Tranditionally, VLBI has been done with circularly polarized feeds since the amplitude of the cross-correlation of circularly polarized signals does not depend on the relative parallactic angles of the antennas (which varies with geographic location and pointing direction when using AZ-EL antenna positioners).
Unfortunately, most broadband feeds are linearly polarized. To get around the problem of amplitude variation wrt relative parallactic angle, it is possible to correlate all products of the linearly polarized signals and then combined those products during post-correlation fringe detection to produce a pseudo total intensity observable. Another option though is to combine the linearly polarized signals at the antenna to produce circular polarization. Accurate broadband hybrids are in fact available that would make this possible but these cannot be used prior to the LNA since they will severly degrade system noise temperature (Tsys). On the other hand, phase and amplitude imbalance of the LNAs leads to poor polarization purity if the combination is done after the LNAs. It may be possible however to compensate for the amplitude and phase imbalance using variable gain and delay components:
- post-LNA linear to circular polarization converter (insert link to Chris's Shanghai paper)