Circularly Polarised Yagi Antenna

Antenna Requirements

The Vela pulsar signal is almost completely polarised, with linear polarisation being the dominant mode. This means there is the potential for the pulsar signal and a linearly-polarised receiving antenna to be cross-polarised, resulting in the received signal dropping to zero - possibly for the whole observation period. In the worse case this condition could last for a numbers of days - fortunately unlikely. An extra complication is that the polarisation angle swings about 45° during the pulse on phase.

Note that even though the PA (position angle) of the polarisation at the peak of the pulse of Vela is fixed in space, at 436 MHz the last stage of the signal's journey through space (Earth's ionosphere) introduces a variable amount of polarisation twist due to Faraday Rotation.

Therefore, an alignment of the polarisation of a linear-polarised antenna to be coincident to the fixed orientation in space of the Vela pulsar signal is made ineffective by this last "twist".

To avoid the effects of an unknown polarisation orientation for the incoming Vela signal it has been decided to utilise a circularly-polarised antenna - which responds to linearly-polarised signals from all orientations.