DIVERSITY RECEIVE and CIRCULAR POLARIATION TRANSMIT WITH THE 432 MHz FEEDHORN

 

The design of the horn permits a useful connection configuration to alleviate one of the most frustrating aspects of working 70 cm moonbounce; namely deep fading that results from unfortunate Faraday rotation conditions. With separate monopoles for the two orthogonal polarization angles on the horn, it is possible to arrange things so that the transmit waves are circularly polarized to reduce some of the dramatic fading effects that frequently occur between linearly polarized stations. Circular polarization on transmit from my station allows the signal to be more constant at linearly polarized receiving stations than a linearly polarized transmission from me would be to them, as circular polarization eliminates any effects of Faraday rotation. Of course, using circular polarization on transmit to a linearly polarized receiving station will drop the received signal strength they observe from me by 3dB relative to the very rare situation when both transmit and receive stations (linearly polarized) are perfectly aligned. The advantage of using circular polarizatiion for transmit is that my signal will never be "locked out" at the receiving station due to any cross polarization effect, as sometimes occurs between stations that communicate using linear polarization. Indeed, the transmitted signal from me, as received by a linearly polarized station, is more constant in level than is a station transmitting with linear polarization during periods when the degree of Faraday rotation is changing.

As the horn contains two independent and orthogonal monopole antennas, it is possible to configure the receive system in a polarization diversity arrangement. Using polarization diversity on receive substantially improves my ability to receive signals, with much less fading due to polarization-angle mis-alignments, because with polarization diversity the receive system is uniformly sensitive to all waves regardless of the polarization angle of the incoming wave. In addition, of course, I will always be able to hear my own echo if I use polarization diversity reception, no matter what Faraday rotation conditions may exist.

A configuration of the feedhorn connections to achieve these two conditions is shown below; this is now the "normal" configuration for my station for 70 cm EME communications:

This configuration requires four coaxial lines between the shack and the dish; one for high power Tx, one for the local oscillator for the mixers, and one for each of the two Rx IF lines back to the shack. Further, for wide-bandwidth diversity receive, it is necessary that the two IF lines back to the shack are coherent paths for the rf, i.e., identical in lengths, etc, insofar as it is reasonably possible/practical. These RXn lines were measured out together and installed together to ensure that the two rf paths are as similar as possible. Fine phasing adjustments to make the two lines coherent are done in the shack.

In the shack the two IF Rx lines are each connected to separate Mercury receiver boards in an HPSDR (High Performance Software Defined Radio) radio that has been modified to accept two Mercury receiver boards on the single Atlas bus simultaneously to provide a data stream of interleaved IQ values from each board to the PC (personal computer) for diversity combining, and digital delay for fine phasing if desired, in the PC.