- Discussion of polarization issues (link to Brian's memo)
- Generation of pseudo total intensity observable (link to short memo that hasn't been written yet?)
- VGOS feed comparison and summary (link to Bill's feed comparison)
- Eleven Feed
- QRFH
- Italian Feed
- Yebes circularly polarized conical feed
The feed is the part of the front end that converts electromatic waves propagating in free space into electrical signals in a wire. Technically it is part of the antenna and not the receiver.
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, all products of the linearly polarized signals will be correlated and then those products will be combined during post-correlation fringe detection to produce a pseudo total intensity observable.
Some antenna feeds are single ended so that only one low noise amplifier (LNA) is required per polarization. Other feeds have multiple outputs per polarization and these outputs need to be combined. To avoid losses in the combining circuits, one LNA is required per output.
Recently broadband circular polarized feeds are being developed based on a conical topology. Even with circularly polarized feeds, all polarization products will be correlated to help calibrate delay biases related to polarization impurity.