Context : The origin of broad-absorption-line quasars ( BAL QSOs ) is still an open issue .
Accounting for \sim 20 \% of the QSO population , these objects present broad absorption lines in their optical spectra generated from outflows with velocities up to 0.2 c .
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 ) .

Aims : 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 .
We also consider a third model in which BALs are due to polar jets driven by radiation pressure .

Methods : 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 spectral indices give clues to the orientation of these objects , while the determination of the peak frequency can constrain their age , and test the evolutionary scenario , in which BAL QSOs are young QSOs .
The fractional polarisation and the rotation measure in part reflect the local magnetic field strength and particle density .

Results : The fractions of resolved sources in the BAL and non-BAL QSO samples are similar ( 16 % vs 12 % ) .
The resolved sources in the two samples have similar linear sizes ( 20 to 400 kpc ) and morphology .
There is weak evidence that the fraction of variable sources amongst BAL QSOs is lower .
The fractions of candidate GHz-peaked sources are similar in the two samples ( 36 \pm 12 % vs 23 \pm 8 % ) , suggesting that BAL QSOs are not generally younger than non-BAL QSOs .
BAL and non-BAL QSOs show a large range of spectral indices , including flat-spectrum and steep-spectrum sources , 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 ( \geq 97 % ) , the spectra of BAL QSOs are not flatter than those of non-BAL QSOs , which suggests that a polar orientation is not preferred .
The distributions of fractional polarisation in the two samples are similar , median values 1-3 % .
The distributions of rotation measure are also similar , the only outlier being the BAL QSO 1624+37 , with an extreme rest-frame Rotation Measure ( from the literature ) of - 18350 \pm 570 rad m ^ { -2 } .

Conclusions :