined transmit/receive antennas

The use of combined transmit/receive antennas on spacecraft can reduce the mass by a considerable amount. Such a configuration normally requires the diplexing of the receive and transmit bands onto a common feed. Due to the difference in power levels, problems can arise if passive intermodulation (PIM) products from the transmit carriers fall within the receive band at power levels of the same order as the receive carriers. The design and performance of a low-PIM diplexer is described. It is shown that a transmission line model may be used for the precise design of rectangular waveguide manifold diplexers which maintain the Chebyshev transfer function properties of the low-pass prototype. The use of mode matching in the final stages of realisation allows the unit to be constructed in two halves and the performance maintained without the need for tuning screws. By reducing the number of metal to metal contacts lower PIM levels can be achieved. It is shown that for two 100 W carriers input to the transmit port, the 3rd Order APIM level is lower than -140 dBm, i.e. -190 dBC, over a temperature range of -40 deg C to +120 deg C. The variation of PIM level with both power and PIM order is presented