  Polarisation measurements of solar flares at millimetre-waves were used to
investigate the magnetic field configuration of the emitting sources. We
analyse two solar flares (SOL2013-02-17 and SOL2013-11-05) observed by the
POlarisation Emission of Millimetre Activity at the Sun (POEMAS) at 45 and 90
GHz, at microwaves from 1 - 15 GHz by the Radio Solar Telescope Network (RSTN),
and at high frequencies (212 GHz) by the Solar Submillimetre Telescope (SST).
Also, hard X-rays from these flares were simultaneously detected by the Reuven
Ramaty High-Energy Solar Spectroscopic Imager (RHESSI). The flux and
polarisation radio spectra were fit using a model that simulates
gyrosynchrotron emission in a spatially-varying 3D magnetic field loop
structure. For the {modelling}, the magnetic loop geometry was fixed and the
field strength was the only free parameter of the magnetic field. In addition,
a uniform electron distribution was {assumed by} the model, with the number
density of energetic electrons and the electron spectral index as free
parameters. The fitted model reproduced reasonably well the observed degree of
polarisation and radio flux spectra for each event yielding the physical
parameters of the loop and flaring sources. Our results indicate that the high
degree of polarisation during a solar flare can be explained by two sources
located at the {footpoints} of highly asymmetric magnetic loops whereas low
polarisation degrees arise from footpoint sources of symmetric magnetic loops.
