Radio spectra and polarisation properties of a bright sample of
  Radio-Loud Broad Absorption Line Quasars
  The origin of broad-absorption-line quasars (BAL QSOs) is still an open
issue. Accounting for ~20% of the QSO population, these objects present broad
absorption lines in their optical spectra generated from outflows with
velocities up to 0.2c. In this work we present the results of a multi-frequency
study of a well-defined radio-loud BAL QSO sample, and a comparison sample of
radio-loud non-BAL QSOs, both selected from the Sloan Digital Sky Survey
(SDSS).
  We aim to test which of the currently-popular models for the BAL phenomenon -
`orientation' or 'evolutionary' - best accounts for the radio properties of BAL
quasars. Observations from 1.4 to 43 GHz have been obtained with the VLA and
Effelsberg telescopes, and data from 74 to 408 MHz have been compiled from the
literature.
  The fractions of candidate GHz-peaked sources are similar in the two samples
(36\pm12% vs 23\pm8%), suggesting that BAL QSOs are not generally younger than
non-BAL QSOs. BAL and non-BAL QSOs show a large range of spectral indices,
consistent with a broad range of orientations. There is weak evidence (91%
confidence) that the spectral indices of the BAL QSOs are steeper than those of
non-BAL QSOs, mildly favouring edge-on orientations. At a higher level of
significance (\geq97%), the spectra of BAL QSOs are not flatter than those of
non-BAL QSOs, which suggests that a polar orientation is not preferred.
