Fractional polarization of the radio filaments. The points at 85 and 200 cm are upper limits (see text). The 21 cm value was computed at the same resolution as the 85 cm observations. Given the low resolution of the 2 m observations, the measurement at 150 MHz was not taken into account during the discussion. The horizontal bars indicate the λ² coverage of the observations. The sinc function corresponds to a Burn slab of Faraday thickness of 4 rad m^-2, normalized to the 21 cm fractional polarization (14%) at the 85 cm resolution.