The radial artefact I'm talking about extends for hundreds of km (see attached recent example, still visible days later), on the National radar scan, which is a composite image. It consistently extends from Melbourne to the NE corner of Tasmania. This cannot be explained by birds or insects. I've done a bit of reading on the web and dug up these points:
1. Modern weather radars are pulsed Doppler radars operated at several standard angles of elevation, the lowest usually being 0.5deg, and this is so low it is often blocked by trees, hills and man-made structures. This gives a low return signal ("no dbZ") and for reasons not explained but presumably due to the heavy post-acquisition signal processing, this often causes long radial spikes as clearly shown in Fig 7 of this PDF:
http://www.weather.gov/media/btv/research/Radar Artifacts and Associated Signatures.pdf
and also in the figure in the "Blocking Beam" section of this site:
http://www.ec.gc.ca/meteo-weather/defau ... 2B931828-1
These will be seen even in the base, non-composite scans. It seems to me that the beam-blocking is a physical event, but the generation of radial lines on the scan is not.
2. Next possibility is EM interference, and C-band EMI from wireless devices can cause sharp radial spikes: see the figure in the final section on EMI in the 2nd reference.
3. Next possibility is artifact due to the program that stitches together the scans at different elevations to produce the composite mode scans often displayed. This is a 3D modelling artifact.
4. Final possibility is artifacts in the stitching program which combines and interpolates images from different geographical sites to produce the 512km composite and the National scans. I feel this might be the cause here, in part because consistent artifact lines in these 2 larger scale scans are not seen in the 64, 128 and 256km scans on the same day.
Most of these (except the EMI case) appear to be due to software artifacts rather than physical effects on the radar hardware. However I still haven't found any info that explains how a low signal, or database inconsistencies between sources of a composite image, creates a linear software artifact.