Circularly Polarised Yagi Antenna
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.
Sourcing a Circularly Polarised Antenna
I decided to go down the path of purchasing a commercial circularly-polarised antenna. A good example is one intended for amateur radio use in the 70 cm band (432 MHz).
This entailed considerable expense, but the fact that, if it failed to deliver success w.r.t. detecting Vela, it can be used for its original design purpose of amateur radio activities took away some of the financial pain - and avoided yet another "white elephant" antenna lying around.
The antenna decided upon is the M2 436CP42UG circularly-polarised antenna.
There have been several configurations built.
Configuration #2 is the current configuration.
Configuration #1 was the initial configuration (see this configuration for Yagi construction details)
M2 436CP42UG Circularly-Polarised Antenna